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@Ci{$Id: manual.of,v 1.166 2016/12/22 15:46:25 roberto Exp roberto $}
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@manual{

@sect1{@title{Introduction}

Lua est un langage de script puissant, efficace, léger, embarquable.
Il supporte la programmation procédurale,
la programmation orientée objet, la programmation fonctionnelle,
la programmation data-driven et la description de données.

Lua combine une syntaxe procédurale simple avec une puissante construction de description des données
basée sur des tableaux associatifs et une sémantique extensible.
Lua est typé dynamiquement,
basé sur l'interprétation d'un bytecode et
une machine virtuelle.
Il possède une gestion de la mémoire avec
un garbage collector incrémental,
le rendant idéal comme outil de configuration, pour l'écriture de script
ainsi que pour le prototypage rapide.

Lua est implémenté comme une bibliothèque, écrite en @emphx{clean C},
le sous-ensemble issue du @N{Standard C} et C++.
La distribution du Lua comprend un programme hôte nommé @id{lua},
qui utilise la bibliothèque Lua pour offrir un interpréteur Lua complet,
et autonome,
à utiliser interactivement ou comme interpréteur.
Lua est à la fois pensé pour être un langage de script puissant et léger
pour les besoins d'extension de tout programme,
et comme un langage autonome puissant, léger, et efficace.

En tant que langage d'extension, Lua n'a pas de fonction principale @Q{main} :
il fonctionne comme un langage embarqué sur un hôte,
appelé @emph{programme embarqué}, ou simplement @emphx{hôte}.
(Souvent, cet hôte est un programme @id{lua} autonome.)
Le programme hôte peut appeler des fonctions pour exécuter des blocs (@emph{chunk}) de code Lua,
peut écrire et lire des variables Lua,
peut enregistrer des @N{fonctions C} pour être appelées avec du code Lua.
Avec l'utilisation de @N{fonctions C}, Lua peut être utilisé dans
de nombreux domaines de programmation,
et ainsi créer des langages de programmation partageant une même base syntaxique.

Lua est un logiciel libre,
et est fourni sans aucune garantie,
comme indiqué dans sa licence.
L'implémentation décrite dans ce manuel est disponible
sur le site officiel de Lua, @id{www.lua.org}.

Comme n'importe quel autre manuel de référence,
ce document n'a pas pour but d'enseigner, mais de renseigner.
Pour discuter des décisions à propos du design de Lua,
voir les documents techniques disponibles sur le site de Lua.
Pour une introduction technique à la programmation en Lua,
voir le livre de Roberto, @emphx{Programming in Lua}.

}


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@sect1{basic| @title{Concepts de base}

Cette section décrit les concepts de base du langage.

@sect2{TypesSec| @title{Valeurs et types}

Lua est un @emph{langage typé dynamiquement}.
Cela signifie que
les variables n'ont pas de type, seules les valeurs en ont un.
Il n'y a pas de définition de type dans le langage.
Toutes les valeurs portent leur propre type.

Toutes les valeurs dans Lua sont @emph{des valeurs de première classe}.
Cela signifie que toutes les valeurs sont stockées dans des variables,
passées comme arguments à des fonctions, et renvoyées comme résultat.

Il y a huit @x{types de base} en Lua :
@def{nil}, @def{boolean}, @def{number},
@def{string}, @def{function}, @def{userdata},
@def{thread}, et @def{table}.
Le type @emph{nil} ne contient qu'une valeur, @nil,
qui a la principale propriété d'être différente de toutes les autres valeurs ;
il représente usuellement l'absence de valeur utile.
Le type @emph{boolean} contient deux valeurs, @false et @true.
@nil et @false sont des conditions fausses ;
toutes les autres valeurs sont des conditions vraies.
Le type @emph{number} représente à la fois
les nombres entiers et réels.
Le type @emph{string} représente des séquences d'octets immuables.
@index{eight-bin clean}
Lua est 8-bit clean :
Les strings peuvent contenir n'importes quelles valeurs de 8 bits,
incluant le @x{caractère nul} (@Char{\0}).
Lua est aussi indépendant de l'encodage ;
aucune supposition n'est faite sur le contenu d'un string.

Le type @emph{number} utilise deux représentations internes,
ou deux @x{sous-types},
l'une appelée @def{integer}, et une autre appelée @def{float}.
Lua a des règles explicites quant à la représentation à utiliser,
mais peut également les convertir automatiquement quand cela est nécessaire @see{coercion}.
Par conséquent,
le programmeur peut choisir d'ignorer la différence
entre les integers et les floats la plupart du temps,
ou bien contrôler complètement quel type utiliser.
Le Lua standard utilise des integers sur 64 bits et des floats à double précision (64 bits),
mais vous pouvez aussi compiler Lua pour le
faire utiliser des integers sur 32 bits et/ou des floats à simple précision (32 bits).
Le fait d'utiliser 32 bits pour et les integers, et les floats
est particulièrement avantageux
pour les petites machines, et les systèmes embarqués
(voir la macro @id{LUA_32BITS} dans le fichier @id{luaconf.h}).

Lua peut appeler (et manipuler) des fonctions écrites en Lua, et des fonctions écrites en C @see{functioncall}.
Les deux sont représentées par le type @emph{function}.

Le type @emph{userdata} est capable de stocker des @N{données C} dans des variables Lua.
Une variable de type userdata représente un bloc mémoire.
Il y a deux types d'userdata :
@emphx{full userdata}, qui est un objet dont l'emplacement en mémoire est géré par Lua,
et @emphx{light userdata}, qui est simplement une valeur de type @N{pointeur C}.
Le type userdata n'a pas d'opérations prédéfinies en Lua, en dehors de l'affectation et des tests d'identité.
En utilisant les @emph{metatables}, le programmeur peut définir des opérations sur les valeurs de type full userdata @see{metatable}.
Les valeurs de type userdata ne peuvent pas être créer ou modifier en Lua, mais uniquement en utilisant @N{l'API C}.
Cela garantit l'intégrité des données propres au programme hôte.

Le type @def{thread} représente des threads d'exécution indépendants de l'exécution et est utilisé pour implémenter des coroutines @see{coroutine}.
Les threads Lua ne sont pas liés aux threads du système d'exploitation.
Lua supporte les coroutines sur tous les systèmes, même sur ceux qui ne supportent pas les threads nativement.

Le type @emph{table} implémente les @x{tableaux associatifs} c'est à dire des @x{tableaux} qui peuvent être indexés non seulement avec des nombres, mais avec n'importe quelle valeur Lua exceptés @nil et @x{NaN} (@emphx{Not a Number} est une valeur spéciale utilisée pour représenter des résultats numériques indéfinis ou non représentables, comme par exemple @T{0/0}).
Les tables peuvent être @emph{hétérogènes} ; c'est à dire qu'elles peuvent contenir des valeurs de tous les types (excepté @nil).
Les clés de valeur associée @nil ne sont pas considérées comme faisant partie de la table.
Réciproquement, toute clé qui ne fait pas partie de la table a pour valeur associée @nil.

Les tables sont le seul mécanisme de structures de donnée propre à Lua ; elles peuvent être utilisées pour représenter des tableaux ordinaires, des listes, des tables de symboles, des dictionnaires, des enregistrements, des graphes, des arbres, etc.
Pour représenter les @x{enregistrements}, Lua utilise le nom du champs comme un index.
Le langage supporte cette représentation en fournissant un identifiant @id{a.name} comme sucre syntaxique pour @T{a["name"]}.
Il y a plusieurs façons pratiques de créer des tables en Lua @see{tableconstructor}.

Tout comme les indices, les valeurs des champs d'une table peuvent être de tous les types.
En particulier, comme les fonctions sont des valeurs de première classe, elles peuvent être associées aux champs d'une table.
Ainsi, les tables peuvent contenir des @emph{méthodes} @see{func-def}.

L'indexation des tables suit la définition de l'égalité pure dans le langage. Les expressions @T{a[i]} et @T{a[j]} indiquent le même élément de la table si et seulement si @id{i} et @id{j} sont égaux (c'est à dire, égaux sans méta-méthodes).
En particulier, les flottants avec des valeurs entières sont égaux à leur représentation entière respective (e.g., @T{1.0 == 1}).
Afin d'éviter les ambiguïtés, tous les flottants avec une valeur entière sont automatiquement convertis vers leur forme entière respective.
Par exemple, si vous écrivez @T{a[2.0] = true}, la clé insérée dans la table sera l'entier @T{2} (d'un autre côté, 2 et @St{2} sont deux valeurs différentes et pointent ainsi vers deux entrées de la table différentes).

Les valeurs de type table, fonction, thread et (full) userdata sont des @emph{objets} :
les variables ne @emph{contiennent} pas ces valeurs, elles @emph{renvoient} vers elles.
Les affectations, passages de paramètres, et retours de fonctions manipulent toujours des références vers ces valeurs ; ces opérations n'impliquent pas une quelconque copie.

La fonction de bibliothèque @Lid{type} renvoie une chaîne de caractères décrivant le type de la valeur passée en argument @see{predefined}.

}

@sect2{globalenv| @title{Environnements et environnement global}

Comme nous le verrons dans @refsec{variables} et @refsec{assignment},
chaque référence à un nom libre
(c'est à dire, un nom qui n'est lié à aucune déclaration) @id{var}
est syntaxiquement transformé en @T{_ENV.var}.
De plus, chaque bloc de code (@emph{chunk}) est compilé dans la portée d'une
variable locale externe nommée @id{_ENV} @see{chunks},
donc @id{_ENV} lui-même n'est jamais un nom libre.

Malgré l'existence de cette variable externe @id{_ENV} et
la traduction de noms disponible,
@id{_ENV} est un nom complètement normal.
En particulier, vous pouvez définir de nouvelles variables et paramètres avec ce nom.
Chaque référence à un nom disponible utilise le @id{_ENV} qui est
visible à ce stade du programme,
suivant les règles habituelles de visibilité de Lua @see{visibility}.

Toute table utilisée comme valeur de @id{_ENV} s'appelle un @def{environnement}.

Lua garde un environnement distinct appelé @def{environnement global}.
Cette valeur est conservée dans un index spécial dans un registre C @see{registry}.
En Lua, la variable globale @Lid{_G} est initialisée avec cette même valeur.
(@Lid{_G} n'est jamais utilisé en interne.)

Quand Lua charge un chunk,
la valeur par défaut de la variable @id{_ENV}
est l'environnement global @seeF{load}.
Par conséquent, par défaut,
les noms disponibles dans le code Lua se réfèrent aux entrées dans l'environnement global
(et, par conséquent, ils sont également appelés @def{variables globales}).
En outre, toutes les bibliothèques standard sont chargées dans l'environnement global
et certaines fonctions modifient cet environnement.
Les fonctions @Lid{load} (et @Lid{loadfile})
permettent de charger un chunk en spécifiant un environnement.
(En C, vous devez charger le chunk puis modifier la valeur de sa première upvalue. @seeF{lua_load})

}

@sect2{error| @title{Gestion des erreurs}

Parce que Lua est un langage d'extension embarqué,
toutes les actions Lua commencent à partir de @N{code C} dans le programme hôte
appelant une fonction de la bibliothèque Lua.
(Lorsque vous utilisez Lua en mode autonome, l'application @id{lua} est le programme hôte.)
Chaque fois qu'une erreur survient pendant
la compilation ou l'exécution d'un chunk de Lua,
le contrôle revient à l'hôte,
qui peut prendre des mesures appropriées
(comme l'impression d'un message d'erreur).

Le code Lua peut générer explicitement une erreur en appelant la fonction @Lid{error}.
Si vous devez capturer des erreurs en Lua,
vous pouvez utiliser @Lid{pcall} ou @Lid{xpcall}
pour appeler une fonction donnée dans le @emphx{mode protégé}.

Chaque fois qu'il y a une erreur,
un @def{objet d'erreur} (également appelé @def{message d'erreur})
se propage avec des informations sur l'erreur.

Lua lui même génère seulement des erreurs dont l'objet est une chaîne,
mais les programmes peuvent générer des erreurs avec
n'importe quelle valeur comme objet d'erreur.
C'est au programme Lua ou à son hôte de gérer de tels objets d'erreur.

Lorsque vous utilisez @Lid{xpcall} ou @Lid{lua_pcall},
vous pouvez indiquer un @def{gestionnaire de message}
à appeler en cas d'erreurs.
Cette fonction est appelée avec l'objet d'erreur d'origine
et renvoie un nouvel objet d'erreur.
Il est appelé avant que l'erreur déroule la pile,
afin qu'il puisse recueillir plus d'informations sur l'erreur,
par exemple en inspectant la pile et en créant une trace de la pile.
Ce gestionnaire de message est toujours protégé par un appel protégé;
donc, une erreur dans le gestionnaire de messages
appellera à nouveau le gestionnaire de messages.
Si cette boucle dure trop longtemps,
Lua la casse et renvoie un message approprié.
(Le gestionnaire de messages est uniquement appelé pour des erreurs d'exécution régulières.
Il n'est appelé ni pour des erreurs d'allocation mémoire
ni pour des erreurs lors de l'exécution de finalizers @see{finalizers}.)

}

@sect2{metatable| @title{Metatbles et Metamethods}

Toute valeur en Lua peut avoir une @emph{metatable}.
Cette @emph{metatable} est une table Lua ordinaire
qui définit le comportement de la valeur d'origine
sous certaines opérations spéciales.
Vous pouvez modifier plusieurs aspects du comportement
des opérations sur une valeur en définissant des champs spécifiques dans sa metatable.
Par exemple, lorsqu'une valeur non numérique est l'opérande d'une addition,
Lua regarde si le champ de @St{__add} de la metatable de la valeur a pour valeur associée une fonction.
Le cas échéant, Lua appelle cette fonction pour effectuer l'addition.

La clé pour chaque événement dans une metatable est une chaîne
avec le nom de l'événement préfixé par deux caractères de soulignement;
les valeurs correspondantes sont appelées @def{metamethods}.
Dans l'exemple précédent, la clé est @St{__add}
et la metamethod est la fonction qui effectue l'addition.

Vous pouvez obtenir la metatable de n'importe quelle valeur
en utilisant la fonction @Lid{getmetatable}.
Lua interroge les metamethods dans les metatables en utilisant un accès brut @seeF{rawget}.
Donc, pour récupérer la metamethod pour l'événement @id{ev} dans l'objet @id{o},
Lua fait l'équivalent du coude suivant:
@verbatim{
rawget(getmetatable(@rep{o}) or {}, "__@rep{ev}")
}

Vous pouvez remplacer les metatables de tables en utilisant la fonction @Lid{setmetatable}.
Vous ne pouvez pas modifier les metatables d'autres types depuis du code Lua
(sauf en utilisant la @link{debuglib|bibliothèque debug});
vous devrez utiliser @N{l'API C} pour cela.

Les tables et les full userdata ont chacun leur propre metatable
(plusieurs tables ou userdata peuvent cependant partager une même metatable).
Les valeurs des autres types partagent la même metatable par type;
il y a une table pour tous les nombres (type number), une pour les chaines de caractères (type string) etc.
Par défaut, une valeur n'a pas de metatable définie,
cependant la bibliothèque string définit une metatable pour le type string @see{strlib}.

Une metatable contrôle comment se comporte un objet dans les opérations
arithmétiques, les opérations bit à bit, les comparaisons, les concaténations,
les appels, l'indexation et les opérations de longueur.
Elle peut également définir une fonction appelé quand une userdata
ou une table passe au @link{GC|garbage collector}.

Dans le cas des opérateurs unaires (négation, longueur, non bit à bit),
la metamethod est appelé avec un second argument factice égal au premier.
Cet argument supplémentaire est uniquement présent pour simplifier
l'implémentation de Lua (en confondant les opérateurs unaires et binaires)
et peut disparaître dans de prochaines versions de Lua.
(Dans la plupart des cas, cet argument n'a pas d'utilité.)

Voici une liste détaillée des éventements contrôlés par les metatables.
Chaque opération est identifiée par sa clé correspondante.

@description{
@item{@idx{__add}|
Opération d'addition(@T{+}).
Si un opérande pour une addition n'est pas un nombre
(ni une chaîne assimilable à un nombre),
Lua essaiera d'appeler une metamethod.
Tout d'abord, Lua vérifie le première opérande (même s'il est valide).
Si cet opérande ne définit pas une metamethod pour @idx{__add},
alors Lua vérifie le second opérande.
Si Lua peut trouver une metamethod,
il appelle la metamethod avec les deux opérandes comme arguments
et le résultat de l'appel
(ajusté à une valeur)
est le résultat de l'opération.
Autrement,
cela soulève une erreur.
}

@item{@idx{__sub}|
Opération de soustraction (@T{-}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__mul}|
Opération de multiplication (@T{*}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__div}|
Opération de division (@T{/}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__mod}|
Opération modulo (@T{%}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__pow}|
Opération d'exponentiation (@T{^}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__unm}|
Opération négation (unary @T{-}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__idiv}|
Opération floor (@T{//}).
Comportement similaire à l'opération d'addition.
}

@item{@idx{__band}|
Opération AND (@T{&}) bit à bit.
Comportement semblable à l'opération d'addition,
sauf que Lua va essayer une metamethod
si un  opérande n'est ni un nombre entier
ni une valeur assimilable à un entier @see{coercion}.
}

@item{@idx{__bor}|
Opération OR (@T{|} bit à bit.
Comportement semblable à l'opération AND bit à bit.
}

@item{@idx{__ bxor}|
Opération OU (binaire @T{~}) externe bit à bit.
Comportement semblable à l'opération AND bit à bit.
}

@item{@idx{__bnot}|
Opération NOT (unaire @T{~}) bit à bit.
Comportement semblable à l'opération AND bit à bit.
}

@item{@idx{__shl}|
Opération de décalage vers la gauche (@T{<<}).
Comportement semblable à l'opération AND bit à bit.
}

@item{@idx{__shr}|
Opération de décalage à droite (@T{>>}).
Comportement semblable à l'opération AND bit à bit.
}

@item{@idx{__concat}|
Opération de concaténation (@T{..}).
Comportement semblable à l'opération d'addition,
sauf que Lua va essayer une metamethod
si un opérande n'est ni une chaîne ni un nombre
(qui est toujours coercible à une chaîne).
}

@item{@idx{__len}|
Opération de longueur (@T{#}).
Si l'objet n'est pas une chaîne,
Lua va essayer sa metamethod.
S'il y a une metamethod,
Lua l'appelle avec l'objet comme argument,
et le résultat de l'appel
(toujours ajusté à une valeur)
est le résultat de l'opération.
S'il n'y a pas de metamethod mais que l'objet est une table,
alors Lua utilise l'opération de longueur de table @see{len-op}.
Sinon, Lua soulève une erreur.
}

@item{@idx{__eq}|
Opération égale (@T{==}).
Comportement semblable à l'opération d'addition,
sauf que Lua essaiera une metamethod uniquement
lorsque les valeurs en comparaison sont soit deux
table, soit deux userdata
et s'ils ne désignent pas la même valeur égaux.
Le résultat de l'appel est toujours
converti en booléen.
}

@item{@idx{__lt}|
Opération moins (@T{<}).
Comportement semblable à l'opération d'addition,
sauf que Lua essaiera une metamethod uniquement lorsque les valeurs
en comparaison ne sont ni les mêmes, ni deux chaînes.
Le résultat de l'appel est toujours converti en booléen.
}

@item{@idx{__le}|
L'opération inférieur ou égal (@T{<=}) operation.
Contrairement aux autres opérations,
l'opération inférieur ou égal peut utiliser deux évenements différents.
Premièrement, Lua cherche la métaméthode @idx{__le} dans les deux operandes,
comme dans l'opération strictement inférieur.
Si il ne peut pas trouver cette métaméthode,
alors il va essayer la métaméthode @idx{__lt},
en supposant que @T{a <= b} est équivalent à @T{not (b < a)}.
Comme les autres opérateurs de comparaison,
le résultat est toujours un booléen.
(Cette utilisation de l'évenement @idx{__lt} peut être supprimée dans des versions futures;
elle est aussi plus lente qu'une vraie métaméthode @idx{__le}.)
}

@item{@idx{__index}|
L'accès par clé @T{table[key]}.
Cet événement arrive quand @id{table} n'est pas une table ou
quand @id{key} n'est pas présent dans @id{table}.

Malgré son nom,
la métaméthode pour cet événement peut être une fonction ou une table.
Si c'est une fonction,
elle est appellée avec @id{table} et @id{key} comme arguments,
et le résultat de l'appel
(ajusté à une seule valeur)
est le résultat de l'opération.
Si c'est une table,
le résultat final est le résultat de l'indexation de cette table avec @id{key}.
(Cet indexation est régulière, pas brute,
et peut donc déclencher une autre métamethode.)
}

@item{@idx{__newindex}|
Modification par indexation @T{table[key] = value}.
Comme l’événement d'indexation,
cet événement arrive quand @id{table} n'est pas une table ou
quand @id{key} n'est pas présent dans @id{table}.

Comme l'indexation,
la métamethode pour cet évenement peut être soit une fonction, soit une table.
Si c'est une fonction,
elle est appellée avec @id{table}, @id{key}, et @id{value} comme arguments.
Si c'est une table,
Lua effectue la même opération mais à cette table avec les même clé et valeur.
(Cet affectation est régulier,
et peut donc déclencher une autre métamethode.)

Dès lors qu'il existe une métaméthode @idx{__newindex},
Lua ne fait aucune affectation par lui même.
(Si necessaire,
la métamethode elle même peut appeller @Lid{rawset}
pour faire l'affectation.)
}

@item{@idx{__call}|
L'opération d'appel @T{func(args)}.
Cet évenement arrive quand Lua essaye d'appeler une valeur qui n'est pas une fonction
(c'est à dire, quand @id{func} n'est pas une fonction).
La métaméthode est cherchée dans la metatable de @id{func}.
Si présente,
la métaméthode est appelée avec @id{func} en tant que premier argument,
suivi par les arguments de l'appel original (@id{args}).
Tous les résultats de l'appel
sont le résultat de l'opération.
(C'est la seule métaméthode qui permet plusieurs résultats.)
}

}

C'est une bonne pratique d'ajouter toutes les metamethod nécessaires à une table
avant de la configurer comme metatable d'un objet quelconque.
En particulier, la metamethod @idx{__gc} fonctionne uniquement dans ce cas @see{finalizers}

Comme les metatables sont des tableaux réguliers,
ils peuvent contenir des champs arbitraires
en plus des noms d'événements définis ci-dessus.
Certaines fonctions de la bibliothèque standard
(e.g., @Lid{tostring})
utilises d'autres champs de la metatable pour leur propre fonctionnement.

}

@sect2{GC| @title{Garbage Collection}

Lua effectue une gestion automatique de la mémoire.
Ceci veut dire que
vous n'avez pas à vous soucier d'allouer de la mémoire pour les nouveaux objets
ou de la libérer lorsque les objets ne sont plus nécessaires.
Lua gère automatiquement la mémoire en exécutant
un @def{garbage collector} pour collecter tous les @emph{objets morts}
(c'est à dire les objets qui ne sont plus accessible depuis Lua).
L'intégralité de la mémoire utilisée par Lua est soumise à une gestion automatique :
chaînes, tables, userdata, fonctions, threads, structures internes, etc.

Lua met en œuvre un collecteur incrémental de mark-and-sweep.
Il utilise deux nombres pour contrôler ses cycles de garbage-collection:
le @def{garbage-collector pause} et
le @def{garbage-collector step multiplier}.
Les deux utilisent des points de pourcentage comme des unités
(par exemple, une valeur de 100 signifie une valeur interne de 1).

Le garbage-collector pause
contrôle la durée pendant laquelle le collecteur attend avant de commencer un nouveau cycle.
Des valeurs plus importantes rendent le collecteur moins agressif.
Les valeurs inférieures à 100 signifient que le collecteur n'attendra pas pour
démarrer un nouveau cycle.
Une valeur de 200 signifie que le collecteur attend que le double de la mémoire total soit utilisé
avant de commencer un nouveau cycle.

Le garbage-collector step multiplier
contrôle la vitesse relative du collecteur par rapport à
l'allocation mémoire.
Des valeurs plus importantes rendent le collecteur plus agressif
mais augmentent aussi la charge de chaque étape d'un cycle de collecte.
Il est déconseillé d'utiliser des valeurs inférieures à 100,
car elles rendent le collecteur trop lent et
il est probable que le collecteur ne terminera jamais un cycle.
La valeur par défaut est de 200,
ce qui signifie que le collecteur fonctionne à @Q{deux fois}
la vitesse d'allocation de la mémoire.

Si vous définissez le multiplicateur de pas à un très grand nombre
(plus de 10% du nombre maximum
d'octets que le programme peut utiliser),
le collecteur se comporte comme un collecteur de stop-the-world
(c'est à dire qu'un pas réalise un cycle complet).
Si vous réglez la pause sur 200,
le collecteur se comporte comme dans les anciennes versions de Lua,
faire une collection complète chaque fois que Lua double son
utilisation mémoire.

Vous pouvez modifier ces nombres en appelant @Lid{lua_gc} en C
ou @Lid{collectgarbage} en Lua.
Vous pouvez également utiliser ces fonctions pour contrôler directement
le collecteur (par exemple, pour l'arrêter et le redémarrer).


@sect3{finalizers| @title{Garbage-Collection Metamethods}

Vous pouvez définir des metamethods pour le garbage-collector dans le cas des tables
et dans le cas des full userdata avec l'@N{API C}.
Ces metamethods sont également appelées @def{finalizers}.
Les finalizers permettent de coordonner le garbage-collector
de Lua avec la gestion de ressources externes, comme la fermeture
de fichiers, celle de connexions réseaux ou sur des bases de donnée
ou bien la libération de la mémoire que vous avez vous même allouée.

Afin qu'un objet (table ou userdata) soit bien finalisé lors du passage
du garbage-collector, il doit être @emph{marqué}.
@index{mark (for finalization)}
Pour marquer un objet, il faut que sa metatable ait un champ
de clé @St{__gc} lors de son attribution.
Si la clé @idx{__gc} n'est présente lors de l'association de la metatable
à un objet, alors celui-ci ne sera pas marqué,
même si cette clé est rajoutée à la metatable ensuite.
Quand un objet marqué devient un objet mort,
il n'est pas immédiatement collecté.
Lua le place tout d'abord dans une liste et,
à la suite de la collecte, Lua parcourt cette liste.
Pour chaque objet, il vérifie le type de sa metamethod @idx{__gc}.
S'il s'agit bien d'une fonction,
Lua l'appelle avec l'objet comme unique argument,
et l'ignore dans le cas contraire.

À la fin du cycle de collecte,
les finalizers des objets sont appelés dans
l'ordre inverse dans lequel les objets ont été marqués.
Autrement dit, le premier objet dont le finalizer est appelé
est le dernier à avoir été marqué dans le programme.
L'exécution d'un finalizer peut intervenir à tout moment
lors de l'exécution d'un programme.

Puisqu'un objet collecté peut encore être utilisé dans son finalizer,
cet objet (et ceux accessibles par son intermédiaire) peuvent
échapper à la collecte.@index{resurrection}
En règle générale, ceci n'est que temporaire avant
le cycle de collecte suivant.
Cependant, si le finalizer stocke l'objet dans un espace global
alors ce dernier peut ne plus être collecté jusqu'à la fin du programme.
De plus, si le finalizer marque à nouveau l'objet, son (nouveau) finalizer
sera appelé au cycle suivant alors que l'objet est inaccessible.
Dans tous les cas,
la mémoire est libéré uniquement quand l'objet est à la fois inaccessible
et non marqué.

Quand un état Lua est fermé @seeF{lua_close},
Lua appelle les finalizers de tous les objets marqués dans
l'ordre inverse de celui dans lequel ils ont été marqués.
Les objets marqués à nouveau lors de cette phase sont ignorés.

}

@sect3{weak-table| @title{Weak Tables}

Une @def{weak table} est une table
dont les éléments sont des @def{weak references}.
Une référence faible est ignorée par le garbage collector.
En d'autres termes, si les seules références à un objet
sont des références faibles, alors le garbage collector
collectera cet objet

Une weak table peut avoir des clés faibles, des valeurs faibles
ou les deux.
Une weak table permet de collecter ses valeurs mais empêche
la collecte de ses clés.

Une table avec, à la fois des clés faibles et des valeurs faibles
permet la collecte de clés valeurs.

Dans tous les cas, si la clé ou la valeur est collectée,
toute la paire est retirée de la table.
La faiblesse d'une table est contrôlée par les @idx{__mode}
domaines de sa métatable.
Si le champ @idx{__mode} est une chaîne contenant le
@N{caractère @Char{k}}, les clés de la table sont faibles.
Si @idx{__mode} contient @Char{v}, les valeurs dans
le tableau sont faibles.

Une table avec des clés faibles et des valeurs fortes est également
appelé @def{ephemoron table}.
Dans une table ephemeron, une valeur n'est considérée comme
accessible que si sa clé est accessible.
En particulier,  si la seule référence à une clé vient de sa valeur,
la paire est supprimée.

Toute modification de la faiblesse d'une table peut prendre effet
uniquement au prochain cycle de collecte.
En particulier, si vous modifiez la faiblesse en mode plus fort,
Lua peut encore recueillir des objets de cette table
avant l'entrée en vigueur de la modification.

Seuls les objets ayant une construction explicite sont retirées
des weak tables.
Les valeurs, telles que les nombres et @x{light@N{fonctions C}},
ne sont pas assujettis à la collecte du garbage collector
et ne sont donc pas supprimées des weak tables
(à moins que leurs valeurs associées ne soient collectées).
Bien que les chaînes soient assujetties à la collecte,
elles n'ont pas de construction explicite et ne sont
donc pas supprimées des weak tables.

Les objets ressuscités (c'est à dire que les objets
sont finalisés et accessibles uniquement à travers
des objets en cours de finalisation)
ont un comportement spécial dans les weak tables.
Ils sont retirées des valeurs faibles avant
d'exécuter leurs finaliseurs mais sont retirées
des clés faibles uniquement dans la prochaine collecte
après avoir exécuté leurs finalisations lorsque de tels
objets sont effectivement libérés.
Ce comportement permet et finaliseurs d'accéder
aux propriétés associées à l'objet par des weak tables.

Si une weak tables est parmi les objets ressuscités
dans un cycle de collecte,
il se peut qu'il ne soit pas correctement
effacé jusqu'au prochain cycle.

}

}

@sect2{coroutine|@title{Les coroutines}

Lua supporte les coroutines, également appelé
@emphx{multithreading collaboratif}.
Une coroutine en Lua représente un fil d'exécution indépendant.
Contrairement aux threads dans les systèmes multithread,
une coroutine ne suspend que son exécution en appelant
explicitement la fonction yield.

Vous créez une coroutine en appelant @Lid{coroutine.create}.
Son seul argument est une fonction qui est la fonction principale de la coroutine.
La fonction @id{create} crée uniquement une nouvelle coroutine et
lui renvoie une poignée (objet du type @emph{thread});
Il ne démarre pas la coroutine.

Vous exécutez une coroutine en appelant @Lid{coroutine.resume}.
Lorsque vous appelez tout d'abord @Lid{coroutine.resume},
en passant comme premier argument un thread renvoyé par
@Lid{coroutine.create},
la coroutine commence son exécution en appelant sa fonction principale.
Les arguments supplémentaires transmis à @Lid{coroutine.resume}
sont passés comme arguments de cotte fonction.
Un fois que la coroutine commence son exécution en appelant
sa fonction principale.
Les arguments supplémentaires transmis à @Lid{coroutine.resume}
sont passés comme arguments à cette fonction.
Une fois que la coroutine commence son exécution,
elle s'exécute jusqu'à se terminer ou rencontrer @emph{yields}.

Une coroutine peut terminer son exécution de deux façons:
normalement, lorsque sa fonction principale se termine
(explicitement ou implicitement après la dernière instruction);
et anormalement, s'il existe une erreur non protégée.
En cas de fin normale, @Lid{coroutine.resume} renvoie @true,
plus toutes les valeurs renvoyées par la fonction principale de la coroutine.
En cas d'erreurs, @Lid{coroutine.resume} renvoie @false
pour un objet d'erreur.

A coroutine yields by calling @Lid{coroutine.yield}.
When a coroutine yields,
the corresponding @Lid{coroutine.resume} returns immediately,
even if the yield happens inside nested function calls
(that is, not in the main function,
but in a function directly or indirectly called by the main function).
In the case of a yield, @Lid{coroutine.resume} also returns @true,
plus any values passed to @Lid{coroutine.yield}.
The next time you resume the same coroutine,
it continues its execution from the point where it yielded,
with the call to @Lid{coroutine.yield} returning any extra
arguments passed to @Lid{coroutine.resume}.

Like @Lid{coroutine.create},
the @Lid{coroutine.wrap} function also creates a coroutine,
but instead of returning the coroutine itself,
it returns a function that, when called, resumes the coroutine.
Any arguments passed to this function
go as extra arguments to @Lid{coroutine.resume}.
@Lid{coroutine.wrap} returns all the values returned by @Lid{coroutine.resume},
except the first one (the boolean error code).
Unlike @Lid{coroutine.resume},
@Lid{coroutine.wrap} does not catch errors;
any error is propagated to the caller.

As an example of how coroutines work,
consider the following code:
@verbatim{
function foo (a)
  print("foo", a)
  return coroutine.yield(2*a)
end

co = coroutine.create(function (a,b)
      print("co-body", a, b)
      local r = foo(a+1)
      print("co-body", r)
      local r, s = coroutine.yield(a+b, a-b)
      print("co-body", r, s)
      return b, "end"
end)

print("main", coroutine.resume(co, 1, 10))
print("main", coroutine.resume(co, "r"))
print("main", coroutine.resume(co, "x", "y"))
print("main", coroutine.resume(co, "x", "y"))
}
When you run it, it produces the following output:
@verbatim{
co-body 1       10
foo     2
main    true    4
co-body r
main    true    11      -9
co-body x       y
main    true    10      end
main    false   cannot resume dead coroutine
}

You can also create and manipulate coroutines through the C API:
see functions @Lid{lua_newthread}, @Lid{lua_resume},
and @Lid{lua_yield}.

}

}


@C{-------------------------------------------------------------------------}
@sect1{language| @title{The Language}

This section describes the lexis, the syntax, and the semantics of Lua.
In other words,
this section describes
which tokens are valid,
how they can be combined,
and what their combinations mean.

Language constructs will be explained using the usual extended BNF notation,
in which
@N{@bnfrep{@rep{a}} means 0} or more @rep{a}'s, and
@N{@bnfopt{@rep{a}} means} an optional @rep{a}.
Non-terminals are shown like @bnfNter{non-terminal},
keywords are shown like @rw{kword},
and other terminal symbols are shown like @bnfter{=}.
The complete syntax of Lua can be found in @refsec{BNF}
at the end of this manual.

@sect2{lexical| @title{Lexical Conventions}

Lua est un langage @x{free-form}.
Il ignore les espaces (incluant les retours à la ligne) et les commentaires
entre des éléments lexicaux (@x{tokens}),
sauf entre des séparateurs entre des @x{noms} et des @x{mots clés}.

En Lua, les @def{Noms}
(aussi appelés @def{identifiers})
peuvent être n'importe quel combinaison de lettres,
chiffres, et tirets bas,
qui ne commencent ni par un chiffre,
ni par un mot réservé.
Les identifiers sont utilisés pour nommer les variables, les champs de tables et les labels.

Les @def{mots clés} suivants sont réservés
et ne peuvent pas être utilisés comme noms :
@index{reserved words}
@verbatim{
and       break     do        else      elseif    end
false     for       function  goto      if        in
local     nil       not       or        repeat    return
then      true      until     while
}

Lua est sensible à la casse :
@id{and} est un mot (clé?) réservé, mais @id{And} et @id{AND}
sont deux noms différents et valides.
En tant que convention,
les programmes devraient éviter de créer
des noms qui commencent par un tiret bas suivi par
une ou plusieurs lettres majuscules (comme @Lid{_VERSION}).

Voici quelques autres exemples de @x{tokens} :
@verbatim{
+     -     *     /     %     ^     #
&     ~     |     <<    >>    //
==    ~=    <=    >=    <     >     =
(     )     {     }     [     ]     ::
;     :     ,     .     ..    ...
}

Une @def{chaine de caractères littérale courte}
peut être délimitée par des guillemets ou apostrophes (la même chose des deux cotés),
et peut contenir les séquences d'échappement de C suivantes :
@Char{\a} (caractère d'appel [bell]),
@Char{\b} (retour arrière [backspace]),
@Char{\f} (saut de page [form feed]),
@Char{\n} (saut de ligne [line feed]),
@Char{\r} (retour chariot [carriage return]),
@Char{\t} (tabulation horizontale [horizontal tab]),
@Char{\v} (tabulation verticale [vertical tab]),
@Char{\\} (anti-slash [backslash]),
@Char{\"} (guillemets [quotation mark, double quote]),
et @Char{\'} (apostrophe [single quote]).
Un anti-slash suivi par un retour à la ligne
crée un saut de ligne dans la chaîne de caractère.
La séquence d'échappement @Char{\z} saute la prochaine suite
d'espaces, y compris les retours à la ligne;
c'est particulièrement utile pour séparer ou indenter une longue chaîne de caractères littérale
en plusieurs lignes sans ajouter les sauts de lignes et espaces
dans le contenu de la chaîne.
Une chaîne de caractères littérale courte ne peut pas contenir de retours à la ligne non échappés
ni des échappements qui ne forment pas une séquence d'échappement correcte.

On peut spécifier n'importe quel octet dans une chaîne de caractères littérale courte avec sa valeur numérique
(y compris les @x{caractères nuls}).
On peut le faire
avec la séquence d'échappement @T{\x@rep{XX}},
où @rep{XX} est une séquence d’exactement deux chiffres en hexadécimal
ou avec la séquence d'échappement @T{\ddd}},
où @rep{ddd} est une suite de jusqu'à trois chiffres décimaux.
(Notez que si une séquence d'échappement décimale doit être suivie par un chiffre,
elle doit être construite avec exactement trois chiffres.)

L'encodage @x{UTF-8} d'un caractère @x{Unicode}
peut être inséré dans une chaîne de caractères littérale avec
la séquence d'échappement @T{\u{@rep{XXX}}}
(Notez les accolades obligatoires),
où @rep{XXX} est une suite de un ou plusieurs chiffres hexadécimaux
représentant le code point du caractère.

Les chaînes de caractères littérales peuvent aussi être définies avec un format long
entourées par des @x{long brackets}.
On définit un @def{long bracket ouvrant de niveau @rep{n}} comme un crochet
ouvrant suivi par @rep{n} signes égal suivis par un autre crochet ouvrant.
Donc, un long bracket ouvrant de @N{niveau 0} est écrit @T{[[}, @C{]]}
un long bracket ouvrant de @N{niveau 1} est écrit @T{[=[}, @C{]]}
etc…
Un @emph{long bracket fermant} est défini de façon similaire ;
par exemple,
un long bracket fermant s'écrit @C{[[} @T{]====]}.
Une @def{chaîne de caractères littérale longue} commence avec un long bracket ouvrant de n'importe quel niveau et
se termine avec un long bracket? fermant du même niveau.
Elle peut contenir n'importe quel texte à part un long bracket fermant du même niveau.
Les chaînes littérales dans cette forme peuvent continuer sur plusieurs lignes,
n'interprètent pas les séquences d'échappement,
et ignorent les long brackets de n'importe quel autre niveau.
n'importe quel type de séquence de fin de ligne
(retour charriot, saut de ligne, retour charriot suivi par un saut de ligne,
ou saut de ligne suivi par un retour charriot)
est convertit en un seul saut de ligne.

Pour la simplicité,
quand un long bracket ouvrant est immédiatement suivi par un saut de ligne,
le saut de ligne n'est pas inclus dans la chaîne de caractères.
Par exemple, dans un système qui utilise ASCII
(dans lequel @Char{a} est @N{97},
un saut de ligne est @N{10}, et @Char{1} est @N{49}),
les cinq chaînes de caractères littérales suivantes sont équivalentes :
@verbatim{
a = 'alo\n123"'
a = "alo\n123\""
a = '\97lo\10\04923"'
a = [[alo
123"]]
a = [==[
alo
123"]==]
}

N'importe quel octet dans une chaîne de caractère littérale qui n'est pas
spécifiquement affectée par les règles précédentes est représenté par lui même.
Cependant, Lua ouvre les fichiers à analyser en mode texte,
et les fonctions de fichiers du système peuvent avoir des problèmes avec
certains caractères de contrôle.
Donc, c'est plus sûr de représenter des données n'étant pas du texte comme une chaîne de caractères littérale
en utilisant explicitent des séquences d'échappement pour les caractères n'étant pas du texte.

Une @def{constante numérique} (ou @def{nombre})
peut être écrite avec une partie fractionnelle
et une partie décimale optionnelle,
marquée par une lettre @Char{e} ou @Char{E}.
Lua accepte aussi les @x{constantes hexadécimales},
qui commencent par @T{0x} ou @T{0X}.
Les constantes hexadécimales acceptent aussi une partie fractionnelle optionnelle
plus une partie optionnelle d'exposant,
marquée par une lettre @Char{p} ou @Char{P}.
Une constante numérique avec une racine ou un exposant
deviendra un nombre flottant ;
sinon,
si sa valeur rentre dans un entier,
elle deviendra un entier.
Voici des exemples de constantes de nombre entier valides
@verbatim{
3   345   0xff   0xBEBADA
}
Voici des exemples de constantes à virgule flottante valides
@verbatim{
3.0     3.1416     314.16e-2     0.31416E1     34e1
0x0.1E  0xA23p-4   0X1.921FB54442D18P+1
}

Un @def{commentaire} commence avec deux tirets (@T{--})
n'importe où à l'extérieur d'une chaîne de caractères.
Si le texte immédiatement après @T{--} n'est pas un long bracket ouvrant,
alors le commentaire est un @def{commentaire court},
qui se termine à la fin de la ligne.
Sinon, c'est un @def{commentaire long},
qui se termine au long bracket fermant correspondant.
Les commentaires longs sont souvent utilisés pour désactiver du code temporairement.

}

@sect2{variables| @title{Variables}

Les variables servent à stocker des valeurs.
En Lua, il y a trois types de variables :
les variables globales, les variables locales et les champs de table.

Un nom de variable peut désigner selon le contexte une variable globale ou locale
(ou un argument de fonction, qui est un cas particulier de variable locale) :
@Produc{
@producname{var}@producbody{@bnfNter{Name}}
}
@bnfNter{Name} correspond aux identifiants, comme défini en @See{lexical}.

Tout nom de variable est supposé globale à moins d'être explicitement déclaré local @see{localvar}.
@x{Les variable locales} sont contraintes par leur @emph{portée lexicale} :
elles sont accessibles depuis toutes fonctions définies dans une portée inférieur
@see{visibility}.

Une variable a pour valeur @nil avant sa première affectation.

Le crochet sont utilisés pour accéder au champ d'une table :
@Produc{
@producname{var}@producbody{prefixexp @bnfter{[} exp @bnfter{]}}
}

L'utilisation de metatables permet de modifier la nature de l'accès aux champs d'une table.
Un accès à un tableau @T{t[i]} peut être équivalente à
l'appel à une fonction @T{gettable_event(t,i)}.
(Voir @See{metatable} pour une description des metamethodes permettant cela)

La syntaxe @id{var.Name} est simplement du sucre syntaxique pour
@T{var["Name"]} :
@Produc{
@producname{var}@producbody{prefixexp @bnfter{.} @bnfNter{Name}}
}

L'accès à une variable globale @id{x}
est équivalent à @id{_ENV.x}.
La variable $id{_ENV} n'est jamais une variable globale @see{globalenv}
du fait de la manière dont les blocs de codes sont compilés.

}

@sect2{stats| @title{Statements}

Lua supports an almost conventional set of @x{statements},
similar to those in Pascal or C.
This set includes
assignments, control structures, function calls,
and variable declarations.

@sect3{@title{Blocks}

A @x{block} is a list of statements,
which are executed sequentially:
@Produc{
@producname{block}@producbody{@bnfrep{stat}}
}
Lua has @def{empty statements}
that allow you to separate statements with semicolons,
start a block with a semicolon
or write two semicolons in sequence:
@Produc{
@producname{stat}@producbody{@bnfter{;}}
}

Function calls and assignments
can start with an open parenthesis.
This possibility leads to an ambiguity in Lua's grammar.
Consider the following fragment:
@verbatim{
a = b + c
(print or io.write)('done')
}
The grammar could see it in two ways:
@verbatim{
a = b + c(print or io.write)('done')

a = b + c; (print or io.write)('done')
}
The current parser always sees such constructions
in the first way,
interpreting the open parenthesis
as the start of the arguments to a call.
To avoid this ambiguity,
it is a good practice to always precede with a semicolon
statements that start with a parenthesis:
@verbatim{
;(print or io.write)('done')
}

A block can be explicitly delimited to produce a single statement:
@Produc{
@producname{stat}@producbody{@Rw{do} block @Rw{end}}
}
Explicit blocks are useful
to control the scope of variable declarations.
Explicit blocks are also sometimes used to
add a @Rw{return} statement in the middle
of another block @see{control}.

}

@sect3{chunks| @title{Chunks}

The unit of compilation of Lua is called a @def{chunk}.
Syntactically,
a chunk is simply a block:
@Produc{
@producname{chunk}@producbody{block}
}

Lua handles a chunk as the body of an anonymous function
with a variable number of arguments
@see{func-def}.
As such, chunks can define local variables,
receive arguments, and return values.
Moreover, such anonymous function is compiled as in the
scope of an external local variable called @id{_ENV} @see{globalenv}.
The resulting function always has @id{_ENV} as its only upvalue,
even if it does not use that variable.

A chunk can be stored in a file or in a string inside the host program.
To execute a chunk,
Lua first @emph{loads} it,
precompiling the chunk's code into instructions for a virtual machine,
and then Lua executes the compiled code
with an interpreter for the virtual machine.

Chunks can also be precompiled into binary form;
see program @idx{luac} and function @Lid{string.dump} for details.
Programs in source and compiled forms are interchangeable;
Lua automatically detects the file type and acts accordingly @seeF{load}.

}

@sect3{assignment| @title{Assignment}

Lua allows @x{multiple assignments}.
Therefore, the syntax for assignment
defines a list of variables on the left side
and a list of expressions on the right side.
The elements in both lists are separated by commas:
@Produc{
@producname{stat}@producbody{varlist @bnfter{=} explist}
@producname{varlist}@producbody{var @bnfrep{@bnfter{,} var}}
@producname{explist}@producbody{exp @bnfrep{@bnfter{,} exp}}
}
Expressions are discussed in @See{expressions}.

Before the assignment,
the list of values is @emph{adjusted} to the length of
the list of variables.@index{adjustment}
If there are more values than needed,
the excess values are thrown away.
If there are fewer values than needed,
the list is extended with as many  @nil's as needed.
If the list of expressions ends with a function call,
then all values returned by that call enter the list of values,
before the adjustment
(except when the call is enclosed in parentheses; see @See{expressions}).

The assignment statement first evaluates all its expressions
and only then the assignments are performed.
Thus the code
@verbatim{
i = 3
i, a[i] = i+1, 20
}
sets @T{a[3]} to 20, without affecting @T{a[4]}
because the @id{i} in @T{a[i]} is evaluated (to 3)
before it is @N{assigned 4}.
Similarly, the line
@verbatim{
x, y = y, x
}
exchanges the values of @id{x} and @id{y},
and
@verbatim{
x, y, z = y, z, x
}
cyclically permutes the values of @id{x}, @id{y}, and @id{z}.

The meaning of assignments to global variables
and table fields can be changed via metatables.
An assignment to an indexed variable @T{t[i] = val} is equivalent to
@T{settable_event(t,i,val)}.
(See @See{metatable} for a complete description of the
@id{settable_event} function.
This function is not defined or callable in Lua.
We use it here only for explanatory purposes.)

An assignment to a global name @T{x = val}
is equivalent to the assignment
@T{_ENV.x = val} @see{globalenv}.

}

@sect3{control| @title{Control Structures}
The control structures
@Rw{if}, @Rw{while}, and @Rw{repeat} have the usual meaning and
familiar syntax:
@index{while-do statement}
@index{repeat-until statement}
@index{if-then-else statement}
@Produc{
@producname{stat}@producbody{@Rw{while} exp @Rw{do} block @Rw{end}}
@producname{stat}@producbody{@Rw{repeat} block @Rw{until} exp}
@producname{stat}@producbody{@Rw{if} exp @Rw{then} block
  @bnfrep{@Rw{elseif} exp @Rw{then} block}
   @bnfopt{@Rw{else} block} @Rw{end}}
}
Lua also has a @Rw{for} statement, in two flavors @see{for}.

The @x{condition expression} of a
control structure can return any value.
Both @false and @nil are considered false.
All values different from @nil and @false are considered true
(in particular, the number 0 and the empty string are also true).

In the @Rw{repeat}@En@Rw{until} loop,
the inner block does not end at the @Rw{until} keyword,
but only after the condition.
So, the condition can refer to local variables
declared inside the loop block.

The @Rw{goto} statement transfers the program control to a label.
For syntactical reasons,
labels in Lua are considered statements too:
@index{goto statement}
@index{label}
@Produc{
@producname{stat}@producbody{@Rw{goto} Name}
@producname{stat}@producbody{label}
@producname{label}@producbody{@bnfter{::} Name @bnfter{::}}
}

A label is visible in the entire block where it is defined,
except
inside nested blocks where a label with the same name is defined and
inside nested functions.
A goto may jump to any visible label as long as it does not
enter into the scope of a local variable.

Labels and empty statements are called @def{void statements},
as they perform no actions.

The @Rw{break} statement terminates the execution of a
@Rw{while}, @Rw{repeat}, or @Rw{for} loop,
skipping to the next statement after the loop:
@index{break statement}
@Produc{
@producname{stat}@producbody{@Rw{break}}
}
A @Rw{break} ends the innermost enclosing loop.

The @Rw{return} statement is used to return values
from a function or a chunk
(which is an anonymous function).
@index{return statement}
Functions can return more than one value,
so the syntax for the @Rw{return} statement is
@Produc{
@producname{stat}@producbody{@Rw{return} @bnfopt{explist} @bnfopt{@bnfter{;}}}
}

The @Rw{return} statement can only be written
as the last statement of a block.
If it is really necessary to @Rw{return} in the middle of a block,
then an explicit inner block can be used,
as in the idiom @T{do return end},
because now @Rw{return} is the last statement in its (inner) block.

}

@sect3{for| @title{For Statement}

@index{for statement}
The @Rw{for} statement has two forms:
one numerical and one generic.

The numerical @Rw{for} loop repeats a block of code while a
control variable runs through an arithmetic progression.
It has the following syntax:
@Produc{
@producname{stat}@producbody{@Rw{for} @bnfNter{Name} @bnfter{=}
  exp @bnfter{,} exp @bnfopt{@bnfter{,} exp} @Rw{do} block @Rw{end}}
}
The @emph{block} is repeated for @emph{name} starting at the value of
the first @emph{exp}, until it passes the second @emph{exp} by steps of the
third @emph{exp}.
More precisely, a @Rw{for} statement like
@verbatim{
for v = @rep{e1}, @rep{e2}, @rep{e3} do @rep{block} end
}
is equivalent to the code:
@verbatim{
do
  local @rep{var}, @rep{limit}, @rep{step} = tonumber(@rep{e1}), tonumber(@rep{e2}), tonumber(@rep{e3})
  if not (@rep{var} and @rep{limit} and @rep{step}) then error() end
  @rep{var} = @rep{var} - @rep{step}
  while true do
    @rep{var} = @rep{var} + @rep{step}
    if (@rep{step} >= 0 and @rep{var} > @rep{limit}) or (@rep{step} < 0 and @rep{var} < @rep{limit}) then
      break
    end
    local v = @rep{var}
    @rep{block}
  end
end
}

Note the following:
@itemize{

@item{
All three control expressions are evaluated only once,
before the loop starts.
They must all result in numbers.
}

@item{
@T{@rep{var}}, @T{@rep{limit}}, and @T{@rep{step}} are invisible variables.
The names shown here are for explanatory purposes only.
}

@item{
If the third expression (the step) is absent,
then a step @N{of 1} is used.
}

@item{
You can use @Rw{break} and @Rw{goto} to exit a @Rw{for} loop.
}

@item{
The loop variable @T{v} is local to the loop body.
If you need its value after the loop,
assign it to another variable before exiting the loop.
}

}

The generic @Rw{for} statement works over functions,
called @def{iterators}.
On each iteration, the iterator function is called to produce a new value,
stopping when this new value is @nil.
The generic @Rw{for} loop has the following syntax:
@Produc{
@producname{stat}@producbody{@Rw{for} namelist @Rw{in} explist
                    @Rw{do} block @Rw{end}}
@producname{namelist}@producbody{@bnfNter{Name} @bnfrep{@bnfter{,} @bnfNter{Name}}}
}
A @Rw{for} statement like
@verbatim{
for @rep{var_1}, @Cdots, @rep{var_n} in @rep{explist} do @rep{block} end
}
is equivalent to the code:
@verbatim{
do
  local @rep{f}, @rep{s}, @rep{var} = @rep{explist}
  while true do
    local @rep{var_1}, @Cdots, @rep{var_n} = @rep{f}(@rep{s}, @rep{var})
    if @rep{var_1} == nil then break end
    @rep{var} = @rep{var_1}
    @rep{block}
  end
end
}
Note the following:
@itemize{

@item{
@T{@rep{explist}} is evaluated only once.
Its results are an @emph{iterator} function,
a @emph{state},
and an initial value for the first @emph{iterator variable}.
}

@item{
@T{@rep{f}}, @T{@rep{s}}, and @T{@rep{var}} are invisible variables.
The names are here for explanatory purposes only.
}

@item{
You can use @Rw{break} to exit a @Rw{for} loop.
}

@item{
The loop variables @T{@rep{var_i}} are local to the loop;
you cannot use their values after the @Rw{for} ends.
If you need these values,
then assign them to other variables before breaking or exiting the loop.
}

}

}

@sect3{funcstat| @title{Function Calls as Statements}
To allow possible side-effects,
function calls can be executed as statements:
@Produc{
@producname{stat}@producbody{functioncall}
}
In this case, all returned values are thrown away.
Function calls are explained in @See{functioncall}.

}

@sect3{localvar| @title{Local Declarations}
@x{Local variables} can be declared anywhere inside a block.
The declaration can include an initial assignment:
@Produc{
@producname{stat}@producbody{@Rw{local} namelist @bnfopt{@bnfter{=} explist}}
}
If present, an initial assignment has the same semantics
of a multiple assignment @see{assignment}.
Otherwise, all variables are initialized with @nil.

A chunk is also a block @see{chunks},
and so local variables can be declared in a chunk outside any explicit block.

The visibility rules for local variables are explained in @See{visibility}.

}

}

@sect2{expressions| @title{Expressions}

The basic expressions in Lua are the following:
@Produc{
@producname{exp}@producbody{prefixexp}
@producname{exp}@producbody{@Rw{nil} @Or @Rw{false} @Or @Rw{true}}
@producname{exp}@producbody{@bnfNter{Numeral}}
@producname{exp}@producbody{@bnfNter{LiteralString}}
@producname{exp}@producbody{functiondef}
@producname{exp}@producbody{tableconstructor}
@producname{exp}@producbody{@bnfter{...}}
@producname{exp}@producbody{exp binop exp}
@producname{exp}@producbody{unop exp}
@producname{prefixexp}@producbody{var @Or functioncall @Or
                                  @bnfter{(} exp @bnfter{)}}
}

Numerals and literal strings are explained in @See{lexical};
variables are explained in @See{variables};
function definitions are explained in @See{func-def};
function calls are explained in @See{functioncall};
table constructors are explained in @See{tableconstructor}.
Vararg expressions,
denoted by three dots (@Char{...}), can only be used when
directly inside a vararg function;
they are explained in @See{func-def}.

Binary operators comprise arithmetic operators @see{arith},
bitwise operators @see{bitwise},
relational operators @see{rel-ops}, logical operators @see{logic},
and the concatenation operator @see{concat}.
Unary operators comprise the unary minus @see{arith},
the unary bitwise NOT @see{bitwise},
the unary logical @Rw{not} @see{logic},
and the unary @def{length operator} @see{len-op}.

Both function calls and vararg expressions can result in multiple values.
If a function call is used as a statement @see{funcstat},
then its return list is adjusted to zero elements,
thus discarding all returned values.
If an expression is used as the last (or the only) element
of a list of expressions,
then no adjustment is made
(unless the expression is enclosed in parentheses).
In all other contexts,
Lua adjusts the result list to one element,
either discarding all values except the first one
or adding a single @nil if there are no values.

Here are some examples:
@verbatim{
f()                -- adjusted to 0 results
g(f(), x)          -- f() is adjusted to 1 result
g(x, f())          -- g gets x plus all results from f()
a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)
a,b = ...          -- a gets the first vararg parameter, b gets
                   -- the second (both a and b can get nil if there
                   -- is no corresponding vararg parameter)

a,b,c = x, f()     -- f() is adjusted to 2 results
a,b,c = f()        -- f() is adjusted to 3 results
return f()         -- returns all results from f()
return ...         -- returns all received vararg parameters
return x,y,f()     -- returns x, y, and all results from f()
{f()}              -- creates a list with all results from f()
{...}              -- creates a list with all vararg parameters
{f(), nil}         -- f() is adjusted to 1 result
}

Any expression enclosed in parentheses always results in only one value.
Thus,
@T{(f(x,y,z))} is always a single value,
even if @id{f} returns several values.
(The value of @T{(f(x,y,z))} is the first value returned by @id{f}
or @nil if @id{f} does not return any values.)



@sect3{arith| @title{Arithmetic Operators}
Lua supports the following @x{arithmetic operators}:
@description{
@item{@T{+}|addition}
@item{@T{-}|subtraction}
@item{@T{*}|multiplication}
@item{@T{/}|float division}
@item{@T{//}|floor division}
@item{@T{%}|modulo}
@item{@T{^}|exponentiation}
@item{@T{-}|unary minus}
}

With the exception of exponentiation and float division,
the arithmetic operators work as follows:
If both operands are integers,
the operation is performed over integers and the result is an integer.
Otherwise, if both operands are numbers
or strings that can be converted to
numbers @see{coercion},
then they are converted to floats,
the operation is performed following the usual rules
for floating-point arithmetic
(usually the @x{IEEE 754} standard),
and the result is a float.

Exponentiation and float division (@T{/})
always convert their operands to floats
and the result is always a float.
Exponentiation uses the @ANSI{pow},
so that it works for non-integer exponents too.

Floor division (@T{//}) is a division
that rounds the quotient towards minus infinity,
that is, the floor of the division of its operands.

Modulo is defined as the remainder of a division
that rounds the quotient towards minus infinity (floor division).

In case of overflows in integer arithmetic,
all operations @emphx{wrap around},
according to the usual rules of two-complement arithmetic.
(In other words,
they return the unique representable integer
that is equal modulo @M{2@sp{64}} to the mathematical result.)
}

@sect3{bitwise| @title{Bitwise Operators}
Lua supports the following @x{bitwise operators}:
@description{
@item{@T{&}|bitwise AND}
@item{@T{@VerBar}|bitwise OR}
@item{@T{~}|bitwise exclusive OR}
@item{@T{>>}|right shift}
@item{@T{<<}|left shift}
@item{@T{~}|unary bitwise NOT}
}

All bitwise operations convert its operands to integers
@see{coercion},
operate on all bits of those integers,
and result in an integer.

Both right and left shifts fill the vacant bits with zeros.
Negative displacements shift to the other direction;
displacements with absolute values equal to or higher than
the number of bits in an integer
result in zero (as all bits are shifted out).

}

@sect3{coercion| @title{Coercions and Conversions}
Lua provides some automatic conversions between some
types and representations at run time.
Bitwise operators always convert float operands to integers.
Exponentiation and float division
always convert integer operands to floats.
All other arithmetic operations applied to mixed numbers
(integers and floats) convert the integer operand to a float;
this is called the @def{usual rule}.
The C API also converts both integers to floats and
floats to integers, as needed.
Moreover, string concatenation accepts numbers as arguments,
besides strings.

Lua also converts strings to numbers,
whenever a number is expected.

In a conversion from integer to float,
if the integer value has an exact representation as a float,
that is the result.
Otherwise,
the conversion gets the nearest higher or
the nearest lower representable value.
This kind of conversion never fails.

The conversion from float to integer
checks whether the float has an exact representation as an integer
(that is, the float has an integral value and
it is in the range of integer representation).
If it does, that representation is the result.
Otherwise, the conversion fails.

The conversion from strings to numbers goes as follows:
First, the string is converted to an integer or a float,
following its syntax and the rules of the Lua lexer.
(The string may have also leading and trailing spaces and a sign.)
Then, the resulting number (float or integer)
is converted to the type (float or integer) required by the context
(e.g., the operation that forced the conversion).

All conversions from strings to numbers
accept both a dot and the current locale mark
as the radix character.
(The Lua lexer, however, accepts only a dot.)

The conversion from numbers to strings uses a
non-specified human-readable format.
For complete control over how numbers are converted to strings,
use the @id{format} function from the string library
@seeF{string.format}.

}

@sect3{rel-ops| @title{Relational Operators}
Lua supports the following @x{relational operators}:
@description{
@item{@T{==}|equality}
@item{@T{~=}|inequality}
@item{@T{<}|less than}
@item{@T{>}|greater than}
@item{@T{<=}|less or equal}
@item{@T{>=}|greater or equal}
}
These operators always result in @false or @true.

Equality (@T{==}) first compares the type of its operands.
If the types are different, then the result is @false.
Otherwise, the values of the operands are compared.
Strings are compared in the obvious way.
Numbers are equal if they denote the same mathematical value.

Tables, userdata, and threads
are compared by reference:
two objects are considered equal only if they are the same object.
Every time you create a new object
(a table, userdata, or thread),
this new object is different from any previously existing object.
Closures with the same reference are always equal.
Closures with any detectable difference
(different behavior, different definition) are always different.

You can change the way that Lua compares tables and userdata
by using the @Q{eq} metamethod @see{metatable}.

Equality comparisons do not convert strings to numbers
or vice versa.
Thus, @T{"0"==0} evaluates to @false,
and @T{t[0]} and @T{t["0"]} denote different
entries in a table.

The operator @T{~=} is exactly the negation of equality (@T{==}).

The order operators work as follows.
If both arguments are numbers,
then they are compared according to their mathematical values
(regardless of their subtypes).
Otherwise, if both arguments are strings,
then their values are compared according to the current locale.
Otherwise, Lua tries to call the @Q{lt} or the @Q{le}
metamethod @see{metatable}.
A comparison @T{a > b} is translated to @T{b < a}
and @T{a >= b} is translated to @T{b <= a}.

Following the @x{IEEE 754} standard,
@x{NaN} is considered neither smaller than,
nor equal to, nor greater than any value (including itself).

}

@sect3{logic| @title{Logical Operators}
The @x{logical operators} in Lua are
@Rw{and}, @Rw{or}, and @Rw{not}.
Like the control structures @see{control},
all logical operators consider both @false and @nil as false
and anything else as true.

The negation operator @Rw{not} always returns @false or @true.
The conjunction operator @Rw{and} returns its first argument
if this value is @false or @nil;
otherwise, @Rw{and} returns its second argument.
The disjunction operator @Rw{or} returns its first argument
if this value is different from @nil and @false;
otherwise, @Rw{or} returns its second argument.
Both @Rw{and} and @Rw{or} use @x{short-circuit evaluation};
that is,
the second operand is evaluated only if necessary.
Here are some examples:
@verbatim{
10 or 20            --> 10
10 or error()       --> 10
nil or "a"          --> "a"
nil and 10          --> nil
false and error()   --> false
false and nil       --> false
false or nil        --> nil
10 and 20           --> 20
}
(In this manual,
@T{-->} indicates the result of the preceding expression.)

}

@sect3{concat| @title{Concatenation}
The string @x{concatenation} operator in Lua is
denoted by two dots (@Char{..}).
If both operands are strings or numbers, then they are converted to
strings according to the rules described in @See{coercion}.
Otherwise, the @idx{__concat} metamethod is called @see{metatable}.

}

@sect3{len-op| @title{The Length Operator}

The length operator is denoted by the unary prefix operator @T{#}.

The length of a string is its number of bytes
(that is, the usual meaning of string length when each
character is one byte).

The length operator applied on a table
returns a @x{border} in that table.
A @def{border} in a table @id{t} is any natural number
that satisfies the following condition:
@verbatim{
(border == 0 or t[border] ~= nil) and t[border + 1] == nil
}
In words,
a border is any (natural) index in a table
where a non-nil value is followed by a nil value
(or zero, when index 1 is nil).

A table with exactly one border is called a @def{sequence}.
For instance, the table @T{{10, 20, 30, 40, 50}} is a sequence,
as it has only one border (5).
The table @T{{10, 20, 30, nil, 50}} has two borders (3 and 5),
and therefore it is not a sequence.
The table @T{{nil, 20, 30, nil, nil, 60, nil}}
has three borders (0, 3, and 6),
so it is not a sequence, too.
The table @T{{}} is a sequence with border 0.
Note that non-natural keys do not interfere
with whether a table is a sequence.

When @id{t} is a sequence,
@T{#t} returns its only border,
which corresponds to the intuitive notion of the length of the sequence.
When @id{t} is not a sequence,
@T{#t} can return any of its borders.
(The exact one depends on details of
the internal representation of the table,
which in turn can depend on how the table was populated and
the memory addresses of its non-numeric keys.)

The computation of the length of a table
has a guaranteed worst time of @M{O(log n)},
where @M{n} is the largest natural key in the table.

A program can modify the behavior of the length operator for
any value but strings through the @idx{__len} metamethod @see{metatable}.

}

@sect3{prec| @title{Precedence}
@x{Operator precedence} in Lua follows the table below,
from lower to higher priority:
@verbatim{
or
and
<     >     <=    >=    ~=    ==
|
~
&
<<    >>
..
+     -
*     /     //    %
unary operators (not   #     -     ~)
^
}
As usual,
you can use parentheses to change the precedences of an expression.
The concatenation (@Char{..}) and exponentiation (@Char{^})
operators are right associative.
All other binary operators are left associative.

}

@sect3{tableconstructor| @title{Table Constructors}
Table @x{constructors} are expressions that create tables.
Every time a constructor is evaluated, a new table is created.
A constructor can be used to create an empty table
or to create a table and initialize some of its fields.
The general syntax for constructors is
@Produc{
@producname{tableconstructor}@producbody{@bnfter{@Open} @bnfopt{fieldlist} @bnfter{@Close}}
@producname{fieldlist}@producbody{field @bnfrep{fieldsep field} @bnfopt{fieldsep}}
@producname{field}@producbody{@bnfter{[} exp @bnfter{]} @bnfter{=} exp @Or
               @bnfNter{Name} @bnfter{=} exp @Or exp}
@producname{fieldsep}@producbody{@bnfter{,} @Or @bnfter{;}}
}

Each field of the form @T{[exp1] = exp2} adds to the new table an entry
with key @id{exp1} and value @id{exp2}.
A field of the form @T{name = exp} is equivalent to
@T{["name"] = exp}.
Finally, fields of the form @id{exp} are equivalent to
@T{[i] = exp}, where @id{i} are consecutive integers
starting with 1.
Fields in the other formats do not affect this counting.
For example,
@verbatim{
a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }
}
is equivalent to
@verbatim{
do
  local t = {}
  t[f(1)] = g
  t[1] = "x"         -- 1st exp
  t[2] = "y"         -- 2nd exp
  t.x = 1            -- t["x"] = 1
  t[3] = f(x)        -- 3rd exp
  t[30] = 23
  t[4] = 45          -- 4th exp
  a = t
end
}

The order of the assignments in a constructor is undefined.
(This order would be relevant only when there are repeated keys.)

If the last field in the list has the form @id{exp}
and the expression is a function call or a vararg expression,
then all values returned by this expression enter the list consecutively
@see{functioncall}.

The field list can have an optional trailing separator,
as a convenience for machine-generated code.

}

@sect3{functioncall| @title{Function Calls}
A @x{function call} in Lua has the following syntax:
@Produc{
@producname{functioncall}@producbody{prefixexp args}
}
In a function call,
first @bnfNter{prefixexp} and @bnfNter{args} are evaluated.
If the value of @bnfNter{prefixexp} has type @emph{function},
then this function is called
with the given arguments.
Otherwise, the @bnfNter{prefixexp} @Q{call} metamethod is called,
having as first parameter the value of @bnfNter{prefixexp},
followed by the original call arguments
@see{metatable}.

The form
@Produc{
@producname{functioncall}@producbody{prefixexp @bnfter{:} @bnfNter{Name} args}
}
can be used to call @Q{methods}.
A call @T{v:name(@rep{args})}
is syntactic sugar for @T{v.name(v,@rep{args})},
except that @id{v} is evaluated only once.

Arguments have the following syntax:
@Produc{
@producname{args}@producbody{@bnfter{(} @bnfopt{explist} @bnfter{)}}
@producname{args}@producbody{tableconstructor}
@producname{args}@producbody{@bnfNter{LiteralString}}
}
All argument expressions are evaluated before the call.
A call of the form @T{f{@rep{fields}}} is
syntactic sugar for @T{f({@rep{fields}})};
that is, the argument list is a single new table.
A call of the form @T{f'@rep{string}'}
(or @T{f"@rep{string}"} or @T{f[[@rep{string}]]})
is syntactic sugar for @T{f('@rep{string}')};
that is, the argument list is a single literal string.

A call of the form @T{return @rep{functioncall}} is called
a @def{tail call}.
Lua implements @def{proper tail calls}
(or @emph{proper tail recursion}):
in a tail call,
the called function reuses the stack entry of the calling function.
Therefore, there is no limit on the number of nested tail calls that
a program can execute.
However, a tail call erases any debug information about the
calling function.
Note that a tail call only happens with a particular syntax,
where the @Rw{return} has one single function call as argument;
this syntax makes the calling function return exactly
the returns of the called function.
So, none of the following examples are tail calls:
@verbatim{
return (f(x))        -- results adjusted to 1
return 2 * f(x)
return x, f(x)       -- additional results
f(x); return         -- results discarded
return x or f(x)     -- results adjusted to 1
}

}

@sect3{func-def| @title{Function Definitions}

The syntax for function definition is
@Produc{
@producname{functiondef}@producbody{@Rw{function} funcbody}
@producname{funcbody}@producbody{@bnfter{(} @bnfopt{parlist} @bnfter{)} block @Rw{end}}
}

The following syntactic sugar simplifies function definitions:
@Produc{
@producname{stat}@producbody{@Rw{function} funcname funcbody}
@producname{stat}@producbody{@Rw{local} @Rw{function} @bnfNter{Name} funcbody}
@producname{funcname}@producbody{@bnfNter{Name} @bnfrep{@bnfter{.} @bnfNter{Name}} @bnfopt{@bnfter{:} @bnfNter{Name}}}
}
The statement
@verbatim{
function f () @rep{body} end
}
translates to
@verbatim{
f = function () @rep{body} end
}
The statement
@verbatim{
function t.a.b.c.f () @rep{body} end
}
translates to
@verbatim{
t.a.b.c.f = function () @rep{body} end
}
The statement
@verbatim{
local function f () @rep{body} end
}
translates to
@verbatim{
local f; f = function () @rep{body} end
}
not to
@verbatim{
local f = function () @rep{body} end
}
(This only makes a difference when the body of the function
contains references to @id{f}.)

A function definition is an executable expression,
whose value has type @emph{function}.
When Lua precompiles a chunk,
all its function bodies are precompiled too.
Then, whenever Lua executes the function definition,
the function is @emph{instantiated} (or @emph{closed}).
This function instance (or @emphx{closure})
is the final value of the expression.

Parameters act as local variables that are
initialized with the argument values:
@Produc{
@producname{parlist}@producbody{namelist @bnfopt{@bnfter{,} @bnfter{...}} @Or
  @bnfter{...}}
}
When a function is called,
the list of @x{arguments} is adjusted to
the length of the list of parameters,
unless the function is a @def{vararg function},
which is indicated by three dots (@Char{...})
at the end of its parameter list.
A vararg function does not adjust its argument list;
instead, it collects all extra arguments and supplies them
to the function through a @def{vararg expression},
which is also written as three dots.
The value of this expression is a list of all actual extra arguments,
similar to a function with multiple results.
If a vararg expression is used inside another expression
or in the middle of a list of expressions,
then its return list is adjusted to one element.
If the expression is used as the last element of a list of expressions,
then no adjustment is made
(unless that last expression is enclosed in parentheses).


As an example, consider the following definitions:
@verbatim{
function f(a, b) end
function g(a, b, ...) end
function r() return 1,2,3 end
}
Then, we have the following mapping from arguments to parameters and
to the vararg expression:
@verbatim{
CALL            PARAMETERS

f(3)             a=3, b=nil
f(3, 4)          a=3, b=4
f(3, 4, 5)       a=3, b=4
f(r(), 10)       a=1, b=10
f(r())           a=1, b=2

g(3)             a=3, b=nil, ... -->  (nothing)
g(3, 4)          a=3, b=4,   ... -->  (nothing)
g(3, 4, 5, 8)    a=3, b=4,   ... -->  5  8
g(5, r())        a=5, b=1,   ... -->  2  3
}

Results are returned using the @Rw{return} statement @see{control}.
If control reaches the end of a function
without encountering a @Rw{return} statement,
then the function returns with no results.

@index{multiple return}
There is a system-dependent limit on the number of values
that a function may return.
This limit is guaranteed to be larger than 1000.

The @emphx{colon} syntax
is used for defining @def{methods},
that is, functions that have an implicit extra parameter @idx{self}.
Thus, the statement
@verbatim{
function t.a.b.c:f (@rep{params}) @rep{body} end
}
is syntactic sugar for
@verbatim{
t.a.b.c.f = function (self, @rep{params}) @rep{body} end
}

}

}

@sect2{visibility| @title{Visibility Rules}

@index{visibility}
Lua is a lexically scoped language.
The scope of a local variable begins at the first statement after
its declaration and lasts until the last non-void statement
of the innermost block that includes the declaration.
Consider the following example:
@verbatim{
x = 10                -- global variable
do                    -- new block
  local x = x         -- new 'x', with value 10
  print(x)            --> 10
  x = x+1
  do                  -- another block
    local x = x+1     -- another 'x'
    print(x)          --> 12
  end
  print(x)            --> 11
end
print(x)              --> 10  (the global one)
}

Notice that, in a declaration like @T{local x = x},
the new @id{x} being declared is not in scope yet,
and so the second @id{x} refers to the outside variable.

Because of the @x{lexical scoping} rules,
local variables can be freely accessed by functions
defined inside their scope.
A local variable used by an inner function is called
an @def{upvalue}, or @emphx{external local variable},
inside the inner function.

Notice that each execution of a @Rw{local} statement
defines new local variables.
Consider the following example:
@verbatim{
a = {}
local x = 20
for i=1,10 do
  local y = 0
  a[i] = function () y=y+1; return x+y end
end
}
The loop creates ten closures
(that is, ten instances of the anonymous function).
Each of these closures uses a different @id{y} variable,
while all of them share the same @id{x}.

}

}


@C{-------------------------------------------------------------------------}
@sect1{API| @title{The Application Program Interface}

@index{C API}
This section describes the @N{C API} for Lua, that is,
the set of @N{C functions} available to the host program to communicate
with Lua.
All API functions and related types and constants
are declared in the header file @defid{lua.h}.

Even when we use the term @Q{function},
any facility in the API may be provided as a macro instead.
Except where stated otherwise,
all such macros use each of their arguments exactly once
(except for the first argument, which is always a Lua state),
and so do not generate any hidden side-effects.

As in most @N{C libraries},
the Lua API functions do not check their arguments for validity or consistency.
However, you can change this behavior by compiling Lua
with the macro @defid{LUA_USE_APICHECK} defined.

The Lua library is fully reentrant:
it has no global variables.
It keeps all information it needs in a dynamic structure,
called the @def{Lua state}.

Each Lua state has one or more threads,
which correspond to independent, cooperative lines of execution.
The type @Lid{lua_State} (despite its name) refers to a thread.
(Indirectly, through the thread, it also refers to the
Lua state associated to the thread.)

A pointer to a thread must be passed as the first argument to
every function in the library, except to @Lid{lua_newstate},
which creates a Lua state from scratch and returns a pointer
to the @emph{main thread} in the new state.


@sect2{@title{The Stack}

Lua uses a @emph{virtual stack} to pass values to and from C.
Each element in this stack represents a Lua value
(@nil, number, string, etc.).
Functions in the API can access this stack through the
Lua state parameter that they receive.

Whenever Lua calls C, the called function gets a new stack,
which is independent of previous stacks and of stacks of
@N{C functions} that are still active.
This stack initially contains any arguments to the @N{C function}
and it is where the @N{C function} can store temporary
Lua values and must push its results
to be returned to the caller @seeC{lua_CFunction}.

For convenience,
most query operations in the API do not follow a strict stack discipline.
Instead, they can refer to any element in the stack
by using an @emph{index}:@index{index (API stack)}
A positive index represents an absolute stack position
(starting @N{at 1});
a negative index represents an offset relative to the top of the stack.
More specifically, if the stack has @rep{n} elements,
then @N{index 1} represents the first element
(that is, the element that was pushed onto the stack first)
and
@N{index @rep{n}} represents the last element;
@N{index @num{-1}} also represents the last element
(that is, the element at @N{the top})
and index @M{-n} represents the first element.

}

@sect2{stacksize| @title{Stack Size}

When you interact with the Lua API,
you are responsible for ensuring consistency.
In particular,
@emph{you are responsible for controlling stack overflow}.
You can use the function @Lid{lua_checkstack}
to ensure that the stack has enough space for pushing new elements.

Whenever Lua calls C,
it ensures that the stack has space for
at least @defid{LUA_MINSTACK} extra slots.
@id{LUA_MINSTACK} is defined as 20,
so that usually you do not have to worry about stack space
unless your code has loops pushing elements onto the stack.

When you call a Lua function
without a fixed number of results @seeF{lua_call},
Lua ensures that the stack has enough space for all results,
but it does not ensure any extra space.
So, before pushing anything in the stack after such a call
you should use @Lid{lua_checkstack}.

}

@sect2{@title{Valid and Acceptable Indices}

Any function in the API that receives stack indices
works only with @emphx{valid indices} or @emphx{acceptable indices}.

A @def{valid index} is an index that refers to a
position that stores a modifiable Lua value.
It comprises stack indices @N{between 1} and the stack top
(@T{1 @leq abs(index) @leq top})
@index{stack index}
plus @def{pseudo-indices},
which represent some positions that are accessible to @N{C code}
but that are not in the stack.
Pseudo-indices are used to access the registry @see{registry}
and the upvalues of a @N{C function} @see{c-closure}.

Functions that do not need a specific mutable position,
but only a value (e.g., query functions),
can be called with acceptable indices.
An @def{acceptable index} can be any valid index,
but it also can be any positive index after the stack top
within the space allocated for the stack,
that is, indices up to the stack size.
(Note that 0 is never an acceptable index.)
Except when noted otherwise,
functions in the API work with acceptable indices.

Acceptable indices serve to avoid extra tests
against the stack top when querying the stack.
For instance, a @N{C function} can query its third argument
without the need to first check whether there is a third argument,
that is, without the need to check whether 3 is a valid index.

For functions that can be called with acceptable indices,
any non-valid index is treated as if it
contains a value of a virtual type @defid{LUA_TNONE},
which behaves like a nil value.

}

@sect2{c-closure| @title{C Closures}

When a @N{C function} is created,
it is possible to associate some values with it,
thus creating a @def{@N{C closure}}
@seeC{lua_pushcclosure};
these values are called @def{upvalues} and are
accessible to the function whenever it is called.

Whenever a @N{C function} is called,
its upvalues are located at specific pseudo-indices.
These pseudo-indices are produced by the macro
@Lid{lua_upvalueindex}.
The first upvalue associated with a function is at index
@T{lua_upvalueindex(1)}, and so on.
Any access to @T{lua_upvalueindex(@rep{n})},
where @rep{n} is greater than the number of upvalues of the
current function
(but not greater than 256,
which is one plus the maximum number of upvalues in a closure),
produces an acceptable but invalid index.

}

@sect2{registry| @title{Registry}

Lua provides a @def{registry},
a predefined table that can be used by any @N{C code} to
store whatever Lua values it needs to store.
The registry table is always located at pseudo-index
@defid{LUA_REGISTRYINDEX}.
Any @N{C library} can store data into this table,
but it must take care to choose keys
that are different from those used
by other libraries, to avoid collisions.
Typically, you should use as key a string containing your library name,
or a light userdata with the address of a @N{C object} in your code,
or any Lua object created by your code.
As with variable names,
string keys starting with an underscore followed by
uppercase letters are reserved for Lua.

The integer keys in the registry are used
by the reference mechanism @seeC{luaL_ref}
and by some predefined values.
Therefore, integer keys must not be used for other purposes.

When you create a new Lua state,
its registry comes with some predefined values.
These predefined values are indexed with integer keys
defined as constants in @id{lua.h}.
The following constants are defined:
@description{
@item{@defid{LUA_RIDX_MAINTHREAD}| At this index the registry has
the main thread of the state.
(The main thread is the one created together with the state.)
}

@item{@defid{LUA_RIDX_GLOBALS}| At this index the registry has
the @x{global environment}.
}
}

}

@sect2{C-error|@title{Error Handling in C}

Internally, Lua uses the C @id{longjmp} facility to handle errors.
(Lua will use exceptions if you compile it as C++;
search for @id{LUAI_THROW} in the source code for details.)
When Lua faces any error
(such as a @x{memory allocation error} or a type error)
it @emph{raises} an error;
that is, it does a long jump.
A @emphx{protected environment} uses @id{setjmp}
to set a recovery point;
any error jumps to the most recent active recovery point.

Inside a @N{C function} you can raise an error by calling @Lid{lua_error}.

Most functions in the API can raise an error,
for instance due to a @x{memory allocation error}.
The documentation for each function indicates whether
it can raise errors.

If an error happens outside any protected environment,
Lua calls a @def{panic function} (see @Lid{lua_atpanic})
and then calls @T{abort},
thus exiting the host application.
Your panic function can avoid this exit by
never returning
(e.g., doing a long jump to your own recovery point outside Lua).

The panic function,
as its name implies,
is a mechanism of last resort.
Programs should avoid it.
As a general rule,
when a @N{C function} is called by Lua with a Lua state,
it can do whatever it wants on that Lua state,
as it should be already protected.
However,
when C code operates on other Lua states
(e.g., a Lua parameter to the function,
a Lua state stored in the registry, or
the result of @Lid{lua_newthread}),
it should use them only in API calls that cannot raise errors.

The panic function runs as if it were a @x{message handler} @see{error};
in particular, the error object is at the top of the stack.
However, there is no guarantee about stack space.
To push anything on the stack,
the panic function must first check the available space @see{stacksize}.

}

@sect2{continuations|@title{Handling Yields in C}

Internally, Lua uses the C @id{longjmp} facility to yield a coroutine.
Therefore, if a @N{C function} @id{foo} calls an API function
and this API function yields
(directly or indirectly by calling another function that yields),
Lua cannot return to @id{foo} any more,
because the @id{longjmp} removes its frame from the C stack.

To avoid this kind of problem,
Lua raises an error whenever it tries to yield across an API call,
except for three functions:
@Lid{lua_yieldk}, @Lid{lua_callk}, and @Lid{lua_pcallk}.
All those functions receive a @def{continuation function}
(as a parameter named @id{k}) to continue execution after a yield.

We need to set some terminology to explain continuations.
We have a @N{C function} called from Lua which we will call
the @emph{original function}.
This original function then calls one of those three functions in the C API,
which we will call the @emph{callee function},
that then yields the current thread.
(This can happen when the callee function is @Lid{lua_yieldk},
or when the callee function is either @Lid{lua_callk} or @Lid{lua_pcallk}
and the function called by them yields.)

Suppose the running thread yields while executing the callee function.
After the thread resumes,
it eventually will finish running the callee function.
However,
the callee function cannot return to the original function,
because its frame in the C stack was destroyed by the yield.
Instead, Lua calls a @def{continuation function},
which was given as an argument to the callee function.
As the name implies,
the continuation function should continue the task
of the original function.

As an illustration, consider the following function:
@verbatim{
int original_function (lua_State *L) {
  ...     /* code 1 */
  status = lua_pcall(L, n, m, h);  /* calls Lua */
  ...     /* code 2 */
}
}
Now we want to allow
the Lua code being run by @Lid{lua_pcall} to yield.
First, we can rewrite our function like here:
@verbatim{
int k (lua_State *L, int status, lua_KContext ctx) {
  ...  /* code 2 */
}

int original_function (lua_State *L) {
  ...     /* code 1 */
  return k(L, lua_pcall(L, n, m, h), ctx);
}
}
In the above code,
the new function @id{k} is a
@emph{continuation function} (with type @Lid{lua_KFunction}),
which should do all the work that the original function
was doing after calling @Lid{lua_pcall}.
Now, we must inform Lua that it must call @id{k} if the Lua code
being executed by @Lid{lua_pcall} gets interrupted in some way
(errors or yielding),
so we rewrite the code as here,
replacing @Lid{lua_pcall} by @Lid{lua_pcallk}:
@verbatim{
int original_function (lua_State *L) {
  ...     /* code 1 */
  return k(L, lua_pcallk(L, n, m, h, ctx2, k), ctx1);
}
}
Note the external, explicit call to the continuation:
Lua will call the continuation only if needed, that is,
in case of errors or resuming after a yield.
If the called function returns normally without ever yielding,
@Lid{lua_pcallk} (and @Lid{lua_callk}) will also return normally.
(Of course, instead of calling the continuation in that case,
you can do the equivalent work directly inside the original function.)

Besides the Lua state,
the continuation function has two other parameters:
the final status of the call plus the context value (@id{ctx}) that
was passed originally to @Lid{lua_pcallk}.
(Lua does not use this context value;
it only passes this value from the original function to the
continuation function.)
For @Lid{lua_pcallk},
the status is the same value that would be returned by @Lid{lua_pcallk},
except that it is @Lid{LUA_YIELD} when being executed after a yield
(instead of @Lid{LUA_OK}).
For @Lid{lua_yieldk} and @Lid{lua_callk},
the status is always @Lid{LUA_YIELD} when Lua calls the continuation.
(For these two functions,
Lua will not call the continuation in case of errors,
because they do not handle errors.)
Similarly, when using @Lid{lua_callk},
you should call the continuation function
with @Lid{LUA_OK} as the status.
(For @Lid{lua_yieldk}, there is not much point in calling
directly the continuation function,
because @Lid{lua_yieldk} usually does not return.)

Lua treats the continuation function as if it were the original function.
The continuation function receives the same Lua stack
from the original function,
in the same state it would be if the callee function had returned.
(For instance,
after a @Lid{lua_callk} the function and its arguments are
removed from the stack and replaced by the results from the call.)
It also has the same upvalues.
Whatever it returns is handled by Lua as if it were the return
of the original function.

}

@sect2{@title{Functions and Types}

Here we list all functions and types from the @N{C API} in
alphabetical order.
Each function has an indicator like this:
@apii{o,p,x}

The first field, @T{o},
is how many elements the function pops from the stack.
The second field, @T{p},
is how many elements the function pushes onto the stack.
(Any function always pushes its results after popping its arguments.)
A field in the form @T{x|y} means the function can push (or pop)
@T{x} or @T{y} elements,
depending on the situation;
an interrogation mark @Char{?} means that
we cannot know how many elements the function pops/pushes
by looking only at its arguments
(e.g., they may depend on what is on the stack).
The third field, @T{x},
tells whether the function may raise errors:
@Char{-} means the function never raises any error;
@Char{m} means the function may raise out-of-memory errors
and errors running a @idx{__gc} metamethod;
@Char{e} means the function may raise any errors
(it can run arbitrary Lua code,
either directly or through metamethods);
@Char{v} means the function may raise an error on purpose.


@APIEntry{int lua_absindex (lua_State *L, int idx);|
@apii{0,0,-}

Converts the @x{acceptable index} @id{idx}
into an equivalent @x{absolute index}
(that is, one that does not depend on the stack top).

}


@APIEntry{
typedef void * (*lua_Alloc) (void *ud,
                             void *ptr,
                             size_t osize,
                             size_t nsize);|

The type of the @x{memory-allocation function} used by Lua states.
The allocator function must provide a
functionality similar to @id{realloc},
but not exactly the same.
Its arguments are
@id{ud}, an opaque pointer passed to @Lid{lua_newstate};
@id{ptr}, a pointer to the block being allocated/reallocated/freed;
@id{osize}, the original size of the block or some code about what
is being allocated;
and @id{nsize}, the new size of the block.

When @id{ptr} is not @id{NULL},
@id{osize} is the size of the block pointed by @id{ptr},
that is, the size given when it was allocated or reallocated.

When @id{ptr} is @id{NULL},
@id{osize} encodes the kind of object that Lua is allocating.
@id{osize} is any of
@Lid{LUA_TSTRING}, @Lid{LUA_TTABLE}, @Lid{LUA_TFUNCTION},
@Lid{LUA_TUSERDATA}, or @Lid{LUA_TTHREAD} when (and only when)
Lua is creating a new object of that type.
When @id{osize} is some other value,
Lua is allocating memory for something else.

Lua assumes the following behavior from the allocator function:

When @id{nsize} is zero,
the allocator must behave like @id{free}
and return @id{NULL}.

When @id{nsize} is not zero,
the allocator must behave like @id{realloc}.
The allocator returns @id{NULL}
if and only if it cannot fulfill the request.
Lua assumes that the allocator never fails when
@T{osize >= nsize}.

Here is a simple implementation for the @x{allocator function}.
It is used in the auxiliary library by @Lid{luaL_newstate}.
@verbatim{
static void *l_alloc (void *ud, void *ptr, size_t osize,
                                           size_t nsize) {
  (void)ud;  (void)osize;  /* not used */
  if (nsize == 0) {
    free(ptr);
    return NULL;
  }
  else
    return realloc(ptr, nsize);
}
}
Note that @N{Standard C} ensures
that @T{free(NULL)} has no effect and that
@T{realloc(NULL,size)} is equivalent to @T{malloc(size)}.
This code assumes that @id{realloc} does not fail when shrinking a block.
(Although @N{Standard C} does not ensure this behavior,
it seems to be a safe assumption.)

}

@APIEntry{void lua_arith (lua_State *L, int op);|
@apii{2|1,1,e}

Performs an arithmetic or bitwise operation over the two values
(or one, in the case of negations)
at the top of the stack,
with the value at the top being the second operand,
pops these values, and pushes the result of the operation.
The function follows the semantics of the corresponding Lua operator
(that is, it may call metamethods).

The value of @id{op} must be one of the following constants:
@description{

@item{@defid{LUA_OPADD}| performs addition (@T{+})}
@item{@defid{LUA_OPSUB}| performs subtraction (@T{-})}
@item{@defid{LUA_OPMUL}| performs multiplication (@T{*})}
@item{@defid{LUA_OPDIV}| performs float division (@T{/})}
@item{@defid{LUA_OPIDIV}| performs floor division (@T{//})}
@item{@defid{LUA_OPMOD}| performs modulo (@T{%})}
@item{@defid{LUA_OPPOW}| performs exponentiation (@T{^})}
@item{@defid{LUA_OPUNM}| performs mathematical negation (unary @T{-})}
@item{@defid{LUA_OPBNOT}| performs bitwise NOT (@T{~})}
@item{@defid{LUA_OPBAND}| performs bitwise AND (@T{&})}
@item{@defid{LUA_OPBOR}| performs bitwise OR (@T{|})}
@item{@defid{LUA_OPBXOR}| performs bitwise exclusive OR (@T{~})}
@item{@defid{LUA_OPSHL}| performs left shift (@T{<<})}
@item{@defid{LUA_OPSHR}| performs right shift (@T{>>})}

}

}

@APIEntry{lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);|
@apii{0,0,-}

Sets a new panic function and returns the old one @see{C-error}.

}

@APIEntry{void lua_call (lua_State *L, int nargs, int nresults);|
@apii{nargs+1,nresults,e}


Calls a function.

To call a function you must use the following protocol:
first, the function to be called is pushed onto the stack;
then, the arguments to the function are pushed
in direct order;
that is, the first argument is pushed first.
Finally you call @Lid{lua_call};
@id{nargs} is the number of arguments that you pushed onto the stack.
All arguments and the function value are popped from the stack
when the function is called.
The function results are pushed onto the stack when the function returns.
The number of results is adjusted to @id{nresults},
unless @id{nresults} is @defid{LUA_MULTRET}.
In this case, all results from the function are pushed;
Lua takes care that the returned values fit into the stack space,
but it does not ensure any extra space in the stack.
The function results are pushed onto the stack in direct order
(the first result is pushed first),
so that after the call the last result is on the top of the stack.

Any error inside the called function is propagated upwards
(with a @id{longjmp}).

The following example shows how the host program can do the
equivalent to this Lua code:
@verbatim{
a = f("how", t.x, 14)
}
Here it is @N{in C}:
@verbatim{
lua_getglobal(L, "f");                  /* function to be called */
lua_pushliteral(L, "how");                       /* 1st argument */
lua_getglobal(L, "t");                    /* table to be indexed */
lua_getfield(L, -1, "x");        /* push result of t.x (2nd arg) */
lua_remove(L, -2);                  /* remove 't' from the stack */
lua_pushinteger(L, 14);                          /* 3rd argument */
lua_call(L, 3, 1);     /* call 'f' with 3 arguments and 1 result */
lua_setglobal(L, "a");                         /* set global 'a' */
}
Note that the code above is @emph{balanced}:
at its end, the stack is back to its original configuration.
This is considered good programming practice.

}

@APIEntry{
void lua_callk (lua_State *L,
                int nargs,
                int nresults,
                lua_KContext ctx,
                lua_KFunction k);|
@apii{nargs + 1,nresults,e}

This function behaves exactly like @Lid{lua_call},
but allows the called function to yield @see{continuations}.

}

@APIEntry{typedef int (*lua_CFunction) (lua_State *L);|

Type for @N{C functions}.

In order to communicate properly with Lua,
a @N{C function} must use the following protocol,
which defines the way parameters and results are passed:
a @N{C function} receives its arguments from Lua in its stack
in direct order (the first argument is pushed first).
So, when the function starts,
@T{lua_gettop(L)} returns the number of arguments received by the function.
The first argument (if any) is at index 1
and its last argument is at index @T{lua_gettop(L)}.
To return values to Lua, a @N{C function} just pushes them onto the stack,
in direct order (the first result is pushed first),
and returns the number of results.
Any other value in the stack below the results will be properly
discarded by Lua.
Like a Lua function, a @N{C function} called by Lua can also return
many results.

As an example, the following function receives a variable number
of numeric arguments and returns their average and their sum:
@verbatim{
static int foo (lua_State *L) {
  int n = lua_gettop(L);    /* number of arguments */
  lua_Number sum = 0.0;
  int i;
  for (i = 1; i <= n; i++) {
    if (!lua_isnumber(L, i)) {
      lua_pushliteral(L, "incorrect argument");
      lua_error(L);
    }
    sum += lua_tonumber(L, i);
  }
  lua_pushnumber(L, sum/n);        /* first result */
  lua_pushnumber(L, sum);         /* second result */
  return 2;                   /* number of results */
}
}



}


@APIEntry{int lua_checkstack (lua_State *L, int n);|
@apii{0,0,-}

Ensures that the stack has space for at least @id{n} extra slots
(that is, that you can safely push up to @id{n} values into it).
It returns false if it cannot fulfill the request,
either because it would cause the stack
to be larger than a fixed maximum size
(typically at least several thousand elements) or
because it cannot allocate memory for the extra space.
This function never shrinks the stack;
if the stack already has space for the extra slots,
it is left unchanged.

}

@APIEntry{void lua_close (lua_State *L);|
@apii{0,0,-}

Destroys all objects in the given Lua state
(calling the corresponding garbage-collection metamethods, if any)
and frees all dynamic memory used by this state.
On several platforms, you may not need to call this function,
because all resources are naturally released when the host program ends.
On the other hand, long-running programs that create multiple states,
such as daemons or web servers,
will probably need to close states as soon as they are not needed.

}

@APIEntry{int lua_compare (lua_State *L, int index1, int index2, int op);|
@apii{0,0,e}

Compares two Lua values.
Returns 1 if the value at index @id{index1} satisfies @id{op}
when compared with the value at index @id{index2},
following the semantics of the corresponding Lua operator
(that is, it may call metamethods).
Otherwise @N{returns 0}.
Also @N{returns 0} if any of the indices is not valid.

The value of @id{op} must be one of the following constants:
@description{

@item{@defid{LUA_OPEQ}| compares for equality (@T{==})}
@item{@defid{LUA_OPLT}| compares for less than (@T{<})}
@item{@defid{LUA_OPLE}| compares for less or equal (@T{<=})}

}

}

@APIEntry{void lua_concat (lua_State *L, int n);|
@apii{n,1,e}

Concatenates the @id{n} values at the top of the stack,
pops them, and leaves the result at the top.
If @N{@T{n} is 1}, the result is the single value on the stack
(that is, the function does nothing);
if @id{n} is 0, the result is the empty string.
Concatenation is performed following the usual semantics of Lua
@see{concat}.

}

@APIEntry{void lua_copy (lua_State *L, int fromidx, int toidx);|
@apii{0,0,-}

Copies the element at index @id{fromidx}
into the valid index @id{toidx},
replacing the value at that position.
Values at other positions are not affected.

}

@APIEntry{void lua_createtable (lua_State *L, int narr, int nrec);|
@apii{0,1,m}

Creates a new empty table and pushes it onto the stack.
Parameter @id{narr} is a hint for how many elements the table
will have as a sequence;
parameter @id{nrec} is a hint for how many other elements
the table will have.
Lua may use these hints to preallocate memory for the new table.
This preallocation is useful for performance when you know in advance
how many elements the table will have.
Otherwise you can use the function @Lid{lua_newtable}.

}

@APIEntry{int lua_dump (lua_State *L,
                        lua_Writer writer,
                        void *data,
                        int strip);|
@apii{0,0,-}

Dumps a function as a binary chunk.
Receives a Lua function on the top of the stack
and produces a binary chunk that,
if loaded again,
results in a function equivalent to the one dumped.
As it produces parts of the chunk,
@Lid{lua_dump} calls function @id{writer} @seeC{lua_Writer}
with the given @id{data}
to write them.

If @id{strip} is true,
the binary representation may not include all debug information
about the function,
to save space.

The value returned is the error code returned by the last
call to the writer;
@N{0 means} no errors.

This function does not pop the Lua function from the stack.

}

@APIEntry{int lua_error (lua_State *L);|
@apii{1,0,v}

Generates a Lua error,
using the value at the top of the stack as the error object.
This function does a long jump,
and therefore never returns
@seeC{luaL_error}.

}

@APIEntry{int lua_gc (lua_State *L, int what, int data);|
@apii{0,0,m}

Controls the garbage collector.

This function performs several tasks,
according to the value of the parameter @id{what}:
@description{

@item{@id{LUA_GCSTOP}|
stops the garbage collector.
}

@item{@id{LUA_GCRESTART}|
restarts the garbage collector.
}

@item{@id{LUA_GCCOLLECT}|
performs a full garbage-collection cycle.
}

@item{@id{LUA_GCCOUNT}|
returns the current amount of memory (in Kbytes) in use by Lua.
}

@item{@id{LUA_GCCOUNTB}|
returns the remainder of dividing the current amount of bytes of
memory in use by Lua by 1024.
}

@item{@id{LUA_GCSTEP}|
performs an incremental step of garbage collection.
}

@item{@id{LUA_GCSETPAUSE}|
sets @id{data} as the new value
for the @emph{pause} of the collector @see{GC}
and returns the previous value of the pause.
}

@item{@id{LUA_GCSETSTEPMUL}|
sets @id{data} as the new value for the @emph{step multiplier} of
the collector @see{GC}
and returns the previous value of the step multiplier.
}

@item{@id{LUA_GCISRUNNING}|
returns a boolean that tells whether the collector is running
(i.e., not stopped).
}

}

For more details about these options,
see @Lid{collectgarbage}.

}

@APIEntry{lua_Alloc lua_getallocf (lua_State *L, void **ud);|
@apii{0,0,-}

Returns the @x{memory-allocation function} of a given state.
If @id{ud} is not @id{NULL}, Lua stores in @T{*ud} the
opaque pointer given when the memory-allocator function was set.

}

@APIEntry{int lua_getfield (lua_State *L, int index, const char *k);|
@apii{0,1,e}

Pushes onto the stack the value @T{t[k]},
where @id{t} is the value at the given index.
As in Lua, this function may trigger a metamethod
for the @Q{index} event @see{metatable}.

Returns the type of the pushed value.

}

@APIEntry{void *lua_getextraspace (lua_State *L);|
@apii{0,0,-}

Returns a pointer to a raw memory area associated with the
given Lua state.
The application can use this area for any purpose;
Lua does not use it for anything.

Each new thread has this area initialized with a copy
of the area of the @x{main thread}.

By default, this area has the size of a pointer to void,
but you can recompile Lua with a different size for this area.
(See @id{LUA_EXTRASPACE} in @id{luaconf.h}.)

}

@APIEntry{int lua_getglobal (lua_State *L, const char *name);|
@apii{0,1,e}

Pushes onto the stack the value of the global @id{name}.
Returns the type of that value.

}

@APIEntry{int lua_geti (lua_State *L, int index, lua_Integer i);|
@apii{0,1,e}

Pushes onto the stack the value @T{t[i]},
where @id{t} is the value at the given index.
As in Lua, this function may trigger a metamethod
for the @Q{index} event @see{metatable}.

Returns the type of the pushed value.

}

@APIEntry{int lua_getmetatable (lua_State *L, int index);|
@apii{0,0|1,-}

If the value at the given index has a metatable,
the function pushes that metatable onto the stack and @N{returns 1}.
Otherwise,
the function @N{returns 0} and pushes nothing on the stack.

}

@APIEntry{int lua_gettable (lua_State *L, int index);|
@apii{1,1,e}

Pushes onto the stack the value @T{t[k]},
where @id{t} is the value at the given index
and @id{k} is the value at the top of the stack.

This function pops the key from the stack,
pushing the resulting value in its place.
As in Lua, this function may trigger a metamethod
for the @Q{index} event @see{metatable}.

Returns the type of the pushed value.

}

@APIEntry{int lua_gettop (lua_State *L);|
@apii{0,0,-}

Returns the index of the top element in the stack.
Because indices start @N{at 1},
this result is equal to the number of elements in the stack;
in particular, @N{0 means} an empty stack.

}

@APIEntry{int lua_getuservalue (lua_State *L, int index);|
@apii{0,1,-}

Pushes onto the stack the Lua value associated with the full userdata
at the given index.

Returns the type of the pushed value.

}

@APIEntry{void lua_insert (lua_State *L, int index);|
@apii{1,1,-}

Moves the top element into the given valid index,
shifting up the elements above this index to open space.
This function cannot be called with a pseudo-index,
because a pseudo-index is not an actual stack position.

}

@APIEntry{typedef @ldots lua_Integer;|

The type of integers in Lua.

By default this type is @id{long long},
(usually a 64-bit two-complement integer),
but that can be changed to @id{long} or @id{int}
(usually a 32-bit two-complement integer).
(See @id{LUA_INT_TYPE} in @id{luaconf.h}.)

Lua also defines the constants
@defid{LUA_MININTEGER} and @defid{LUA_MAXINTEGER},
with the minimum and the maximum values that fit in this type.

}

@APIEntry{int lua_isboolean (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a boolean,
and @N{0 otherwise}.

}

@APIEntry{int lua_iscfunction (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a @N{C function},
and @N{0 otherwise}.

}

@APIEntry{int lua_isfunction (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a function
(either C or Lua), and @N{0 otherwise}.

}

@APIEntry{int lua_isinteger (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is an integer
(that is, the value is a number and is represented as an integer),
and @N{0 otherwise}.

}

@APIEntry{int lua_islightuserdata (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a light userdata,
and @N{0 otherwise}.

}

@APIEntry{int lua_isnil (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is @nil,
and @N{0 otherwise}.

}

@APIEntry{int lua_isnone (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the given index is not valid,
and @N{0 otherwise}.

}

@APIEntry{int lua_isnoneornil (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the given index is not valid
or if the value at this index is @nil,
and @N{0 otherwise}.

}

@APIEntry{int lua_isnumber (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a number
or a string convertible to a number,
and @N{0 otherwise}.

}

@APIEntry{int lua_isstring (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a string
or a number (which is always convertible to a string),
and @N{0 otherwise}.

}

@APIEntry{int lua_istable (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a table,
and @N{0 otherwise}.

}

@APIEntry{int lua_isthread (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a thread,
and @N{0 otherwise}.

}

@APIEntry{int lua_isuserdata (lua_State *L, int index);|
@apii{0,0,-}

Returns 1 if the value at the given index is a userdata
(either full or light), and @N{0 otherwise}.

}

@APIEntry{int lua_isyieldable (lua_State *L);|
@apii{0,0,-}

Returns 1 if the given coroutine can yield,
and @N{0 otherwise}.

}

@APIEntry{typedef @ldots lua_KContext;|

The type for continuation-function contexts.
It must be a numeric type.
This type is defined as @id{intptr_t}
when @id{intptr_t} is available,
so that it can store pointers too.
Otherwise, it is defined as @id{ptrdiff_t}.

}

@APIEntry{
typedef int (*lua_KFunction) (lua_State *L, int status, lua_KContext ctx);|

Type for continuation functions @see{continuations}.

}

@APIEntry{void lua_len (lua_State *L, int index);|
@apii{0,1,e}

Returns the length of the value at the given index.
It is equivalent to the @Char{#} operator in Lua @see{len-op} and
may trigger a metamethod for the @Q{length} event @see{metatable}.
The result is pushed on the stack.

}

@APIEntry{
int lua_load (lua_State *L,
              lua_Reader reader,
              void *data,
              const char *chunkname,
              const char *mode);|
@apii{0,1,-}

Loads a Lua chunk without running it.
If there are no errors,
@id{lua_load} pushes the compiled chunk as a Lua
function on top of the stack.
Otherwise, it pushes an error message.

The return values of @id{lua_load} are:
@description{

@item{@Lid{LUA_OK}| no errors;}

@item{@defid{LUA_ERRSYNTAX}|
syntax error during precompilation;}

@item{@Lid{LUA_ERRMEM}|
@x{memory allocation (out-of-memory) error};}

@item{@Lid{LUA_ERRGCMM}|
error while running a @idx{__gc} metamethod.
(This error has no relation with the chunk being loaded.
It is generated by the garbage collector.)
}

}

The @id{lua_load} function uses a user-supplied @id{reader} function
to read the chunk @seeC{lua_Reader}.
The @id{data} argument is an opaque value passed to the reader function.

The @id{chunkname} argument gives a name to the chunk,
which is used for error messages and in debug information @see{debugI}.

@id{lua_load} automatically detects whether the chunk is text or binary
and loads it accordingly (see program @idx{luac}).
The string @id{mode} works as in function @Lid{load},
with the addition that
a @id{NULL} value is equivalent to the string @St{bt}.

@id{lua_load} uses the stack internally,
so the reader function must always leave the stack
unmodified when returning.

If the resulting function has upvalues,
its first upvalue is set to the value of the @x{global environment}
stored at index @id{LUA_RIDX_GLOBALS} in the registry @see{registry}.
When loading main chunks,
this upvalue will be the @id{_ENV} variable @see{globalenv}.
Other upvalues are initialized with @nil.

}

@APIEntry{lua_State *lua_newstate (lua_Alloc f, void *ud);|
@apii{0,0,-}

Creates a new thread running in a new, independent state.
Returns @id{NULL} if it cannot create the thread or the state
(due to lack of memory).
The argument @id{f} is the @x{allocator function};
Lua does all memory allocation for this state
through this function @seeF{lua_Alloc}.
The second argument, @id{ud}, is an opaque pointer that Lua
passes to the allocator in every call.

}

@APIEntry{void lua_newtable (lua_State *L);|
@apii{0,1,m}

Creates a new empty table and pushes it onto the stack.
It is equivalent to @T{lua_createtable(L, 0, 0)}.

}

@APIEntry{lua_State *lua_newthread (lua_State *L);|
@apii{0,1,m}

Creates a new thread, pushes it on the stack,
and returns a pointer to a @Lid{lua_State} that represents this new thread.
The new thread returned by this function shares with the original thread
its global environment,
but has an independent execution stack.

There is no explicit function to close or to destroy a thread.
Threads are subject to garbage collection,
like any Lua object.

}

@APIEntry{void *lua_newuserdata (lua_State *L, size_t size);|
@apii{0,1,m}

This function allocates a new block of memory with the given size,
pushes onto the stack a new full userdata with the block address,
and returns this address.
The host program can freely use this memory.

}

@APIEntry{int lua_next (lua_State *L, int index);|
@apii{1,2|0,e}

Pops a key from the stack,
and pushes a key@En{}value pair from the table at the given index
(the @Q{next} pair after the given key).
If there are no more elements in the table,
then @Lid{lua_next} returns 0 (and pushes nothing).

A typical traversal looks like this:
@verbatim{
/* table is in the stack at index 't' */
lua_pushnil(L);  /* first key */
while (lua_next(L, t) != 0) {
  /* uses 'key' (at index -2) and 'value' (at index -1) */
  printf("%s - %s\n",
         lua_typename(L, lua_type(L, -2)),
         lua_typename(L, lua_type(L, -1)));
  /* removes 'value'; keeps 'key' for next iteration */
  lua_pop(L, 1);
}
}

While traversing a table,
do not call @Lid{lua_tolstring} directly on a key,
unless you know that the key is actually a string.
Recall that @Lid{lua_tolstring} may change
the value at the given index;
this confuses the next call to @Lid{lua_next}.

See function @Lid{next} for the caveats of modifying
the table during its traversal.

}

@APIEntry{typedef @ldots lua_Number;|

The type of floats in Lua.

By default this type is double,
but that can be changed to a single float or a long double.
(See @id{LUA_FLOAT_TYPE} in @id{luaconf.h}.)

}

@APIEntry{int lua_numbertointeger (lua_Number n, lua_Integer *p);|

Converts a Lua float to a Lua integer.
This macro assumes that @id{n} has an integral value.
If that value is within the range of Lua integers,
it is converted to an integer and assigned to @T{*p}.
The macro results in a boolean indicating whether the
conversion was successful.
(Note that this range test can be tricky to do
correctly without this macro,
due to roundings.)

This macro may evaluate its arguments more than once.

}

@APIEntry{int lua_pcall (lua_State *L, int nargs, int nresults, int msgh);|
@apii{nargs + 1,nresults|1,-}

Calls a function in protected mode.

Both @id{nargs} and @id{nresults} have the same meaning as
in @Lid{lua_call}.
If there are no errors during the call,
@Lid{lua_pcall} behaves exactly like @Lid{lua_call}.
However, if there is any error,
@Lid{lua_pcall} catches it,
pushes a single value on the stack (the error object),
and returns an error code.
Like @Lid{lua_call},
@Lid{lua_pcall} always removes the function
and its arguments from the stack.

If @id{msgh} is 0,
then the error object returned on the stack
is exactly the original error object.
Otherwise, @id{msgh} is the stack index of a
@emph{message handler}.
(This index cannot be a pseudo-index.)
In case of runtime errors,
this function will be called with the error object
and its return value will be the object
returned on the stack by @Lid{lua_pcall}.

Typically, the message handler is used to add more debug
information to the error object, such as a stack traceback.
Such information cannot be gathered after the return of @Lid{lua_pcall},
since by then the stack has unwound.

The @Lid{lua_pcall} function returns one of the following constants
(defined in @id{lua.h}):
@description{

@item{@defid{LUA_OK} (0)|
success.}

@item{@defid{LUA_ERRRUN}|
a runtime error.
}

@item{@defid{LUA_ERRMEM}|
@x{memory allocation error}.
For such errors, Lua does not call the @x{message handler}.
}

@item{@defid{LUA_ERRERR}|
error while running the @x{message handler}.
}

@item{@defid{LUA_ERRGCMM}|
error while running a @idx{__gc} metamethod.
For such errors, Lua does not call the @x{message handler}
(as this kind of error typically has no relation
with the function being called).
}

}

}

@APIEntry{
int lua_pcallk (lua_State *L,
                int nargs,
                int nresults,
                int msgh,
                lua_KContext ctx,
                lua_KFunction k);|
@apii{nargs + 1,nresults|1,-}

This function behaves exactly like @Lid{lua_pcall},
but allows the called function to yield @see{continuations}.

}

@APIEntry{void lua_pop (lua_State *L, int n);|
@apii{n,0,-}

Pops @id{n} elements from the stack.

}

@APIEntry{void lua_pushboolean (lua_State *L, int b);|
@apii{0,1,-}

Pushes a boolean value with value @id{b} onto the stack.

}

@APIEntry{void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);|
@apii{n,1,m}

Pushes a new @N{C closure} onto the stack.

When a @N{C function} is created,
it is possible to associate some values with it,
thus creating a @x{@N{C closure}} @see{c-closure};
these values are then accessible to the function whenever it is called.
To associate values with a @N{C function},
first these values must be pushed onto the stack
(when there are multiple values, the first value is pushed first).
Then @Lid{lua_pushcclosure}
is called to create and push the @N{C function} onto the stack,
with the argument @id{n} telling how many values will be
associated with the function.
@Lid{lua_pushcclosure} also pops these values from the stack.

The maximum value for @id{n} is 255.

When @id{n} is zero,
this function creates a @def{light @N{C function}},
which is just a pointer to the @N{C function}.
In that case, it never raises a memory error.

}

@APIEntry{void lua_pushcfunction (lua_State *L, lua_CFunction f);|
@apii{0,1,-}

Pushes a @N{C function} onto the stack.
This function receives a pointer to a @N{C function}
and pushes onto the stack a Lua value of type @id{function} that,
when called, invokes the corresponding @N{C function}.

Any function to be callable by Lua must
follow the correct protocol to receive its parameters
and return its results @seeC{lua_CFunction}.

}

@APIEntry{const char *lua_pushfstring (lua_State *L, const char *fmt, ...);|
@apii{0,1,e}

Pushes onto the stack a formatted string
and returns a pointer to this string.
It is similar to the @ANSI{sprintf},
but has some important differences:
@itemize{

@item{
You do not have to allocate space for the result:
the result is a Lua string and Lua takes care of memory allocation
(and deallocation, through garbage collection).
}

@item{
The conversion specifiers are quite restricted.
There are no flags, widths, or precisions.
The conversion specifiers can only be
@Char{%%} (inserts the character @Char{%}),
@Char{%s} (inserts a zero-terminated string, with no size restrictions),
@Char{%f} (inserts a @Lid{lua_Number}),
@Char{%I} (inserts a @Lid{lua_Integer}),
@Char{%p} (inserts a pointer as a hexadecimal numeral),
@Char{%d} (inserts an @T{int}),
@Char{%c} (inserts an @T{int} as a one-byte character), and
@Char{%U} (inserts a @T{long int} as a @x{UTF-8} byte sequence).
}

}

Unlike other push functions,
this function checks for the stack space it needs,
including the slot for its result.

}

@APIEntry{void lua_pushglobaltable (lua_State *L);|
@apii{0,1,-}

Pushes the @x{global environment} onto the stack.

}

@APIEntry{void lua_pushinteger (lua_State *L, lua_Integer n);|
@apii{0,1,-}

Pushes an integer with value @id{n} onto the stack.

}

@APIEntry{void lua_pushlightuserdata (lua_State *L, void *p);|
@apii{0,1,-}

Pushes a light userdata onto the stack.

Userdata represent @N{C values} in Lua.
A @def{light userdata} represents a pointer, a @T{void*}.
It is a value (like a number):
you do not create it, it has no individual metatable,
and it is not collected (as it was never created).
A light userdata is equal to @Q{any}
light userdata with the same @N{C address}.

}

@APIEntry{const char *lua_pushliteral (lua_State *L, const char *s);|
@apii{0,1,m}

This macro is equivalent to @Lid{lua_pushstring},
but should be used only when @id{s} is a literal string.

}

@APIEntry{const char *lua_pushlstring (lua_State *L, const char *s, size_t len);|
@apii{0,1,m}

Pushes the string pointed to by @id{s} with size @id{len}
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at @id{s} can be freed or reused immediately after
the function returns.
The string can contain any binary data,
including @x{embedded zeros}.

Returns a pointer to the internal copy of the string.

}

@APIEntry{void lua_pushnil (lua_State *L);|
@apii{0,1,-}

Pushes a nil value onto the stack.

}

@APIEntry{void lua_pushnumber (lua_State *L, lua_Number n);|
@apii{0,1,-}

Pushes a float with value @id{n} onto the stack.

}

@APIEntry{const char *lua_pushstring (lua_State *L, const char *s);|
@apii{0,1,m}

Pushes the zero-terminated string pointed to by @id{s}
onto the stack.
Lua makes (or reuses) an internal copy of the given string,
so the memory at @id{s} can be freed or reused immediately after
the function returns.

Returns a pointer to the internal copy of the string.

If @id{s} is @id{NULL}, pushes @nil and returns @id{NULL}.

}

@APIEntry{int lua_pushthread (lua_State *L);|
@apii{0,1,-}

Pushes the thread represented by @id{L} onto the stack.
Returns 1 if this thread is the @x{main thread} of its state.

}

@APIEntry{void lua_pushvalue (lua_State *L, int index);|
@apii{0,1,-}

Pushes a copy of the element at the given index
onto the stack.

}

@APIEntry{
const char *lua_pushvfstring (lua_State *L,
                              const char *fmt,
                              va_list argp);|
@apii{0,1,m}

Equivalent to @Lid{lua_pushfstring}, except that it receives a @id{va_list}
instead of a variable number of arguments.

}

@APIEntry{int lua_rawequal (lua_State *L, int index1, int index2);|
@apii{0,0,-}

Returns 1 if the two values in indices @id{index1} and
@id{index2} are primitively equal
(that is, without calling the @idx{__eq} metamethod).
Otherwise @N{returns 0}.
Also @N{returns 0} if any of the indices are not valid.

}

@APIEntry{int lua_rawget (lua_State *L, int index);|
@apii{1,1,-}

Similar to @Lid{lua_gettable}, but does a raw access
(i.e., without metamethods).

}

@APIEntry{int lua_rawgeti (lua_State *L, int index, lua_Integer n);|
@apii{0,1,-}

Pushes onto the stack the value @T{t[n]},
where @id{t} is the table at the given index.
The access is raw,
that is, it does not invoke the @idx{__index} metamethod.

Returns the type of the pushed value.

}

@APIEntry{int lua_rawgetp (lua_State *L, int index, const void *p);|
@apii{0,1,-}

Pushes onto the stack the value @T{t[k]},
where @id{t} is the table at the given index and
@id{k} is the pointer @id{p} represented as a light userdata.
The access is raw;
that is, it does not invoke the @idx{__index} metamethod.

Returns the type of the pushed value.

}

@APIEntry{size_t lua_rawlen (lua_State *L, int index);|
@apii{0,0,-}

Returns the raw @Q{length} of the value at the given index:
for strings, this is the string length;
for tables, this is the result of the length operator (@Char{#})
with no metamethods;
for userdata, this is the size of the block of memory allocated
for the userdata;
for other values, it @N{is 0}.

}

@APIEntry{void lua_rawset (lua_State *L, int index);|
@apii{2,0,m}

Similar to @Lid{lua_settable}, but does a raw assignment
(i.e., without metamethods).

}

@APIEntry{void lua_rawseti (lua_State *L, int index, lua_Integer i);|
@apii{1,0,m}

Does the equivalent of @T{t[i] = v},
where @id{t} is the table at the given index
and @id{v} is the value at the top of the stack.

This function pops the value from the stack.
The assignment is raw,
that is, it does not invoke the @idx{__newindex} metamethod.

}

@APIEntry{void lua_rawsetp (lua_State *L, int index, const void *p);|
@apii{1,0,m}

Does the equivalent of @T{t[p] = v},
where @id{t} is the table at the given index,
@id{p} is encoded as a light userdata,
and @id{v} is the value at the top of the stack.

This function pops the value from the stack.
The assignment is raw,
that is, it does not invoke @idx{__newindex} metamethod.

}

@APIEntry{
typedef const char * (*lua_Reader) (lua_State *L,
                                    void *data,
                                    size_t *size);|

The reader function used by @Lid{lua_load}.
Every time it needs another piece of the chunk,
@Lid{lua_load} calls the reader,
passing along its @id{data} parameter.
The reader must return a pointer to a block of memory
with a new piece of the chunk
and set @id{size} to the block size.
The block must exist until the reader function is called again.
To signal the end of the chunk,
the reader must return @id{NULL} or set @id{size} to zero.
The reader function may return pieces of any size greater than zero.

}

@APIEntry{void lua_register (lua_State *L, const char *name, lua_CFunction f);|
@apii{0,0,e}

Sets the @N{C function} @id{f} as the new value of global @id{name}.
It is defined as a macro:
@verbatim{
#define lua_register(L,n,f) \
       (lua_pushcfunction(L, f), lua_setglobal(L, n))
}

}

@APIEntry{void lua_remove (lua_State *L, int index);|
@apii{1,0,-}

Removes the element at the given valid index,
shifting down the elements above this index to fill the gap.
This function cannot be called with a pseudo-index,
because a pseudo-index is not an actual stack position.

}

@APIEntry{void lua_replace (lua_State *L, int index);|
@apii{1,0,-}

Moves the top element into the given valid index
without shifting any element
(therefore replacing the value at that given index),
and then pops the top element.

}

@APIEntry{int lua_resume (lua_State *L, lua_State *from, int nargs);|
@apii{?,?,-}

Starts and resumes a coroutine in the given thread @id{L}.

To start a coroutine,
you push onto the thread stack the main function plus any arguments;
then you call @Lid{lua_resume},
with @id{nargs} being the number of arguments.
This call returns when the coroutine suspends or finishes its execution.
When it returns, the stack contains all values passed to @Lid{lua_yield},
or all values returned by the body function.
@Lid{lua_resume} returns
@Lid{LUA_YIELD} if the coroutine yields,
@Lid{LUA_OK} if the coroutine finishes its execution
without errors,
or an error code in case of errors @seeC{lua_pcall}.

In case of errors,
the stack is not unwound,
so you can use the debug API over it.
The error object is on the top of the stack.

To resume a coroutine,
you remove any results from the last @Lid{lua_yield},
put on its stack only the values to
be passed as results from @id{yield},
and then call @Lid{lua_resume}.

The parameter @id{from} represents the coroutine that is resuming @id{L}.
If there is no such coroutine,
this parameter can be @id{NULL}.

}

@APIEntry{void lua_rotate (lua_State *L, int idx, int n);|
@apii{0,0,-}

Rotates the stack elements between the valid index @id{idx}
and the top of the stack.
The elements are rotated @id{n} positions in the direction of the top,
for a positive @id{n},
or @T{-n} positions in the direction of the bottom,
for a negative @id{n}.
The absolute value of @id{n} must not be greater than the size
of the slice being rotated.
This function cannot be called with a pseudo-index,
because a pseudo-index is not an actual stack position.

}

@APIEntry{void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);|
@apii{0,0,-}

Changes the @x{allocator function} of a given state to @id{f}
with user data @id{ud}.

}

@APIEntry{void lua_setfield (lua_State *L, int index, const char *k);|
@apii{1,0,e}

Does the equivalent to @T{t[k] = v},
where @id{t} is the value at the given index
and @id{v} is the value at the top of the stack.

This function pops the value from the stack.
As in Lua, this function may trigger a metamethod
for the @Q{newindex} event @see{metatable}.

}

@APIEntry{void lua_setglobal (lua_State *L, const char *name);|
@apii{1,0,e}

Pops a value from the stack and
sets it as the new value of global @id{name}.

}

@APIEntry{void lua_seti (lua_State *L, int index, lua_Integer n);|
@apii{1,0,e}

Does the equivalent to @T{t[n] = v},
where @id{t} is the value at the given index
and @id{v} is the value at the top of the stack.

This function pops the value from the stack.
As in Lua, this function may trigger a metamethod
for the @Q{newindex} event @see{metatable}.

}

@APIEntry{void lua_setmetatable (lua_State *L, int index);|
@apii{1,0,-}

Pops a table from the stack and
sets it as the new metatable for the value at the given index.

}

@APIEntry{void lua_settable (lua_State *L, int index);|
@apii{2,0,e}

Does the equivalent to @T{t[k] = v},
where @id{t} is the value at the given index,
@id{v} is the value at the top of the stack,
and @id{k} is the value just below the top.

This function pops both the key and the value from the stack.
As in Lua, this function may trigger a metamethod
for the @Q{newindex} event @see{metatable}.

}

@APIEntry{void lua_settop (lua_State *L, int index);|
@apii{?,?,-}

Accepts any index, @N{or 0},
and sets the stack top to this index.
If the new top is larger than the old one,
then the new elements are filled with @nil.
If @id{index} @N{is 0}, then all stack elements are removed.

}

@APIEntry{void lua_setuservalue (lua_State *L, int index);|
@apii{1,0,-}

Pops a value from the stack and sets it as
the new value associated to the full userdata at the given index.

}

@APIEntry{typedef struct lua_State lua_State;|

An opaque structure that points to a thread and indirectly
(through the thread) to the whole state of a Lua interpreter.
The Lua library is fully reentrant:
it has no global variables.
All information about a state is accessible through this structure.

A pointer to this structure must be passed as the first argument to
every function in the library, except to @Lid{lua_newstate},
which creates a Lua state from scratch.

}

@APIEntry{int lua_status (lua_State *L);|
@apii{0,0,-}

Returns the status of the thread @id{L}.

The status can be 0 (@Lid{LUA_OK}) for a normal thread,
an error code if the thread finished the execution
of a @Lid{lua_resume} with an error,
or @defid{LUA_YIELD} if the thread is suspended.

You can only call functions in threads with status @Lid{LUA_OK}.
You can resume threads with status @Lid{LUA_OK}
(to start a new coroutine) or @Lid{LUA_YIELD}
(to resume a coroutine).

}

@APIEntry{size_t lua_stringtonumber (lua_State *L, const char *s);|
@apii{0,1,-}

Converts the zero-terminated string @id{s} to a number,
pushes that number into the stack,
and returns the total size of the string,
that is, its length plus one.
The conversion can result in an integer or a float,
according to the lexical conventions of Lua @see{lexical}.
The string may have leading and trailing spaces and a sign.
If the string is not a valid numeral,
returns 0 and pushes nothing.
(Note that the result can be used as a boolean,
true if the conversion succeeds.)

}

@APIEntry{int lua_toboolean (lua_State *L, int index);|
@apii{0,0,-}

Converts the Lua value at the given index to a @N{C boolean}
value (@N{0 or 1}).
Like all tests in Lua,
@Lid{lua_toboolean} returns true for any Lua value
different from @false and @nil;
otherwise it returns false.
(If you want to accept only actual boolean values,
use @Lid{lua_isboolean} to test the value's type.)

}

@APIEntry{lua_CFunction lua_tocfunction (lua_State *L, int index);|
@apii{0,0,-}

Converts a value at the given index to a @N{C function}.
That value must be a @N{C function};
otherwise, returns @id{NULL}.

}

@APIEntry{lua_Integer lua_tointeger (lua_State *L, int index);|
@apii{0,0,-}

Equivalent to @Lid{lua_tointegerx} with @id{isnum} equal to @id{NULL}.

}

@APIEntry{lua_Integer lua_tointegerx (lua_State *L, int index, int *isnum);|
@apii{0,0,-}

Converts the Lua value at the given index
to the signed integral type @Lid{lua_Integer}.
The Lua value must be an integer,
or a number or string convertible to an integer @see{coercion};
otherwise, @id{lua_tointegerx} @N{returns 0}.

If @id{isnum} is not @id{NULL},
its referent is assigned a boolean value that
indicates whether the operation succeeded.

}

@APIEntry{const char *lua_tolstring (lua_State *L, int index, size_t *len);|
@apii{0,0,m}

Converts the Lua value at the given index to a @N{C string}.
If @id{len} is not @id{NULL},
it sets @T{*len} with the string length.
The Lua value must be a string or a number;
otherwise, the function returns @id{NULL}.
If the value is a number,
then @id{lua_tolstring} also
@emph{changes the actual value in the stack to a string}.
(This change confuses @Lid{lua_next}
when @id{lua_tolstring} is applied to keys during a table traversal.)

@id{lua_tolstring} returns a pointer
to a string inside the Lua state.
This string always has a zero (@Char{\0})
after its last character (as @N{in C}),
but can contain other zeros in its body.

Because Lua has garbage collection,
there is no guarantee that the pointer returned by @id{lua_tolstring}
will be valid after the corresponding Lua value is removed from the stack.

}

@APIEntry{lua_Number lua_tonumber (lua_State *L, int index);|
@apii{0,0,-}

Equivalent to @Lid{lua_tonumberx} with @id{isnum} equal to @id{NULL}.

}

@APIEntry{lua_Number lua_tonumberx (lua_State *L, int index, int *isnum);|
@apii{0,0,-}

Converts the Lua value at the given index
to the @N{C type} @Lid{lua_Number} @seeC{lua_Number}.
The Lua value must be a number or a string convertible to a number
@see{coercion};
otherwise, @Lid{lua_tonumberx} @N{returns 0}.

If @id{isnum} is not @id{NULL},
its referent is assigned a boolean value that
indicates whether the operation succeeded.

}

@APIEntry{const void *lua_topointer (lua_State *L, int index);|
@apii{0,0,-}

Converts the value at the given index to a generic
@N{C pointer} (@T{void*}).
The value can be a userdata, a table, a thread, or a function;
otherwise, @id{lua_topointer} returns @id{NULL}.
Different objects will give different pointers.
There is no way to convert the pointer back to its original value.

Typically this function is used only for hashing and debug information.

}

@APIEntry{const char *lua_tostring (lua_State *L, int index);|
@apii{0,0,m}

Equivalent to @Lid{lua_tolstring} with @id{len} equal to @id{NULL}.

}

@APIEntry{lua_State *lua_tothread (lua_State *L, int index);|
@apii{0,0,-}

Converts the value at the given index to a Lua thread
(represented as @T{lua_State*}).
This value must be a thread;
otherwise, the function returns @id{NULL}.

}

@APIEntry{void *lua_touserdata (lua_State *L, int index);|
@apii{0,0,-}

If the value at the given index is a full userdata,
returns its block address.
If the value is a light userdata,
returns its pointer.
Otherwise, returns @id{NULL}.

}

@APIEntry{int lua_type (lua_State *L, int index);|
@apii{0,0,-}

Returns the type of the value in the given valid index,
or @id{LUA_TNONE} for a non-valid (but acceptable) index.
The types returned by @Lid{lua_type} are coded by the following constants
defined in @id{lua.h}:
@defid{LUA_TNIL} (0),
@defid{LUA_TNUMBER},
@defid{LUA_TBOOLEAN},
@defid{LUA_TSTRING},
@defid{LUA_TTABLE},
@defid{LUA_TFUNCTION},
@defid{LUA_TUSERDATA},
@defid{LUA_TTHREAD},
and
@defid{LUA_TLIGHTUSERDATA}.

}

@APIEntry{const char *lua_typename (lua_State *L, int tp);|
@apii{0,0,-}

Returns the name of the type encoded by the value @id{tp},
which must be one the values returned by @Lid{lua_type}.

}

@APIEntry{typedef @ldots lua_Unsigned;|

The unsigned version of @Lid{lua_Integer}.

}

@APIEntry{int lua_upvalueindex (int i);|
@apii{0,0,-}

Returns the pseudo-index that represents the @id{i}-th upvalue of
the running function @see{c-closure}.

}

@APIEntry{const lua_Number *lua_version (lua_State *L);|
@apii{0,0,-}

Returns the address of the version number
(a C static variable)
stored in the Lua core.
When called with a valid @Lid{lua_State},
returns the address of the version used to create that state.
When called with @id{NULL},
returns the address of the version running the call.

}

@APIEntry{
typedef int (*lua_Writer) (lua_State *L,
                           const void* p,
                           size_t sz,
                           void* ud);|

The type of the writer function used by @Lid{lua_dump}.
Every time it produces another piece of chunk,
@Lid{lua_dump} calls the writer,
passing along the buffer to be written (@id{p}),
its size (@id{sz}),
and the @id{data} parameter supplied to @Lid{lua_dump}.

The writer returns an error code:
@N{0 means} no errors;
any other value means an error and stops @Lid{lua_dump} from
calling the writer again.

}

@APIEntry{void lua_xmove (lua_State *from, lua_State *to, int n);|
@apii{?,?,-}

Exchange values between different threads of the same state.

This function pops @id{n} values from the stack @id{from},
and pushes them onto the stack @id{to}.

}

@APIEntry{int lua_yield (lua_State *L, int nresults);|
@apii{?,?,e}

This function is equivalent to @Lid{lua_yieldk},
but it has no continuation @see{continuations}.
Therefore, when the thread resumes,
it continues the function that called
the function calling @id{lua_yield}.

}


@APIEntry{
int lua_yieldk (lua_State *L,
                int nresults,
                lua_KContext ctx,
                lua_KFunction k);|
@apii{?,?,e}

Yields a coroutine (thread).

When a @N{C function} calls @Lid{lua_yieldk},
the running coroutine suspends its execution,
and the call to @Lid{lua_resume} that started this coroutine returns.
The parameter @id{nresults} is the number of values from the stack
that will be passed as results to @Lid{lua_resume}.

When the coroutine is resumed again,
Lua calls the given @x{continuation function} @id{k} to continue
the execution of the @N{C function} that yielded @see{continuations}.
This continuation function receives the same stack
from the previous function,
with the @id{n} results removed and
replaced by the arguments passed to @Lid{lua_resume}.
Moreover,
the continuation function receives the value @id{ctx}
that was passed to @Lid{lua_yieldk}.

Usually, this function does not return;
when the coroutine eventually resumes,
it continues executing the continuation function.
However, there is one special case,
which is when this function is called
from inside a line or a count hook @see{debugI}.
In that case, @id{lua_yieldk} should be called with no continuation
(probably in the form of @Lid{lua_yield}) and no results,
and the hook should return immediately after the call.
Lua will yield and,
when the coroutine resumes again,
it will continue the normal execution
of the (Lua) function that triggered the hook.

This function can raise an error if it is called from a thread
with a pending C call with no continuation function,
or it is called from a thread that is not running inside a resume
(e.g., the main thread).

}

}

@sect2{debugI| @title{The Debug Interface}

Lua has no built-in debugging facilities.
Instead, it offers a special interface
by means of functions and @emph{hooks}.
This interface allows the construction of different
kinds of debuggers, profilers, and other tools
that need @Q{inside information} from the interpreter.


@APIEntry{
typedef struct lua_Debug {
  int event;
  const char *name;           /* (n) */
  const char *namewhat;       /* (n) */
  const char *what;           /* (S) */
  const char *source;         /* (S) */
  int currentline;            /* (l) */
  int linedefined;            /* (S) */
  int lastlinedefined;        /* (S) */
  unsigned char nups;         /* (u) number of upvalues */
  unsigned char nparams;      /* (u) number of parameters */
  char isvararg;              /* (u) */
  char istailcall;            /* (t) */
  char short_src[LUA_IDSIZE]; /* (S) */
  /* private part */
  @rep{other fields}
} lua_Debug;
|

A structure used to carry different pieces of
information about a function or an activation record.
@Lid{lua_getstack} fills only the private part
of this structure, for later use.
To fill the other fields of @Lid{lua_Debug} with useful information,
call @Lid{lua_getinfo}.

The fields of @Lid{lua_Debug} have the following meaning:
@description{

@item{@id{source}|
the name of the chunk that created the function.
If @T{source} starts with a @Char{@At},
it means that the function was defined in a file where
the file name follows the @Char{@At}.
If @T{source} starts with a @Char{=},
the remainder of its contents describe the source in a user-dependent manner.
Otherwise,
the function was defined in a string where
@T{source} is that string.
}

@item{@id{short_src}|
a @Q{printable} version of @T{source}, to be used in error messages.
}

@item{@id{linedefined}|
the line number where the definition of the function starts.
}

@item{@id{lastlinedefined}|
the line number where the definition of the function ends.
}

@item{@id{what}|
the string @T{"Lua"} if the function is a Lua function,
@T{"C"} if it is a @N{C function},
@T{"main"} if it is the main part of a chunk.
}

@item{@id{currentline}|
the current line where the given function is executing.
When no line information is available,
@T{currentline} is set to @num{-1}.
}

@item{@id{name}|
a reasonable name for the given function.
Because functions in Lua are first-class values,
they do not have a fixed name:
some functions can be the value of multiple global variables,
while others can be stored only in a table field.
The @T{lua_getinfo} function checks how the function was
called to find a suitable name.
If it cannot find a name,
then @id{name} is set to @id{NULL}.
}

@item{@id{namewhat}|
explains the @T{name} field.
The value of @T{namewhat} can be
@T{"global"}, @T{"local"}, @T{"method"},
@T{"field"}, @T{"upvalue"}, or @T{""} (the empty string),
according to how the function was called.
(Lua uses the empty string when no other option seems to apply.)
}

@item{@id{istailcall}|
true if this function invocation was called by a tail call.
In this case, the caller of this level is not in the stack.
}

@item{@id{nups}|
the number of upvalues of the function.
}

@item{@id{nparams}|
the number of fixed parameters of the function
(always @N{0 for} @N{C functions}).
}

@item{@id{isvararg}|
true if the function is a vararg function
(always true for @N{C functions}).
}

}

}

@APIEntry{lua_Hook lua_gethook (lua_State *L);|
@apii{0,0,-}

Returns the current hook function.

}

@APIEntry{int lua_gethookcount (lua_State *L);|
@apii{0,0,-}

Returns the current hook count.

}

@APIEntry{int lua_gethookmask (lua_State *L);|
@apii{0,0,-}

Returns the current hook mask.

}

@APIEntry{int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);|
@apii{0|1,0|1|2,e}

Gets information about a specific function or function invocation.

To get information about a function invocation,
the parameter @id{ar} must be a valid activation record that was
filled by a previous call to @Lid{lua_getstack} or
given as argument to a hook @seeC{lua_Hook}.

To get information about a function you push it onto the stack
and start the @id{what} string with the character @Char{>}.
(In that case,
@id{lua_getinfo} pops the function from the top of the stack.)
For instance, to know in which line a function @id{f} was defined,
you can write the following code:
@verbatim{
lua_Debug ar;
lua_getglobal(L, "f");  /* get global 'f' */
lua_getinfo(L, ">S", &ar);
printf("%d\n", ar.linedefined);
}

Each character in the string @id{what}
selects some fields of the structure @id{ar} to be filled or
a value to be pushed on the stack:
@description{

@item{@Char{n}| fills in the field @id{name} and @id{namewhat};
}

@item{@Char{S}|
fills in the fields @id{source}, @id{short_src},
@id{linedefined}, @id{lastlinedefined}, and @id{what};
}

@item{@Char{l}| fills in the field @id{currentline};
}

@item{@Char{t}| fills in the field @id{istailcall};
}

@item{@Char{u}| fills in the fields
@id{nups}, @id{nparams}, and @id{isvararg};
}

@item{@Char{f}|
pushes onto the stack the function that is
running at the given level;
}

@item{@Char{L}|
pushes onto the stack a table whose indices are the
numbers of the lines that are valid on the function.
(A @emph{valid line} is a line with some associated code,
that is, a line where you can put a break point.
Non-valid lines include empty lines and comments.)

If this option is given together with option @Char{f},
its table is pushed after the function.
}

}

This function returns 0 on error
(for instance, an invalid option in @id{what}).

}

@APIEntry{const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n);|
@apii{0,0|1,-}

Gets information about a local variable of
a given activation record or a given function.

In the first case,
the parameter @id{ar} must be a valid activation record that was
filled by a previous call to @Lid{lua_getstack} or
given as argument to a hook @seeC{lua_Hook}.
The index @id{n} selects which local variable to inspect;
see @Lid{debug.getlocal} for details about variable indices
and names.

@Lid{lua_getlocal} pushes the variable's value onto the stack
and returns its name.

In the second case, @id{ar} must be @id{NULL} and the function
to be inspected must be at the top of the stack.
In this case, only parameters of Lua functions are visible
(as there is no information about what variables are active)
and no values are pushed onto the stack.

Returns @id{NULL} (and pushes nothing)
when the index is greater than
the number of active local variables.

}

@APIEntry{int lua_getstack (lua_State *L, int level, lua_Debug *ar);|
@apii{0,0,-}

Gets information about the interpreter runtime stack.

This function fills parts of a @Lid{lua_Debug} structure with
an identification of the @emph{activation record}
of the function executing at a given level.
@N{Level 0} is the current running function,
whereas level @M{n+1} is the function that has called level @M{n}
(except for tail calls, which do not count on the stack).
When there are no errors, @Lid{lua_getstack} returns 1;
when called with a level greater than the stack depth,
it returns 0.

}

@APIEntry{const char *lua_getupvalue (lua_State *L, int funcindex, int n);|
@apii{0,0|1,-}

Gets information about the @id{n}-th upvalue
of the closure at index @id{funcindex}.
It pushes the upvalue's value onto the stack
and returns its name.
Returns @id{NULL} (and pushes nothing)
when the index @id{n} is greater than the number of upvalues.

For @N{C functions}, this function uses the empty string @T{""}
as a name for all upvalues.
(For Lua functions,
upvalues are the external local variables that the function uses,
and that are consequently included in its closure.)

Upvalues have no particular order,
as they are active through the whole function.
They are numbered in an arbitrary order.

}

@APIEntry{typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);|

Type for debugging hook functions.

Whenever a hook is called, its @id{ar} argument has its field
@id{event} set to the specific event that triggered the hook.
Lua identifies these events with the following constants:
@defid{LUA_HOOKCALL}, @defid{LUA_HOOKRET},
@defid{LUA_HOOKTAILCALL}, @defid{LUA_HOOKLINE},
and @defid{LUA_HOOKCOUNT}.
Moreover, for line events, the field @id{currentline} is also set.
To get the value of any other field in @id{ar},
the hook must call @Lid{lua_getinfo}.

For call events, @id{event} can be @id{LUA_HOOKCALL},
the normal value, or @id{LUA_HOOKTAILCALL}, for a tail call;
in this case, there will be no corresponding return event.

While Lua is running a hook, it disables other calls to hooks.
Therefore, if a hook calls back Lua to execute a function or a chunk,
this execution occurs without any calls to hooks.

Hook functions cannot have continuations,
that is, they cannot call @Lid{lua_yieldk},
@Lid{lua_pcallk}, or @Lid{lua_callk} with a non-null @id{k}.

Hook functions can yield under the following conditions:
Only count and line events can yield;
to yield, a hook function must finish its execution
calling @Lid{lua_yield} with @id{nresults} equal to zero
(that is, with no values).

}

@APIEntry{void lua_sethook (lua_State *L, lua_Hook f, int mask, int count);|
@apii{0,0,-}

Sets the debugging hook function.

Argument @id{f} is the hook function.
@id{mask} specifies on which events the hook will be called:
it is formed by a bitwise OR of the constants
@defid{LUA_MASKCALL},
@defid{LUA_MASKRET},
@defid{LUA_MASKLINE},
and @defid{LUA_MASKCOUNT}.
The @id{count} argument is only meaningful when the mask
includes @id{LUA_MASKCOUNT}.
For each event, the hook is called as explained below:
@description{

@item{The call hook| is called when the interpreter calls a function.
The hook is called just after Lua enters the new function,
before the function gets its arguments.
}

@item{The return hook| is called when the interpreter returns from a function.
The hook is called just before Lua leaves the function.
There is no standard way to access the values
to be returned by the function.
}

@item{The line hook| is called when the interpreter is about to
start the execution of a new line of code,
or when it jumps back in the code (even to the same line).
(This event only happens while Lua is executing a Lua function.)
}

@item{The count hook| is called after the interpreter executes every
@T{count} instructions.
(This event only happens while Lua is executing a Lua function.)
}

}

A hook is disabled by setting @id{mask} to zero.

}

@APIEntry{const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n);|
@apii{0|1,0,-}

Sets the value of a local variable of a given activation record.
It assigns the value at the top of the stack
to the variable and returns its name.
It also pops the value from the stack.

Returns @id{NULL} (and pops nothing)
when the index is greater than
the number of active local variables.

Parameters @id{ar} and @id{n} are as in function @Lid{lua_getlocal}.

}

@APIEntry{const char *lua_setupvalue (lua_State *L, int funcindex, int n);|
@apii{0|1,0,-}

Sets the value of a closure's upvalue.
It assigns the value at the top of the stack
to the upvalue and returns its name.
It also pops the value from the stack.

Returns @id{NULL} (and pops nothing)
when the index @id{n} is greater than the number of upvalues.

Parameters @id{funcindex} and @id{n} are as in function @Lid{lua_getupvalue}.

}

@APIEntry{void *lua_upvalueid (lua_State *L, int funcindex, int n);|
@apii{0,0,-}

Returns a unique identifier for the upvalue numbered @id{n}
from the closure at index @id{funcindex}.

These unique identifiers allow a program to check whether different
closures share upvalues.
Lua closures that share an upvalue
(that is, that access a same external local variable)
will return identical ids for those upvalue indices.

Parameters @id{funcindex} and @id{n} are as in function @Lid{lua_getupvalue},
but @id{n} cannot be greater than the number of upvalues.

}

@APIEntry{
void lua_upvaluejoin (lua_State *L, int funcindex1, int n1,
                                    int funcindex2, int n2);|
@apii{0,0,-}

Make the @id{n1}-th upvalue of the Lua closure at index @id{funcindex1}
refer to the @id{n2}-th upvalue of the Lua closure at index @id{funcindex2}.

}

}

}


@C{-------------------------------------------------------------------------}
@sect1{@title{The Auxiliary Library}

@index{lauxlib.h}
The @def{auxiliary library} provides several convenient functions
to interface C with Lua.
While the basic API provides the primitive functions for all
interactions between C and Lua,
the auxiliary library provides higher-level functions for some
common tasks.

All functions and types from the auxiliary library
are defined in header file @id{lauxlib.h} and
have a prefix @id{luaL_}.

All functions in the auxiliary library are built on
top of the basic API,
and so they provide nothing that cannot be done with that API.
Nevertheless, the use of the auxiliary library ensures
more consistency to your code.


Several functions in the auxiliary library use internally some
extra stack slots.
When a function in the auxiliary library uses less than five slots,
it does not check the stack size;
it simply assumes that there are enough slots.

Several functions in the auxiliary library are used to
check @N{C function} arguments.
Because the error message is formatted for arguments
(e.g., @St{bad argument #1}),
you should not use these functions for other stack values.

Functions called @id{luaL_check*}
always raise an error if the check is not satisfied.

@sect2{@title{Functions and Types}

Here we list all functions and types from the auxiliary library
in alphabetical order.


@APIEntry{void luaL_addchar (luaL_Buffer *B, char c);|
@apii{?,?,m}

Adds the byte @id{c} to the buffer @id{B}
@seeC{luaL_Buffer}.

}

@APIEntry{void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);|
@apii{?,?,m}

Adds the string pointed to by @id{s} with length @id{l} to
the buffer @id{B}
@seeC{luaL_Buffer}.
The string can contain @x{embedded zeros}.

}

@APIEntry{void luaL_addsize (luaL_Buffer *B, size_t n);|
@apii{?,?,-}

Adds to the buffer @id{B} @seeC{luaL_Buffer}
a string of length @id{n} previously copied to the
buffer area @seeC{luaL_prepbuffer}.

}

@APIEntry{void luaL_addstring (luaL_Buffer *B, const char *s);|
@apii{?,?,m}

Adds the zero-terminated string pointed to by @id{s}
to the buffer @id{B}
@seeC{luaL_Buffer}.

}

@APIEntry{void luaL_addvalue (luaL_Buffer *B);|
@apii{1,?,m}

Adds the value at the top of the stack
to the buffer @id{B}
@seeC{luaL_Buffer}.
Pops the value.

This is the only function on string buffers that can (and must)
be called with an extra element on the stack,
which is the value to be added to the buffer.

}

@APIEntry{
void luaL_argcheck (lua_State *L,
                    int cond,
                    int arg,
                    const char *extramsg);|
@apii{0,0,v}

Checks whether @id{cond} is true.
If it is not, raises an error with a standard message @seeF{luaL_argerror}.

}

@APIEntry{int luaL_argerror (lua_State *L, int arg, const char *extramsg);|
@apii{0,0,v}

Raises an error reporting a problem with argument @id{arg}
of the @N{C function} that called it,
using a standard message
that includes @id{extramsg} as a comment:
@verbatim{
bad argument #@rep{arg} to '@rep{funcname}' (@rep{extramsg})
}
This function never returns.

}

@APIEntry{typedef struct luaL_Buffer luaL_Buffer;|

Type for a @def{string buffer}.

A string buffer allows @N{C code} to build Lua strings piecemeal.
Its pattern of use is as follows:
@itemize{

@item{First declare a variable @id{b} of type @Lid{luaL_Buffer}.}

@item{Then initialize it with a call @T{luaL_buffinit(L, &b)}.}

@item{
Then add string pieces to the buffer calling any of
the @id{luaL_add*} functions.
}

@item{
Finish by calling @T{luaL_pushresult(&b)}.
This call leaves the final string on the top of the stack.
}

}

If you know beforehand the total size of the resulting string,
you can use the buffer like this:
@itemize{

@item{First declare a variable @id{b} of type @Lid{luaL_Buffer}.}

@item{Then initialize it and preallocate a space of
size @id{sz} with a call @T{luaL_buffinitsize(L, &b, sz)}.}

@item{Then copy the string into that space.}

@item{
Finish by calling @T{luaL_pushresultsize(&b, sz)},
where @id{sz} is the total size of the resulting string
copied into that space.
}

}

During its normal operation,
a string buffer uses a variable number of stack slots.
So, while using a buffer, you cannot assume that you know where
the top of the stack is.
You can use the stack between successive calls to buffer operations
as long as that use is balanced;
that is,
when you call a buffer operation,
the stack is at the same level
it was immediately after the previous buffer operation.
(The only exception to this rule is @Lid{luaL_addvalue}.)
After calling @Lid{luaL_pushresult} the stack is back to its
level when the buffer was initialized,
plus the final string on its top.

}

@APIEntry{void luaL_buffinit (lua_State *L, luaL_Buffer *B);|
@apii{0,0,-}

Initializes a buffer @id{B}.
This function does not allocate any space;
the buffer must be declared as a variable
@seeC{luaL_Buffer}.

}

@APIEntry{char *luaL_buffinitsize (lua_State *L, luaL_Buffer *B, size_t sz);|
@apii{?,?,m}

Equivalent to the sequence
@Lid{luaL_buffinit}, @Lid{luaL_prepbuffsize}.

}

@APIEntry{int luaL_callmeta (lua_State *L, int obj, const char *e);|
@apii{0,0|1,e}

Calls a metamethod.

If the object at index @id{obj} has a metatable and this
metatable has a field @id{e},
this function calls this field passing the object as its only argument.
In this case this function returns true and pushes onto the
stack the value returned by the call.
If there is no metatable or no metamethod,
this function returns false (without pushing any value on the stack).

}

@APIEntry{void luaL_checkany (lua_State *L, int arg);|
@apii{0,0,v}

Checks whether the function has an argument
of any type (including @nil) at position @id{arg}.

}

@APIEntry{lua_Integer luaL_checkinteger (lua_State *L, int arg);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is an integer
(or can be converted to an integer)
and returns this integer cast to a @Lid{lua_Integer}.

}

@APIEntry{const char *luaL_checklstring (lua_State *L, int arg, size_t *l);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is a string
and returns this string;
if @id{l} is not @id{NULL} fills @T{*l}
with the string's length.

This function uses @Lid{lua_tolstring} to get its result,
so all conversions and caveats of that function apply here.

}

@APIEntry{lua_Number luaL_checknumber (lua_State *L, int arg);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is a number
and returns this number.

}

@APIEntry{
int luaL_checkoption (lua_State *L,
                      int arg,
                      const char *def,
                      const char *const lst[]);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is a string and
searches for this string in the array @id{lst}
(which must be NULL-terminated).
Returns the index in the array where the string was found.
Raises an error if the argument is not a string or
if the string cannot be found.

If @id{def} is not @id{NULL},
the function uses @id{def} as a default value when
there is no argument @id{arg} or when this argument is @nil.

This is a useful function for mapping strings to @N{C enums}.
(The usual convention in Lua libraries is
to use strings instead of numbers to select options.)

}

@APIEntry{void luaL_checkstack (lua_State *L, int sz, const char *msg);|
@apii{0,0,v}

Grows the stack size to @T{top + sz} elements,
raising an error if the stack cannot grow to that size.
@id{msg} is an additional text to go into the error message
(or @id{NULL} for no additional text).

}

@APIEntry{const char *luaL_checkstring (lua_State *L, int arg);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is a string
and returns this string.

This function uses @Lid{lua_tolstring} to get its result,
so all conversions and caveats of that function apply here.

}

@APIEntry{void luaL_checktype (lua_State *L, int arg, int t);|
@apii{0,0,v}

Checks whether the function argument @id{arg} has type @id{t}.
See @Lid{lua_type} for the encoding of types for @id{t}.

}

@APIEntry{void *luaL_checkudata (lua_State *L, int arg, const char *tname);|
@apii{0,0,v}

Checks whether the function argument @id{arg} is a userdata
of the type @id{tname} @seeC{luaL_newmetatable} and
returns the userdata address @seeC{lua_touserdata}.

}

@APIEntry{void luaL_checkversion (lua_State *L);|
@apii{0,0,v}

Checks whether the core running the call,
the core that created the Lua state,
and the code making the call are all using the same version of Lua.
Also checks whether the core running the call
and the core that created the Lua state
are using the same address space.

}

@APIEntry{int luaL_dofile (lua_State *L, const char *filename);|
@apii{0,?,e}

Loads and runs the given file.
It is defined as the following macro:
@verbatim{
(luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))
}
It returns false if there are no errors
or true in case of errors.

}

@APIEntry{int luaL_dostring (lua_State *L, const char *str);|
@apii{0,?,-}

Loads and runs the given string.
It is defined as the following macro:
@verbatim{
(luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))
}
It returns false if there are no errors
or true in case of errors.

}

@APIEntry{int luaL_error (lua_State *L, const char *fmt, ...);|
@apii{0,0,v}

Raises an error.
The error message format is given by @id{fmt}
plus any extra arguments,
following the same rules of @Lid{lua_pushfstring}.
It also adds at the beginning of the message the file name and
the line number where the error occurred,
if this information is available.

This function never returns,
but it is an idiom to use it in @N{C functions}
as @T{return luaL_error(@rep{args})}.

}

@APIEntry{int luaL_execresult (lua_State *L, int stat);|
@apii{0,3,m}

This function produces the return values for
process-related functions in the standard library
(@Lid{os.execute} and @Lid{io.close}).

}

@APIEntry{
int luaL_fileresult (lua_State *L, int stat, const char *fname);|
@apii{0,1|3,m}

This function produces the return values for
file-related functions in the standard library
(@Lid{io.open}, @Lid{os.rename}, @Lid{file:seek}, etc.).

}

@APIEntry{int luaL_getmetafield (lua_State *L, int obj, const char *e);|
@apii{0,0|1,m}

Pushes onto the stack the field @id{e} from the metatable
of the object at index @id{obj} and returns the type of pushed value.
If the object does not have a metatable,
or if the metatable does not have this field,
pushes nothing and returns @id{LUA_TNIL}.

}

@APIEntry{int luaL_getmetatable (lua_State *L, const char *tname);|
@apii{0,1,m}

Pushes onto the stack the metatable associated with name @id{tname}
in the registry @seeC{luaL_newmetatable}
(@nil if there is no metatable associated with that name).
Returns the type of the pushed value.

}

@APIEntry{int luaL_getsubtable (lua_State *L, int idx, const char *fname);|
@apii{0,1,e}

Ensures that the value @T{t[fname]},
where @id{t} is the value at index @id{idx},
is a table,
and pushes that table onto the stack.
Returns true if it finds a previous table there
and false if it creates a new table.

}

@APIEntry{
const char *luaL_gsub (lua_State *L,
                       const char *s,
                       const char *p,
                       const char *r);|
@apii{0,1,m}

Creates a copy of string @id{s} by replacing
any occurrence of the string @id{p}
with the string @id{r}.
Pushes the resulting string on the stack and returns it.

}

@APIEntry{lua_Integer luaL_len (lua_State *L, int index);|
@apii{0,0,e}

Returns the @Q{length} of the value at the given index
as a number;
it is equivalent to the @Char{#} operator in Lua @see{len-op}.
Raises an error if the result of the operation is not an integer.
(This case only can happen through metamethods.)

}

@APIEntry{
int luaL_loadbuffer (lua_State *L,
                     const char *buff,
                     size_t sz,
                     const char *name);|
@apii{0,1,-}

Equivalent to @Lid{luaL_loadbufferx} with @id{mode} equal to @id{NULL}.

}


@APIEntry{
int luaL_loadbufferx (lua_State *L,
                      const char *buff,
                      size_t sz,
                      const char *name,
                      const char *mode);|
@apii{0,1,-}

Loads a buffer as a Lua chunk.
This function uses @Lid{lua_load} to load the chunk in the
buffer pointed to by @id{buff} with size @id{sz}.

This function returns the same results as @Lid{lua_load}.
@id{name} is the chunk name,
used for debug information and error messages.
The string @id{mode} works as in function @Lid{lua_load}.

}


@APIEntry{int luaL_loadfile (lua_State *L, const char *filename);|
@apii{0,1,m}

Equivalent to @Lid{luaL_loadfilex} with @id{mode} equal to @id{NULL}.

}

@APIEntry{int luaL_loadfilex (lua_State *L, const char *filename,
                                            const char *mode);|
@apii{0,1,m}

Loads a file as a Lua chunk.
This function uses @Lid{lua_load} to load the chunk in the file
named @id{filename}.
If @id{filename} is @id{NULL},
then it loads from the standard input.
The first line in the file is ignored if it starts with a @T{#}.

The string @id{mode} works as in function @Lid{lua_load}.

This function returns the same results as @Lid{lua_load},
but it has an extra error code @defid{LUA_ERRFILE}
for file-related errors
(e.g., it cannot open or read the file).

As @Lid{lua_load}, this function only loads the chunk;
it does not run it.

}

@APIEntry{int luaL_loadstring (lua_State *L, const char *s);|
@apii{0,1,-}

Loads a string as a Lua chunk.
This function uses @Lid{lua_load} to load the chunk in
the zero-terminated string @id{s}.

This function returns the same results as @Lid{lua_load}.

Also as @Lid{lua_load}, this function only loads the chunk;
it does not run it.

}


@APIEntry{void luaL_newlib (lua_State *L, const luaL_Reg l[]);|
@apii{0,1,m}

Creates a new table and registers there
the functions in list @id{l}.

It is implemented as the following macro:
@verbatim{
(luaL_newlibtable(L,l), luaL_setfuncs(L,l,0))
}
The array @id{l} must be the actual array,
not a pointer to it.

}

@APIEntry{void luaL_newlibtable (lua_State *L, const luaL_Reg l[]);|
@apii{0,1,m}

Creates a new table with a size optimized
to store all entries in the array @id{l}
(but does not actually store them).
It is intended to be used in conjunction with @Lid{luaL_setfuncs}
@seeF{luaL_newlib}.

It is implemented as a macro.
The array @id{l} must be the actual array,
not a pointer to it.

}

@APIEntry{int luaL_newmetatable (lua_State *L, const char *tname);|
@apii{0,1,m}

If the registry already has the key @id{tname},
returns 0.
Otherwise,
creates a new table to be used as a metatable for userdata,
adds to this new table the pair @T{__name = tname},
adds to the registry the pair @T{[tname] = new table},
and returns 1.
(The entry @idx{__name} is used by some error-reporting functions.)

In both cases pushes onto the stack the final value associated
with @id{tname} in the registry.

}

@APIEntry{lua_State *luaL_newstate (void);|
@apii{0,0,-}

Creates a new Lua state.
It calls @Lid{lua_newstate} with an
allocator based on the @N{standard C} @id{realloc} function
and then sets a panic function @see{C-error} that prints
an error message to the standard error output in case of fatal
errors.

Returns the new state,
or @id{NULL} if there is a @x{memory allocation error}.

}

@APIEntry{void luaL_openlibs (lua_State *L);|
@apii{0,0,e}

Opens all standard Lua libraries into the given state.

}

@APIEntry{
T luaL_opt (L, func, arg, dflt);|
@apii{0,0,e}

This macro is defined as follows:
@verbatim{
(lua_isnoneornil(L,(arg)) ? (dflt) : func(L,(arg)))
}
In words, if the argument @id{arg} is nil or absent,
the macro results in the default @id{dflt}.
Otherwise, it results in the result of calling @id{func}
with the state @id{L} and the argument index @id{arg} as
parameters.
Note that it evaluates the expression @id{dflt} only if needed.

}

@APIEntry{
lua_Integer luaL_optinteger (lua_State *L,
                             int arg,
                             lua_Integer d);|
@apii{0,0,v}

If the function argument @id{arg} is an integer
(or convertible to an integer),
returns this integer.
If this argument is absent or is @nil,
returns @id{d}.
Otherwise, raises an error.

}

@APIEntry{
const char *luaL_optlstring (lua_State *L,
                             int arg,
                             const char *d,
                             size_t *l);|
@apii{0,0,v}

If the function argument @id{arg} is a string,
returns this string.
If this argument is absent or is @nil,
returns @id{d}.
Otherwise, raises an error.

If @id{l} is not @id{NULL},
fills the position @T{*l} with the result's length.
If the result is @id{NULL}
(only possible when returning @id{d} and @T{d == NULL}),
its length is considered zero.

This function uses @Lid{lua_tolstring} to get its result,
so all conversions and caveats of that function apply here.

}

@APIEntry{lua_Number luaL_optnumber (lua_State *L, int arg, lua_Number d);|
@apii{0,0,v}

If the function argument @id{arg} is a number,
returns this number.
If this argument is absent or is @nil,
returns @id{d}.
Otherwise, raises an error.

}

@APIEntry{
const char *luaL_optstring (lua_State *L,
                            int arg,
                            const char *d);|
@apii{0,0,v}

If the function argument @id{arg} is a string,
returns this string.
If this argument is absent or is @nil,
returns @id{d}.
Otherwise, raises an error.

}

@APIEntry{char *luaL_prepbuffer (luaL_Buffer *B);|
@apii{?,?,m}

Equivalent to @Lid{luaL_prepbuffsize}
with the predefined size @defid{LUAL_BUFFERSIZE}.

}

@APIEntry{char *luaL_prepbuffsize (luaL_Buffer *B, size_t sz);|
@apii{?,?,m}

Returns an address to a space of size @id{sz}
where you can copy a string to be added to buffer @id{B}
@seeC{luaL_Buffer}.
After copying the string into this space you must call
@Lid{luaL_addsize} with the size of the string to actually add
it to the buffer.

}

@APIEntry{void luaL_pushresult (luaL_Buffer *B);|
@apii{?,1,m}

Finishes the use of buffer @id{B} leaving the final string on
the top of the stack.

}

@APIEntry{void luaL_pushresultsize (luaL_Buffer *B, size_t sz);|
@apii{?,1,m}

Equivalent to the sequence @Lid{luaL_addsize}, @Lid{luaL_pushresult}.

}

@APIEntry{int luaL_ref (lua_State *L, int t);|
@apii{1,0,m}

Creates and returns a @def{reference},
in the table at index @id{t},
for the object at the top of the stack (and pops the object).

A reference is a unique integer key.
As long as you do not manually add integer keys into table @id{t},
@Lid{luaL_ref} ensures the uniqueness of the key it returns.
You can retrieve an object referred by reference @id{r}
by calling @T{lua_rawgeti(L, t, r)}.
Function @Lid{luaL_unref} frees a reference and its associated object.

If the object at the top of the stack is @nil,
@Lid{luaL_ref} returns the constant @defid{LUA_REFNIL}.
The constant @defid{LUA_NOREF} is guaranteed to be different
from any reference returned by @Lid{luaL_ref}.

}

@APIEntry{
typedef struct luaL_Reg {
  const char *name;
  lua_CFunction func;
} luaL_Reg;
|

Type for arrays of functions to be registered by
@Lid{luaL_setfuncs}.
@id{name} is the function name and @id{func} is a pointer to
the function.
Any array of @Lid{luaL_Reg} must end with a sentinel entry
in which both @id{name} and @id{func} are @id{NULL}.

}

@APIEntry{
void luaL_requiref (lua_State *L, const char *modname,
                    lua_CFunction openf, int glb);|
@apii{0,1,e}

If @id{modname} is not already present in @Lid{package.loaded},
calls function @id{openf} with string @id{modname} as an argument
and sets the call result in @T{package.loaded[modname]},
as if that function has been called through @Lid{require}.

If @id{glb} is true,
also stores the module into global @id{modname}.

Leaves a copy of the module on the stack.

}

@APIEntry{void luaL_setfuncs (lua_State *L, const luaL_Reg *l, int nup);|
@apii{nup,0,m}

Registers all functions in the array @id{l}
@seeC{luaL_Reg} into the table on the top of the stack
(below optional upvalues, see next).

When @id{nup} is not zero,
all functions are created sharing @id{nup} upvalues,
which must be previously pushed on the stack
on top of the library table.
These values are popped from the stack after the registration.

}

@APIEntry{void luaL_setmetatable (lua_State *L, const char *tname);|
@apii{0,0,-}

Sets the metatable of the object at the top of the stack
as the metatable associated with name @id{tname}
in the registry @seeC{luaL_newmetatable}.

}

@APIEntry{
typedef struct luaL_Stream {
  FILE *f;
  lua_CFunction closef;
} luaL_Stream;
|

The standard representation for @x{file handles},
which is used by the standard I/O library.

A file handle is implemented as a full userdata,
with a metatable called @id{LUA_FILEHANDLE}
(where @id{LUA_FILEHANDLE} is a macro with the actual metatable's name).
The metatable is created by the I/O library
@seeF{luaL_newmetatable}.

This userdata must start with the structure @id{luaL_Stream};
it can contain other data after this initial structure.
Field @id{f} points to the corresponding C stream
(or it can be @id{NULL} to indicate an incompletely created handle).
Field @id{closef} points to a Lua function
that will be called to close the stream
when the handle is closed or collected;
this function receives the file handle as its sole argument and
must return either @Rw{true} (in case of success)
or @nil plus an error message (in case of error).
Once Lua calls this field,
it changes the field value to @id{NULL}
to signal that the handle is closed.

}

@APIEntry{void *luaL_testudata (lua_State *L, int arg, const char *tname);|
@apii{0,0,m}

This function works like @Lid{luaL_checkudata},
except that, when the test fails,
it returns @id{NULL} instead of raising an error.

}

@APIEntry{const char *luaL_tolstring (lua_State *L, int idx, size_t *len);|
@apii{0,1,e}

Converts any Lua value at the given index to a @N{C string}
in a reasonable format.
The resulting string is pushed onto the stack and also
returned by the function.
If @id{len} is not @id{NULL},
the function also sets @T{*len} with the string length.

If the value has a metatable with a @idx{__tostring} field,
then @id{luaL_tolstring} calls the corresponding metamethod
with the value as argument,
and uses the result of the call as its result.

}

@APIEntry{
void luaL_traceback (lua_State *L, lua_State *L1, const char *msg,
                     int level);|
@apii{0,1,m}

Creates and pushes a traceback of the stack @id{L1}.
If @id{msg} is not @id{NULL} it is appended
at the beginning of the traceback.
The @id{level} parameter tells at which level
to start the traceback.

}

@APIEntry{const char *luaL_typename (lua_State *L, int index);|
@apii{0,0,-}

Returns the name of the type of the value at the given index.

}

@APIEntry{void luaL_unref (lua_State *L, int t, int ref);|
@apii{0,0,-}

Releases reference @id{ref} from the table at index @id{t}
@seeC{luaL_ref}.
The entry is removed from the table,
so that the referred object can be collected.
The reference @id{ref} is also freed to be used again.

If @id{ref} is @Lid{LUA_NOREF} or @Lid{LUA_REFNIL},
@Lid{luaL_unref} does nothing.

}

@APIEntry{void luaL_where (lua_State *L, int lvl);|
@apii{0,1,m}

Pushes onto the stack a string identifying the current position
of the control at level @id{lvl} in the call stack.
Typically this string has the following format:
@verbatim{
@rep{chunkname}:@rep{currentline}:
}
@N{Level 0} is the running function,
@N{level 1} is the function that called the running function,
etc.

This function is used to build a prefix for error messages.

}

}

}


@C{-------------------------------------------------------------------------}
@sect1{libraries| @title{Standard Libraries}

The standard Lua libraries provide useful functions
that are implemented directly through the @N{C API}.
Some of these functions provide essential services to the language
(e.g., @Lid{type} and @Lid{getmetatable});
others provide access to @Q{outside} services (e.g., I/O);
and others could be implemented in Lua itself,
but are quite useful or have critical performance requirements that
deserve an implementation in C (e.g., @Lid{table.sort}).

All libraries are implemented through the official @N{C API}
and are provided as separate @N{C modules}.
Currently, Lua has the following standard libraries:
@itemize{

@item{@link{predefined|basic library};}

@item{@link{corolib|coroutine library};}

@item{@link{packlib|package library};}

@item{@link{strlib|string manipulation};}

@item{@link{utf8|basic UTF-8 support};}

@item{@link{tablib|table manipulation};}

@item{@link{mathlib|mathematical functions} (sin, log, etc.);}

@item{@link{iolib|input and output};}

@item{@link{oslib|operating system facilities};}

@item{@link{debuglib|debug facilities}.}

}
Except for the basic and the package libraries,
each library provides all its functions as fields of a global table
or as methods of its objects.

To have access to these libraries,
the @N{C host} program should call the @Lid{luaL_openlibs} function,
which opens all standard libraries.
Alternatively,
the host program can open them individually by using
@Lid{luaL_requiref} to call
@defid{luaopen_base} (for the basic library),
@defid{luaopen_package} (for the package library),
@defid{luaopen_coroutine} (for the coroutine library),
@defid{luaopen_string} (for the string library),
@defid{luaopen_utf8} (for the UTF8 library),
@defid{luaopen_table} (for the table library),
@defid{luaopen_math} (for the mathematical library),
@defid{luaopen_io} (for the I/O library),
@defid{luaopen_os} (for the operating system library),
and @defid{luaopen_debug} (for the debug library).
These functions are declared in @defid{lualib.h}.

@sect2{predefined| @title{Basic Functions}

The basic library provides core functions to Lua.
If you do not include this library in your application,
you should check carefully whether you need to provide
implementations for some of its facilities.


@LibEntry{assert (v [, message])|

Calls @Lid{error} if
the value of its argument @id{v} is false (i.e., @nil or @false);
otherwise, returns all its arguments.
In case of error,
@id{message} is the error object;
when absent, it defaults to @St{assertion failed!}

}

@LibEntry{collectgarbage ([opt [, arg]])|

This function is a generic interface to the garbage collector.
It performs different functions according to its first argument, @id{opt}:
@description{

@item{@St{collect}|
performs a full garbage-collection cycle.
This is the default option.
}

@item{@St{stop}|
stops automatic execution of the garbage collector.
The collector will run only when explicitly invoked,
until a call to restart it.
}

@item{@St{restart}|
restarts automatic execution of the garbage collector.
}

@item{@St{count}|
returns the total memory in use by Lua in Kbytes.
The value has a fractional part,
so that it multiplied by 1024
gives the exact number of bytes in use by Lua
(except for overflows).
}

@item{@St{step}|
performs a garbage-collection step.
The step @Q{size} is controlled by @id{arg}.
With a zero value,
the collector will perform one basic (indivisible) step.
For non-zero values,
the collector will perform as if that amount of memory
(in KBytes) had been allocated by Lua.
Returns @Rw{true} if the step finished a collection cycle.
}

@item{@St{setpause}|
sets @id{arg} as the new value for the @emph{pause} of
the collector @see{GC}.
Returns the previous value for @emph{pause}.
}

@item{@St{setstepmul}|
sets @id{arg} as the new value for the @emph{step multiplier} of
the collector @see{GC}.
Returns the previous value for @emph{step}.
}

@item{@St{isrunning}|
returns a boolean that tells whether the collector is running
(i.e., not stopped).
}

}

}

@LibEntry{dofile ([filename])|
Opens the named file and executes its contents as a Lua chunk.
When called without arguments,
@id{dofile} executes the contents of the standard input (@id{stdin}).
Returns all values returned by the chunk.
In case of errors, @id{dofile} propagates the error
to its caller (that is, @id{dofile} does not run in protected mode).

}

@LibEntry{error (message [, level])|
Terminates the last protected function called
and returns @id{message} as the error object.
Function @id{error} never returns.

Usually, @id{error} adds some information about the error position
at the beginning of the message, if the message is a string.
The @id{level} argument specifies how to get the error position.
With @N{level 1} (the default), the error position is where the
@id{error} function was called.
@N{Level 2} points the error to where the function
that called @id{error} was called; and so on.
Passing a @N{level 0} avoids the addition of error position information
to the message.

}

@LibEntry{_G|
A global variable (not a function) that
holds the @x{global environment} @see{globalenv}.
Lua itself does not use this variable;
changing its value does not affect any environment,
nor vice versa.

}

@LibEntry{getmetatable (object)|

If @id{object} does not have a metatable, returns @nil.
Otherwise,
if the object's metatable has a @idx{__metatable} field,
returns the associated value.
Otherwise, returns the metatable of the given object.

}

@LibEntry{ipairs (t)|

Returns three values (an iterator function, the table @id{t}, and 0)
so that the construction
@verbatim{
for i,v in ipairs(t) do @rep{body} end
}
will iterate over the key@En{}value pairs
(@T{1,t[1]}), (@T{2,t[2]}), @ldots,
up to the first nil value.

}

@LibEntry{load (chunk [, chunkname [, mode [, env]]])|

Loads a chunk.

If @id{chunk} is a string, the chunk is this string.
If @id{chunk} is a function,
@id{load} calls it repeatedly to get the chunk pieces.
Each call to @id{chunk} must return a string that concatenates
with previous results.
A return of an empty string, @nil, or no value signals the end of the chunk.

If there are no syntactic errors,
returns the compiled chunk as a function;
otherwise, returns @nil plus the error message.

If the resulting function has upvalues,
the first upvalue is set to the value of @id{env},
if that parameter is given,
or to the value of the @x{global environment}.
Other upvalues are initialized with @nil.
(When you load a main chunk,
the resulting function will always have exactly one upvalue,
the @id{_ENV} variable @see{globalenv}.
However,
when you load a binary chunk created from a function @seeF{string.dump},
the resulting function can have an arbitrary number of upvalues.)
All upvalues are fresh, that is,
they are not shared with any other function.

@id{chunkname} is used as the name of the chunk for error messages
and debug information @see{debugI}.
When absent,
it defaults to @id{chunk}, if @id{chunk} is a string,
or to @St{=(load)} otherwise.

The string @id{mode} controls whether the chunk can be text or binary
(that is, a precompiled chunk).
It may be the string @St{b} (only @x{binary chunk}s),
@St{t} (only text chunks),
or @St{bt} (both binary and text).
The default is @St{bt}.

Lua does not check the consistency of binary chunks.
Maliciously crafted binary chunks can crash
the interpreter.

}

@LibEntry{loadfile ([filename [, mode [, env]]])|

Similar to @Lid{load},
but gets the chunk from file @id{filename}
or from the standard input,
if no file name is given.

}

@LibEntry{next (table [, index])|

Allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
@id{next} returns the next index of the table
and its associated value.
When called with @nil as its second argument,
@id{next} returns an initial index
and its associated value.
When called with the last index,
or with @nil in an empty table,
@id{next} returns @nil.
If the second argument is absent, then it is interpreted as @nil.
In particular,
you can use @T{next(t)} to check whether a table is empty.

The order in which the indices are enumerated is not specified,
@emph{even for numeric indices}.
(To traverse a table in numerical order,
use a numerical @Rw{for}.)

The behavior of @id{next} is undefined if,
during the traversal,
you assign any value to a non-existent field in the table.
You may however modify existing fields.
In particular, you may clear existing fields.

}

@LibEntry{pairs (t)|

If @id{t} has a metamethod @idx{__pairs},
calls it with @id{t} as argument and returns the first three
results from the call.

Otherwise,
returns three values: the @Lid{next} function, the table @id{t}, and @nil,
so that the construction
@verbatim{
for k,v in pairs(t) do @rep{body} end
}
will iterate over all key@En{}value pairs of table @id{t}.

See function @Lid{next} for the caveats of modifying
the table during its traversal.

}

@LibEntry{pcall (f [, arg1, @Cdots])|

Calls function @id{f} with
the given arguments in @def{protected mode}.
This means that any error @N{inside @T{f}} is not propagated;
instead, @id{pcall} catches the error
and returns a status code.
Its first result is the status code (a boolean),
which is true if the call succeeds without errors.
In such case, @id{pcall} also returns all results from the call,
after this first result.
In case of any error, @id{pcall} returns @false plus the error message.

}

@LibEntry{print (@Cdots)|
Receives any number of arguments
and prints their values to @id{stdout},
using the @Lid{tostring} function to convert each argument to a string.
@id{print} is not intended for formatted output,
but only as a quick way to show a value,
for instance for debugging.
For complete control over the output,
use @Lid{string.format} and @Lid{io.write}.

}

@LibEntry{rawequal (v1, v2)|
Checks whether @id{v1} is equal to @id{v2},
without invoking the @idx{__eq} metamethod.
Returns a boolean.

}

@LibEntry{rawget (table, index)|
Gets the real value of @T{table[index]},
without invoking the @idx{__index} metamethod.
@id{table} must be a table;
@id{index} may be any value.

}

@LibEntry{rawlen (v)|
Returns the length of the object @id{v},
which must be a table or a string,
without invoking the @idx{__len} metamethod.
Returns an integer.

}

@LibEntry{rawset (table, index, value)|
Sets the real value of @T{table[index]} to @id{value},
without invoking the @idx{__newindex} metamethod.
@id{table} must be a table,
@id{index} any value different from @nil and @x{NaN},
and @id{value} any Lua value.

This function returns @id{table}.

}

@LibEntry{select (index, @Cdots)|

If @id{index} is a number,
returns all arguments after argument number @id{index};
a negative number indexes from the end (@num{-1} is the last argument).
Otherwise, @id{index} must be the string @T{"#"},
and @id{select} returns the total number of extra arguments it received.

}

@LibEntry{setmetatable (table, metatable)|

Sets the metatable for the given table.
(To change the metatable of other types from Lua code,
you must use the @link{debuglib|debug library}.)
If @id{metatable} is @nil,
removes the metatable of the given table.
If the original metatable has a @idx{__metatable} field,
raises an error.

This function returns @id{table}.

}

@LibEntry{tonumber (e [, base])|

When called with no @id{base},
@id{tonumber} tries to convert its argument to a number.
If the argument is already a number or
a string convertible to a number,
then @id{tonumber} returns this number;
otherwise, it returns @nil.

The conversion of strings can result in integers or floats,
according to the lexical conventions of Lua @see{lexical}.
(The string may have leading and trailing spaces and a sign.)

When called with @id{base},
then @id{e} must be a string to be interpreted as
an integer numeral in that base.
The base may be any integer between 2 and 36, inclusive.
In bases @N{above 10}, the letter @Char{A} (in either upper or lower case)
@N{represents 10}, @Char{B} @N{represents 11}, and so forth,
with @Char{Z} representing 35.
If the string @id{e} is not a valid numeral in the given base,
the function returns @nil.

}

@LibEntry{tostring (v)|
Receives a value of any type and
converts it to a string in a human-readable format.
(For complete control of how numbers are converted,
use @Lid{string.format}.)

If the metatable of @id{v} has a @idx{__tostring} field,
then @id{tostring} calls the corresponding value
with @id{v} as argument,
and uses the result of the call as its result.

}

@LibEntry{type (v)|
Returns the type of its only argument, coded as a string.
The possible results of this function are
@St{nil} (a string, not the value @nil),
@St{number},
@St{string},
@St{boolean},
@St{table},
@St{function},
@St{thread},
and @St{userdata}.

}

@LibEntry{_VERSION|

A global variable (not a function) that
holds a string containing the running Lua version.
The current value of this variable is @St{Lua 5.3}.

}

@LibEntry{xpcall (f, msgh [, arg1, @Cdots])|

This function is similar to @Lid{pcall},
except that it sets a new @x{message handler} @id{msgh}.

}

}

@sect2{corolib| @title{Coroutine Manipulation}

This library comprises the operations to manipulate coroutines,
which come inside the table @defid{coroutine}.
See @See{coroutine} for a general description of coroutines.


@LibEntry{coroutine.create (f)|

Creates a new coroutine, with body @id{f}.
@id{f} must be a function.
Returns this new coroutine,
an object with type @T{"thread"}.

}

@LibEntry{coroutine.isyieldable ()|

Returns true when the running coroutine can yield.

A running coroutine is yieldable if it is not the main thread and
it is not inside a non-yieldable @N{C function}.

}

@LibEntry{coroutine.resume (co [, val1, @Cdots])|

Starts or continues the execution of coroutine @id{co}.
The first time you resume a coroutine,
it starts running its body.
The values @id{val1}, @ldots are passed
as the arguments to the body function.
If the coroutine has yielded,
@id{resume} restarts it;
the values @id{val1}, @ldots are passed
as the results from the yield.

If the coroutine runs without any errors,
@id{resume} returns @true plus any values passed to @id{yield}
(when the coroutine yields) or any values returned by the body function
(when the coroutine terminates).
If there is any error,
@id{resume} returns @false plus the error message.

}

@LibEntry{coroutine.running ()|

Returns the running coroutine plus a boolean,
true when the running coroutine is the main one.

}

@LibEntry{coroutine.status (co)|

Returns the status of coroutine @id{co}, as a string:
@T{"running"},
if the coroutine is running (that is, it called @id{status});
@T{"suspended"}, if the coroutine is suspended in a call to @id{yield},
or if it has not started running yet;
@T{"normal"} if the coroutine is active but not running
(that is, it has resumed another coroutine);
and @T{"dead"} if the coroutine has finished its body function,
or if it has stopped with an error.

}

@LibEntry{coroutine.wrap (f)|

Creates a new coroutine, with body @id{f}.
@id{f} must be a function.
Returns a function that resumes the coroutine each time it is called.
Any arguments passed to the function behave as the
extra arguments to @id{resume}.
Returns the same values returned by @id{resume},
except the first boolean.
In case of error, propagates the error.

}

@LibEntry{coroutine.yield (@Cdots)|

Suspends the execution of the calling coroutine.
Any arguments to @id{yield} are passed as extra results to @id{resume}.

}

}

@sect2{packlib| @title{Modules}

The package library provides basic
facilities for loading modules in Lua.
It exports one function directly in the global environment:
@Lid{require}.
Everything else is exported in a table @defid{package}.


@LibEntry{require (modname)|

Loads the given module.
The function starts by looking into the @Lid{package.loaded} table
to determine whether @id{modname} is already loaded.
If it is, then @id{require} returns the value stored
at @T{package.loaded[modname]}.
Otherwise, it tries to find a @emph{loader} for the module.

To find a loader,
@id{require} is guided by the @Lid{package.searchers} sequence.
By changing this sequence,
we can change how @id{require} looks for a module.
The following explanation is based on the default configuration
for @Lid{package.searchers}.

First @id{require} queries @T{package.preload[modname]}.
If it has a value,
this value (which must be a function) is the loader.
Otherwise @id{require} searches for a Lua loader using the
path stored in @Lid{package.path}.
If that also fails, it searches for a @N{C loader} using the
path stored in @Lid{package.cpath}.
If that also fails,
it tries an @emph{all-in-one} loader @seeF{package.searchers}.

Once a loader is found,
@id{require} calls the loader with two arguments:
@id{modname} and an extra value dependent on how it got the loader.
(If the loader came from a file,
this extra value is the file name.)
If the loader returns any non-nil value,
@id{require} assigns the returned value to @T{package.loaded[modname]}.
If the loader does not return a non-nil value and
has not assigned any value to @T{package.loaded[modname]},
then @id{require} assigns @Rw{true} to this entry.
In any case, @id{require} returns the
final value of @T{package.loaded[modname]}.

If there is any error loading or running the module,
or if it cannot find any loader for the module,
then @id{require} raises an error.

}

@LibEntry{package.config|

A string describing some compile-time configurations for packages.
This string is a sequence of lines:
@itemize{

@item{The first line is the @x{directory separator} string.
Default is @Char{\} for @x{Windows} and @Char{/} for all other systems.}

@item{The second line is the character that separates templates in a path.
Default is @Char{;}.}

@item{The third line is the string that marks the
substitution points in a template.
Default is @Char{?}.}

@item{The fourth line is a string that, in a path in @x{Windows},
is replaced by the executable's directory.
Default is @Char{!}.}

@item{The fifth line is a mark to ignore all text after it
when building the @id{luaopen_} function name.
Default is @Char{-}.}

}

}

@LibEntry{package.cpath|

The path used by @Lid{require} to search for a @N{C loader}.

Lua initializes the @N{C path} @Lid{package.cpath} in the same way
it initializes the Lua path @Lid{package.path},
using the environment variable @defid{LUA_CPATH_5_3},
or the environment variable @defid{LUA_CPATH},
or a default path defined in @id{luaconf.h}.

}

@LibEntry{package.loaded|

A table used by @Lid{require} to control which
modules are already loaded.
When you require a module @id{modname} and
@T{package.loaded[modname]} is not false,
@Lid{require} simply returns the value stored there.

This variable is only a reference to the real table;
assignments to this variable do not change the
table used by @Lid{require}.

}

@LibEntry{package.loadlib (libname, funcname)|

Dynamically links the host program with the @N{C library} @id{libname}.

If @id{funcname} is @St{*},
then it only links with the library,
making the symbols exported by the library
available to other dynamically linked libraries.
Otherwise,
it looks for a function @id{funcname} inside the library
and returns this function as a @N{C function}.
So, @id{funcname} must follow the @Lid{lua_CFunction} prototype
@seeC{lua_CFunction}.

This is a low-level function.
It completely bypasses the package and module system.
Unlike @Lid{require},
it does not perform any path searching and
does not automatically adds extensions.
@id{libname} must be the complete file name of the @N{C library},
including if necessary a path and an extension.
@id{funcname} must be the exact name exported by the @N{C library}
(which may depend on the @N{C compiler} and linker used).

This function is not supported by @N{Standard C}.
As such, it is only available on some platforms
(Windows, Linux, Mac OS X, Solaris, BSD,
plus other Unix systems that support the @id{dlfcn} standard).

}

@LibEntry{package.path|

The path used by @Lid{require} to search for a Lua loader.

At start-up, Lua initializes this variable with
the value of the environment variable @defid{LUA_PATH_5_3} or
the environment variable @defid{LUA_PATH} or
with a default path defined in @id{luaconf.h},
if those environment variables are not defined.
Any @St{;;} in the value of the environment variable
is replaced by the default path.

}

@LibEntry{package.preload|

A table to store loaders for specific modules
@seeF{require}.

This variable is only a reference to the real table;
assignments to this variable do not change the
table used by @Lid{require}.

}

@LibEntry{package.searchers|

A table used by @Lid{require} to control how to load modules.

Each entry in this table is a @def{searcher function}.
When looking for a module,
@Lid{require} calls each of these searchers in ascending order,
with the module name (the argument given to @Lid{require}) as its
sole parameter.
The function can return another function (the module @def{loader})
plus an extra value that will be passed to that loader,
or a string explaining why it did not find that module
(or @nil if it has nothing to say).

Lua initializes this table with four searcher functions.

The first searcher simply looks for a loader in the
@Lid{package.preload} table.

The second searcher looks for a loader as a Lua library,
using the path stored at @Lid{package.path}.
The search is done as described in function @Lid{package.searchpath}.

The third searcher looks for a loader as a @N{C library},
using the path given by the variable @Lid{package.cpath}.
Again,
the search is done as described in function @Lid{package.searchpath}.
For instance,
if the @N{C path} is the string
@verbatim{
"./?.so;./?.dll;/usr/local/?/init.so"
}
the searcher for module @id{foo}
will try to open the files @T{./foo.so}, @T{./foo.dll},
and @T{/usr/local/foo/init.so}, in that order.
Once it finds a @N{C library},
this searcher first uses a dynamic link facility to link the
application with the library.
Then it tries to find a @N{C function} inside the library to
be used as the loader.
The name of this @N{C function} is the string @St{luaopen_}
concatenated with a copy of the module name where each dot
is replaced by an underscore.
Moreover, if the module name has a hyphen,
its suffix after (and including) the first hyphen is removed.
For instance, if the module name is @id{a.b.c-v2.1},
the function name will be @id{luaopen_a_b_c}.

The fourth searcher tries an @def{all-in-one loader}.
It searches the @N{C path} for a library for
the root name of the given module.
For instance, when requiring @id{a.b.c},
it will search for a @N{C library} for @id{a}.
If found, it looks into it for an open function for
the submodule;
in our example, that would be @id{luaopen_a_b_c}.
With this facility, a package can pack several @N{C submodules}
into one single library,
with each submodule keeping its original open function.

All searchers except the first one (preload) return as the extra value
the file name where the module was found,
as returned by @Lid{package.searchpath}.
The first searcher returns no extra value.

}

@LibEntry{package.searchpath (name, path [, sep [, rep]])|

Searches for the given @id{name} in the given @id{path}.

A path is a string containing a sequence of
@emph{templates} separated by semicolons.
For each template,
the function replaces each interrogation mark (if any)
in the template with a copy of @id{name}
wherein all occurrences of @id{sep}
(a dot, by default)
were replaced by @id{rep}
(the system's directory separator, by default),
and then tries to open the resulting file name.

For instance, if the path is the string
@verbatim{
"./?.lua;./?.lc;/usr/local/?/init.lua"
}
the search for the name @id{foo.a}
will try to open the files
@T{./foo/a.lua}, @T{./foo/a.lc}, and
@T{/usr/local/foo/a/init.lua}, in that order.

Returns the resulting name of the first file that it can
open in read mode (after closing the file),
or @nil plus an error message if none succeeds.
(This error message lists all file names it tried to open.)

}

}

@sect2{strlib| @title{String Manipulation}

This library provides generic functions for string manipulation,
such as finding and extracting substrings, and pattern matching.
When indexing a string in Lua, the first character is at @N{position 1}
(not @N{at 0}, as in C).
Indices are allowed to be negative and are interpreted as indexing backwards,
from the end of the string.
Thus, the last character is at position @num{-1}, and so on.

The string library provides all its functions inside the table
@defid{string}.
It also sets a @x{metatable for strings}
where the @idx{__index} field points to the @id{string} table.
Therefore, you can use the string functions in object-oriented style.
For instance, @T{string.byte(s,i)}
can be written as @T{s:byte(i)}.

The string library assumes one-byte character encodings.


@LibEntry{string.byte (s [, i [, j]])|
Returns the internal numeric codes of the characters @T{s[i]},
@T{s[i+1]}, @ldots, @T{s[j]}.
The default value for @id{i} @N{is 1};
the default value for @id{j} @N{is @id{i}}.
These indices are corrected
following the same rules of function @Lid{string.sub}.

Numeric codes are not necessarily portable across platforms.

}

@LibEntry{string.char (@Cdots)|
Receives zero or more integers.
Returns a string with length equal to the number of arguments,
in which each character has the internal numeric code equal
to its corresponding argument.

Numeric codes are not necessarily portable across platforms.

}

@LibEntry{string.dump (function [, strip])|

Returns a string containing a binary representation
(a @emph{binary chunk})
of the given function,
so that a later @Lid{load} on this string returns
a copy of the function (but with new upvalues).
If @id{strip} is a true value,
the binary representation may not include all debug information
about the function,
to save space.

Functions with upvalues have only their number of upvalues saved.
When (re)loaded,
those upvalues receive fresh instances containing @nil.
(You can use the debug library to serialize
and reload the upvalues of a function
in a way adequate to your needs.)

}

@LibEntry{string.find (s, pattern [, init [, plain]])|

Looks for the first match of
@id{pattern} @see{pm} in the string @id{s}.
If it finds a match, then @id{find} returns the indices @N{of @T{s}}
where this occurrence starts and ends;
otherwise, it returns @nil.
A third, optional numeric argument @id{init} specifies
where to start the search;
its default value @N{is 1} and can be negative.
A value of @true as a fourth, optional argument @id{plain}
turns off the pattern matching facilities,
so the function does a plain @Q{find substring} operation,
with no characters in @id{pattern} being considered magic.
Note that if @id{plain} is given, then @id{init} must be given as well.

If the pattern has captures,
then in a successful match
the captured values are also returned,
after the two indices.

}

@LibEntry{string.format (formatstring, @Cdots)|

Returns a formatted version of its variable number of arguments
following the description given in its first argument (which must be a string).
The format string follows the same rules as the @ANSI{sprintf}.
The only differences are that the options/modifiers
@T{*}, @id{h}, @id{L}, @id{l}, @id{n},
and @id{p} are not supported
and that there is an extra option, @id{q}.

The @id{q} option formats a string between double quotes,
using escape sequences when necessary to ensure that
it can safely be read back by the Lua interpreter.
For instance, the call
@verbatim{
string.format('%q', 'a string with "quotes" and \n new line')
}
may produce the string:
@verbatim{
"a string with \"quotes\" and \
 new line"
}

Options
@id{A}, @id{a}, @id{E}, @id{e}, @id{f},
@id{G}, and @id{g} all expect a number as argument.
Options @id{c}, @id{d},
@id{i}, @id{o}, @id{u}, @id{X}, and @id{x}
expect an integer.
When Lua is compiled with a C89 compiler,
options @id{A} and @id{a} (hexadecimal floats)
do not support any modifier (flags, width, length).

Option @id{s} expects a string;
if its argument is not a string,
it is converted to one following the same rules of @Lid{tostring}.
If the option has any modifier (flags, width, length),
the string argument should not contain @x{embedded zeros}.

}

@LibEntry{string.gmatch (s, pattern)|
Returns an iterator function that,
each time it is called,
returns the next captures from @id{pattern} @see{pm}
over the string @id{s}.
If @id{pattern} specifies no captures,
then the whole match is produced in each call.

As an example, the following loop
will iterate over all the words from string @id{s},
printing one per line:
@verbatim{
s = "hello world from Lua"
for w in string.gmatch(s, "%a+") do
  print(w)
end
}
The next example collects all pairs @T{key=value} from the
given string into a table:
@verbatim{
t = {}
s = "from=world, to=Lua"
for k, v in string.gmatch(s, "(%w+)=(%w+)") do
  t[k] = v
end
}

For this function, a caret @Char{^} at the start of a pattern does not
work as an anchor, as this would prevent the iteration.

}

@LibEntry{string.gsub (s, pattern, repl [, n])|
Returns a copy of @id{s}
in which all (or the first @id{n}, if given)
occurrences of the @id{pattern} @see{pm} have been
replaced by a replacement string specified by @id{repl},
which can be a string, a table, or a function.
@id{gsub} also returns, as its second value,
the total number of matches that occurred.
The name @id{gsub} comes from @emph{Global SUBstitution}.

If @id{repl} is a string, then its value is used for replacement.
The @N{character @T{%}} works as an escape character:
any sequence in @id{repl} of the form @T{%@rep{d}},
with @rep{d} between 1 and 9,
stands for the value of the @rep{d}-th captured substring.
The sequence @T{%0} stands for the whole match.
The sequence @T{%%} stands for a @N{single @T{%}}.

If @id{repl} is a table, then the table is queried for every match,
using the first capture as the key.

If @id{repl} is a function, then this function is called every time a
match occurs, with all captured substrings passed as arguments,
in order.

In any case,
if the pattern specifies no captures,
then it behaves as if the whole pattern was inside a capture.

If the value returned by the table query or by the function call
is a string or a number,
then it is used as the replacement string;
otherwise, if it is @Rw{false} or @nil,
then there is no replacement
(that is, the original match is kept in the string).

Here are some examples:
@verbatim{
x = string.gsub("hello world", "(%w+)", "%1 %1")
--> x="hello hello world world"

x = string.gsub("hello world", "%w+", "%0 %0", 1)
--> x="hello hello world"

x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
--> x="world hello Lua from"

x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
--> x="home = /home/roberto, user = roberto"

x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
      return load(s)()
    end)
--> x="4+5 = 9"

local t = {name="lua", version="5.3"}
x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
--> x="lua-5.3.tar.gz"
}

}

@LibEntry{string.len (s)|
Receives a string and returns its length.
The empty string @T{""} has length 0.
Embedded zeros are counted,
so @T{"a\000bc\000"} has length 5.

}

@LibEntry{string.lower (s)|
Receives a string and returns a copy of this string with all
uppercase letters changed to lowercase.
All other characters are left unchanged.
The definition of what an uppercase letter is depends on the current locale.

}

@LibEntry{string.match (s, pattern [, init])|
Looks for the first @emph{match} of
@id{pattern} @see{pm} in the string @id{s}.
If it finds one, then @id{match} returns
the captures from the pattern;
otherwise it returns @nil.
If @id{pattern} specifies no captures,
then the whole match is returned.
A third, optional numeric argument @id{init} specifies
where to start the search;
its default value @N{is 1} and can be negative.

}

@LibEntry{string.pack (fmt, v1, v2, @Cdots)|

Returns a binary string containing the values @id{v1}, @id{v2}, etc.
packed (that is, serialized in binary form)
according to the format string @id{fmt} @see{pack}.

}

@LibEntry{string.packsize (fmt)|

Returns the size of a string resulting from @Lid{string.pack}
with the given format.
The format string cannot have the variable-length options
@Char{s} or @Char{z} @see{pack}.

}

@LibEntry{string.rep (s, n [, sep])|
Returns a string that is the concatenation of @id{n} copies of
the string @id{s} separated by the string @id{sep}.
The default value for @id{sep} is the empty string
(that is, no separator).
Returns the empty string if @id{n} is not positive.

(Note that it is very easy to exhaust the memory of your machine
with a single call to this function.)

}

@LibEntry{string.reverse (s)|
Returns a string that is the string @id{s} reversed.

}

@LibEntry{string.sub (s, i [, j])|
Returns the substring of @id{s} that
starts at @id{i}  and continues until @id{j};
@id{i} and @id{j} can be negative.
If @id{j} is absent, then it is assumed to be equal to @num{-1}
(which is the same as the string length).
In particular,
the call @T{string.sub(s,1,j)} returns a prefix of @id{s}
with length @id{j},
and @T{string.sub(s, -i)} (for a positive @id{i})
returns a suffix of @id{s}
with length @id{i}.

If, after the translation of negative indices,
@id{i} is less than 1,
it is corrected to 1.
If @id{j} is greater than the string length,
it is corrected to that length.
If, after these corrections,
@id{i} is greater than @id{j},
the function returns the empty string.

}

@LibEntry{string.unpack (fmt, s [, pos])|

Returns the values packed in string @id{s} @seeF{string.pack}
according to the format string @id{fmt} @see{pack}.
An optional @id{pos} marks where
to start reading in @id{s} (default is 1).
After the read values,
this function also returns the index of the first unread byte in @id{s}.

}

@LibEntry{string.upper (s)|
Receives a string and returns a copy of this string with all
lowercase letters changed to uppercase.
All other characters are left unchanged.
The definition of what a lowercase letter is depends on the current locale.

}


@sect3{pm| @title{Patterns}

Patterns in Lua are described by regular strings,
which are interpreted as patterns by the pattern-matching functions
@Lid{string.find},
@Lid{string.gmatch},
@Lid{string.gsub},
and @Lid{string.match}.
This section describes the syntax and the meaning
(that is, what they match) of these strings.

@sect4{@title{Character Class:}
A @def{character class} is used to represent a set of characters.
The following combinations are allowed in describing a character class:
@description{

@item{@rep{x}|
(where @rep{x} is not one of the @emphx{magic characters}
@T{^$()%.[]*+-?})
represents the character @emph{x} itself.
}

@item{@T{.}| (a dot) represents all characters.}

@item{@T{%a}| represents all letters.}

@item{@T{%c}| represents all control characters.}

@item{@T{%d}| represents all digits.}

@item{@T{%g}| represents all printable characters except space.}

@item{@T{%l}| represents all lowercase letters.}

@item{@T{%p}| represents all punctuation characters.}

@item{@T{%s}| represents all space characters.}

@item{@T{%u}| represents all uppercase letters.}

@item{@T{%w}| represents all alphanumeric characters.}

@item{@T{%x}| represents all hexadecimal digits.}

@item{@T{%@rep{x}}| (where @rep{x} is any non-alphanumeric character)
represents the character @rep{x}.
This is the standard way to escape the magic characters.
Any non-alphanumeric character
(including all punctuation characters, even the non-magical)
can be preceded by a @Char{%}
when used to represent itself in a pattern.
}

@item{@T{[@rep{set}]}|
represents the class which is the union of all
characters in @rep{set}.
A range of characters can be specified by
separating the end characters of the range,
in ascending order, with a @Char{-}.
All classes @T{%}@emph{x} described above can also be used as
components in @rep{set}.
All other characters in @rep{set} represent themselves.
For example, @T{[%w_]} (or @T{[_%w]})
represents all alphanumeric characters plus the underscore,
@T{[0-7]} represents the octal digits,
and @T{[0-7%l%-]} represents the octal digits plus
the lowercase letters plus the @Char{-} character.

You can put a closing square bracket in a set
by positioning it as the first character in the set.
You can put an hyphen in a set
by positioning it as the first or the last character in the set.
(You can also use an escape for both cases.)

The interaction between ranges and classes is not defined.
Therefore, patterns like @T{[%a-z]} or @T{[a-%%]}
have no meaning.
}

@item{@T{[^@rep{set}]}|
represents the complement of @rep{set},
where @rep{set} is interpreted as above.
}

}
For all classes represented by single letters (@T{%a}, @T{%c}, etc.),
the corresponding uppercase letter represents the complement of the class.
For instance, @T{%S} represents all non-space characters.

The definitions of letter, space, and other character groups
depend on the current locale.
In particular, the class @T{[a-z]} may not be equivalent to @T{%l}.

}

@sect4{@title{Pattern Item:}
A @def{pattern item} can be
@itemize{

@item{
a single character class,
which matches any single character in the class;
}

@item{
a single character class followed by @Char{*},
which matches zero or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
}

@item{
a single character class followed by @Char{+},
which matches one or more repetitions of characters in the class.
These repetition items will always match the longest possible sequence;
}

@item{
a single character class followed by @Char{-},
which also matches zero or more repetitions of characters in the class.
Unlike @Char{*},
these repetition items will always match the shortest possible sequence;
}

@item{
a single character class followed by @Char{?},
which matches zero or one occurrence of a character in the class.
It always matches one occurrence if possible;
}

@item{
@T{%@rep{n}}, for @rep{n} between 1 and 9;
such item matches a substring equal to the @rep{n}-th captured string
(see below);
}

@item{
@T{%b@rep{xy}}, where @rep{x} and @rep{y} are two distinct characters;
such item matches strings that start @N{with @rep{x}}, end @N{with @rep{y}},
and where the @rep{x} and @rep{y} are @emph{balanced}.
This means that, if one reads the string from left to right,
counting @M{+1} for an @rep{x} and @M{-1} for a @rep{y},
the ending @rep{y} is the first @rep{y} where the count reaches 0.
For instance, the item @T{%b()} matches expressions with
balanced parentheses.
}

@item{
@T{%f[@rep{set}]}, a @def{frontier pattern};
such item matches an empty string at any position such that
the next character belongs to @rep{set}
and the previous character does not belong to @rep{set}.
The set @rep{set} is interpreted as previously described.
The beginning and the end of the subject are handled as if
they were the character @Char{\0}.
}

}

}

@sect4{@title{Pattern:}
A @def{pattern} is a sequence of pattern items.
A caret @Char{^} at the beginning of a pattern anchors the match at the
beginning of the subject string.
A @Char{$} at the end of a pattern anchors the match at the
end of the subject string.
At other positions,
@Char{^} and @Char{$} have no special meaning and represent themselves.

}

@sect4{@title{Captures:}
A pattern can contain sub-patterns enclosed in parentheses;
they describe @def{captures}.
When a match succeeds, the substrings of the subject string
that match captures are stored (@emph{captured}) for future use.
Captures are numbered according to their left parentheses.
For instance, in the pattern @T{"(a*(.)%w(%s*))"},
the part of the string matching @T{"a*(.)%w(%s*)"} is
stored as the first capture (and therefore has @N{number 1});
the character matching @St{.} is captured with @N{number 2},
and the part matching @St{%s*} has @N{number 3}.

As a special case, the empty capture @T{()} captures
the current string position (a number).
For instance, if we apply the pattern @T{"()aa()"} on the
string @T{"flaaap"}, there will be two captures: @N{3 and 5}.

}

}


@sect3{pack| @title{Format Strings for Pack and Unpack}

The first argument to @Lid{string.pack},
@Lid{string.packsize}, and @Lid{string.unpack}
is a format string,
which describes the layout of the structure being created or read.

A format string is a sequence of conversion options.
The conversion options are as follows:
@description{
@item{@T{<}|sets little endian}
@item{@T{>}|sets big endian}
@item{@T{=}|sets native endian}
@item{@T{![@rep{n}]}|sets maximum alignment to @id{n}
(default is native alignment)}
@item{@T{b}|a signed byte (@id{char})}
@item{@T{B}|an unsigned byte (@id{char})}
@item{@T{h}|a signed @id{short} (native size)}
@item{@T{H}|an unsigned @id{short} (native size)}
@item{@T{l}|a signed @id{long} (native size)}
@item{@T{L}|an unsigned @id{long} (native size)}
@item{@T{j}|a @id{lua_Integer}}
@item{@T{J}|a @id{lua_Unsigned}}
@item{@T{T}|a @id{size_t} (native size)}
@item{@T{i[@rep{n}]}|a signed @id{int} with @id{n} bytes
(default is native size)}
@item{@T{I[@rep{n}]}|an unsigned @id{int} with @id{n} bytes
(default is native size)}
@item{@T{f}|a @id{float} (native size)}
@item{@T{d}|a @id{double} (native size)}
@item{@T{n}|a @id{lua_Number}}
@item{@T{c@rep{n}}|a fixed-sized string with @id{n} bytes}
@item{@T{z}|a zero-terminated string}
@item{@T{s[@emph{n}]}|a string preceded by its length
coded as an unsigned integer with @id{n} bytes
(default is a @id{size_t})}
@item{@T{x}|one byte of padding}
@item{@T{X@rep{op}}|an empty item that aligns
according to option @id{op}
(which is otherwise ignored)}
@item{@Char{ }|(empty space) ignored}
}
(A @St{[@rep{n}]} means an optional integral numeral.)
Except for padding, spaces, and configurations
(options @St{xX <=>!}),
each option corresponds to an argument (in @Lid{string.pack})
or a result (in @Lid{string.unpack}).

For options @St{!@rep{n}}, @St{s@rep{n}}, @St{i@rep{n}}, and @St{I@rep{n}},
@id{n} can be any integer between 1 and 16.
All integral options check overflows;
@Lid{string.pack} checks whether the given value fits in the given size;
@Lid{string.unpack} checks whether the read value fits in a Lua integer.

Any format string starts as if prefixed by @St{!1=},
that is,
with maximum alignment of 1 (no alignment)
and native endianness.

Alignment works as follows:
For each option,
the format gets extra padding until the data starts
at an offset that is a multiple of the minimum between the
option size and the maximum alignment;
this minimum must be a power of 2.
Options @St{c} and @St{z} are not aligned;
option @St{s} follows the alignment of its starting integer.

All padding is filled with zeros by @Lid{string.pack}
(and ignored by @Lid{string.unpack}).

}

}

@sect2{utf8| @title{UTF-8 Support}

This library provides basic support for @x{UTF-8} encoding.
It provides all its functions inside the table @defid{utf8}.
This library does not provide any support for @x{Unicode} other
than the handling of the encoding.
Any operation that needs the meaning of a character,
such as character classification, is outside its scope.

Unless stated otherwise,
all functions that expect a byte position as a parameter
assume that the given position is either the start of a byte sequence
or one plus the length of the subject string.
As in the string library,
negative indices count from the end of the string.


@LibEntry{utf8.char (@Cdots)|
Receives zero or more integers,
converts each one to its corresponding UTF-8 byte sequence
and returns a string with the concatenation of all these sequences.

}

@LibEntry{utf8.charpattern|
The pattern (a string, not a function) @St{[\0-\x7F\xC2-\xF4][\x80-\xBF]*}
@see{pm},
which matches exactly one UTF-8 byte sequence,
assuming that the subject is a valid UTF-8 string.

}

@LibEntry{utf8.codes (s)|

Returns values so that the construction
@verbatim{
for p, c in utf8.codes(s) do @rep{body} end
}
will iterate over all characters in string @id{s},
with @id{p} being the position (in bytes) and @id{c} the code point
of each character.
It raises an error if it meets any invalid byte sequence.

}

@LibEntry{utf8.codepoint (s [, i [, j]])|
Returns the codepoints (as integers) from all characters in @id{s}
that start between byte position @id{i} and @id{j} (both included).
The default for @id{i} is 1 and for @id{j} is @id{i}.
It raises an error if it meets any invalid byte sequence.

}

@LibEntry{utf8.len (s [, i [, j]])|
Returns the number of UTF-8 characters in string @id{s}
that start between positions @id{i} and @id{j} (both inclusive).
The default for @id{i} is @num{1} and for @id{j} is @num{-1}.
If it finds any invalid byte sequence,
returns a false value plus the position of the first invalid byte.

}

@LibEntry{utf8.offset (s, n [, i])|
Returns the position (in bytes) where the encoding of the
@id{n}-th character of @id{s}
(counting from position @id{i}) starts.
A negative @id{n} gets characters before position @id{i}.
The default for @id{i} is 1 when @id{n} is non-negative
and @T{#s + 1} otherwise,
so that @T{utf8.offset(s, -n)} gets the offset of the
@id{n}-th character from the end of the string.
If the specified character is neither in the subject
nor right after its end,
the function returns @nil.

As a special case,
when @id{n} is 0 the function returns the start of the encoding
of the character that contains the @id{i}-th byte of @id{s}.

This function assumes that @id{s} is a valid UTF-8 string.

}

}

@sect2{tablib| @title{Table Manipulation}

This library provides generic functions for table manipulation.
It provides all its functions inside the table @defid{table}.

Remember that, whenever an operation needs the length of a table,
all caveats about the length operator apply @see{len-op}.
All functions ignore non-numeric keys
in the tables given as arguments.


@LibEntry{table.concat (list [, sep [, i [, j]]])|

Given a list where all elements are strings or numbers,
returns the string @T{list[i]..sep..list[i+1] @Cdots sep..list[j]}.
The default value for @id{sep} is the empty string,
the default for @id{i} is 1,
and the default for @id{j} is @T{#list}.
If @id{i} is greater than @id{j}, returns the empty string.

}

@LibEntry{table.insert (list, [pos,] value)|

Inserts element @id{value} at position @id{pos} in @id{list},
shifting up the elements
@T{list[pos], list[pos+1], @Cdots, list[#list]}.
The default value for @id{pos} is @T{#list+1},
so that a call @T{table.insert(t,x)} inserts @id{x} at the end
of list @id{t}.

}

@LibEntry{table.move (a1, f, e, t [,a2])|

Moves elements from table @id{a1} to table @id{a2},
performing the equivalent to the following
multiple assignment:
@T{a2[t],@Cdots = a1[f],@Cdots,a1[e]}.
The default for @id{a2} is @id{a1}.
The destination range can overlap with the source range.
The number of elements to be moved must fit in a Lua integer.

Returns the destination table @id{a2}.

}

@LibEntry{table.pack (@Cdots)|

Returns a new table with all parameters stored into keys 1, 2, etc.
and with a field @St{n} with the total number of parameters.
Note that the resulting table may not be a sequence.

}

@LibEntry{table.remove (list [, pos])|

Removes from @id{list} the element at position @id{pos},
returning the value of the removed element.
When @id{pos} is an integer between 1 and @T{#list},
it shifts down the elements
@T{list[pos+1], list[pos+2], @Cdots, list[#list]}
and erases element @T{list[#list]};
The index @id{pos} can also be 0 when @T{#list} is 0,
or @T{#list + 1};
in those cases, the function erases the element @T{list[pos]}.

The default value for @id{pos} is @T{#list},
so that a call @T{table.remove(l)} removes the last element
of list @id{l}.

}

@LibEntry{table.sort (list [, comp])|

Sorts list elements in a given order, @emph{in-place},
from @T{list[1]} to @T{list[#list]}.
If @id{comp} is given,
then it must be a function that receives two list elements
and returns true when the first element must come
before the second in the final order
(so that, after the sort,
@T{i < j} implies @T{not comp(list[j],list[i])}).
If @id{comp} is not given,
then the standard Lua operator @T{<} is used instead.

Note that the @id{comp} function must define
a strict partial order over the elements in the list;
that is, it must be asymmetric and transitive.
Otherwise, no valid sort may be possible.

The sort algorithm is not stable:
elements considered equal by the given order
may have their relative positions changed by the sort.

}

@LibEntry{table.unpack (list [, i [, j]])|

Returns the elements from the given list.
This function is equivalent to
@verbatim{
return list[i], list[i+1], @Cdots, list[j]
}
By default, @id{i} @N{is 1} and @id{j} is @T{#list}.

}

}

@sect2{mathlib| @title{Mathematical Functions}

This library provides basic mathematical functions.
It provides all its functions and constants inside the table @defid{math}.
Functions with the annotation @St{integer/float} give
integer results for integer arguments
and float results for float (or mixed) arguments.
Rounding functions
(@Lid{math.ceil}, @Lid{math.floor}, and @Lid{math.modf})
return an integer when the result fits in the range of an integer,
or a float otherwise.

@LibEntry{math.abs (x)|

Returns the absolute value of @id{x}. (integer/float)

}

@LibEntry{math.acos (x)|

Returns the arc cosine of @id{x} (in radians).

}

@LibEntry{math.asin (x)|

Returns the arc sine of @id{x} (in radians).

}

@LibEntry{math.atan (y [, x])|

@index{atan2}
Returns the arc tangent of @T{y/x} (in radians),
but uses the signs of both parameters to find the
quadrant of the result.
(It also handles correctly the case of @id{x} being zero.)

The default value for @id{x} is 1,
so that the call @T{math.atan(y)}
returns the arc tangent of @id{y}.

}

@LibEntry{math.ceil (x)|

Returns the smallest integral value larger than or equal to @id{x}.

}

@LibEntry{math.cos (x)|

Returns the cosine of @id{x} (assumed to be in radians).

}

@LibEntry{math.deg (x)|

Converts the angle @id{x} from radians to degrees.

}

@LibEntry{math.exp (x)|

Returns the value @M{e@sp{x}}
(where @id{e} is the base of natural logarithms).

}

@LibEntry{math.floor (x)|

Returns the largest integral value smaller than or equal to @id{x}.

}

@LibEntry{math.fmod (x, y)|

Returns the remainder of the division of @id{x} by @id{y}
that rounds the quotient towards zero. (integer/float)

}

@LibEntry{math.huge|

The float value @idx{HUGE_VAL},
a value larger than any other numeric value.

}

@LibEntry{math.log (x [, base])|

Returns the logarithm of @id{x} in the given base.
The default for @id{base} is @M{e}
(so that the function returns the natural logarithm of @id{x}).

}

@LibEntry{math.max (x, @Cdots)|

Returns the argument with the maximum value,
according to the Lua operator @T{<}. (integer/float)

}

@LibEntry{math.maxinteger|
An integer with the maximum value for an integer.

}

@LibEntry{math.min (x, @Cdots)|

Returns the argument with the minimum value,
according to the Lua operator @T{<}. (integer/float)

}

@LibEntry{math.mininteger|
An integer with the minimum value for an integer.

}

@LibEntry{math.modf (x)|

Returns the integral part of @id{x} and the fractional part of @id{x}.
Its second result is always a float.

}

@LibEntry{math.pi|

The value of @M{@pi}.

}

@LibEntry{math.rad (x)|

Converts the angle @id{x} from degrees to radians.

}

@LibEntry{math.random ([m [, n]])|

When called without arguments,
returns a pseudo-random float with uniform distribution
in the range @C{(} @M{[0,1)}.  @C{]}
When called with two integers @id{m} and @id{n},
@id{math.random} returns a pseudo-random integer
with uniform distribution in the range @M{[m, n]}.
(The value @M{n-m} cannot be negative and must fit in a Lua integer.)
The call @T{math.random(n)} is equivalent to @T{math.random(1,n)}.

This function is an interface to the underling
pseudo-random generator function provided by C.

}

@LibEntry{math.randomseed (x)|

Sets @id{x} as the @Q{seed}
for the pseudo-random generator:
equal seeds produce equal sequences of numbers.

}

@LibEntry{math.sin (x)|

Returns the sine of @id{x} (assumed to be in radians).

}

@LibEntry{math.sqrt (x)|

Returns the square root of @id{x}.
(You can also use the expression @T{x^0.5} to compute this value.)

}

@LibEntry{math.tan (x)|

Returns the tangent of @id{x} (assumed to be in radians).

}

@LibEntry{math.tointeger (x)|

If the value @id{x} is convertible to an integer,
returns that integer.
Otherwise, returns @nil.

}

@LibEntry{math.type (x)|

Returns @St{integer} if @id{x} is an integer,
@St{float} if it is a float,
or @nil if @id{x} is not a number.

}

@LibEntry{math.ult (m, n)|

Returns a boolean,
true if and only if integer @id{m} is below integer @id{n} when
they are compared as @x{unsigned integers}.

}

}


@sect2{iolib| @title{Input and Output Facilities}

The I/O library provides two different styles for file manipulation.
The first one uses implicit file handles;
that is, there are operations to set a default input file and a
default output file,
and all input/output operations are over these default files.
The second style uses explicit file handles.

When using implicit file handles,
all operations are supplied by table @defid{io}.
When using explicit file handles,
the operation @Lid{io.open} returns a file handle
and then all operations are supplied as methods of the file handle.

The table @id{io} also provides
three predefined file handles with their usual meanings from C:
@defid{io.stdin}, @defid{io.stdout}, and @defid{io.stderr}.
The I/O library never closes these files.

Unless otherwise stated,
all I/O functions return @nil on failure
(plus an error message as a second result and
a system-dependent error code as a third result)
and some value different from @nil on success.
On non-POSIX systems,
the computation of the error message and error code
in case of errors
may be not @x{thread safe},
because they rely on the global C variable @id{errno}.

@LibEntry{io.close ([file])|

Equivalent to @T{file:close()}.
Without a @id{file}, closes the default output file.

}

@LibEntry{io.flush ()|

Equivalent to @T{io.output():flush()}.

}

@LibEntry{io.input ([file])|

When called with a file name, it opens the named file (in text mode),
and sets its handle as the default input file.
When called with a file handle,
it simply sets this file handle as the default input file.
When called without parameters,
it returns the current default input file.

In case of errors this function raises the error,
instead of returning an error code.

}

@LibEntry{io.lines ([filename, @Cdots])|

Opens the given file name in read mode
and returns an iterator function that
works like @T{file:lines(@Cdots)} over the opened file.
When the iterator function detects the end of file,
it returns no values (to finish the loop) and automatically closes the file.

The call @T{io.lines()} (with no file name) is equivalent
to @T{io.input():lines("*l")};
that is, it iterates over the lines of the default input file.
In this case it does not close the file when the loop ends.

In case of errors this function raises the error,
instead of returning an error code.

}

@LibEntry{io.open (filename [, mode])|

This function opens a file,
in the mode specified in the string @id{mode}.
In case of success,
it returns a new file handle.

The @id{mode} string can be any of the following:
@description{
@item{@St{r}| read mode (the default);}
@item{@St{w}| write mode;}
@item{@St{a}| append mode;}
@item{@St{r+}| update mode, all previous data is preserved;}
@item{@St{w+}| update mode, all previous data is erased;}
@item{@St{a+}| append update mode, previous data is preserved,
  writing is only allowed at the end of file.}
}
The @id{mode} string can also have a @Char{b} at the end,
which is needed in some systems to open the file in binary mode.

}

@LibEntry{io.output ([file])|

Similar to @Lid{io.input}, but operates over the default output file.

}

@LibEntry{io.popen (prog [, mode])|

This function is system dependent and is not available
on all platforms.

Starts program @id{prog} in a separated process and returns
a file handle that you can use to read data from this program
(if @id{mode} is @T{"r"}, the default)
or to write data to this program
(if @id{mode} is @T{"w"}).

}

@LibEntry{io.read (@Cdots)|

Equivalent to @T{io.input():read(@Cdots)}.

}

@LibEntry{io.tmpfile ()|

In case of success,
returns a handle for a temporary file.
This file is opened in update mode
and it is automatically removed when the program ends.

}

@LibEntry{io.type (obj)|

Checks whether @id{obj} is a valid file handle.
Returns the string @T{"file"} if @id{obj} is an open file handle,
@T{"closed file"} if @id{obj} is a closed file handle,
or @nil if @id{obj} is not a file handle.

}

@LibEntry{io.write (@Cdots)|

Equivalent to @T{io.output():write(@Cdots)}.


}

@LibEntry{file:close ()|

Closes @id{file}.
Note that files are automatically closed when
their handles are garbage collected,
but that takes an unpredictable amount of time to happen.

When closing a file handle created with @Lid{io.popen},
@Lid{file:close} returns the same values
returned by @Lid{os.execute}.

}

@LibEntry{file:flush ()|

Saves any written data to @id{file}.

}

@LibEntry{file:lines (@Cdots)|

Returns an iterator function that,
each time it is called,
reads the file according to the given formats.
When no format is given,
uses @St{l} as a default.
As an example, the construction
@verbatim{
for c in file:lines(1) do @rep{body} end
}
will iterate over all characters of the file,
starting at the current position.
Unlike @Lid{io.lines}, this function does not close the file
when the loop ends.

In case of errors this function raises the error,
instead of returning an error code.

}

@LibEntry{file:read (@Cdots)|

Reads the file @id{file},
according to the given formats, which specify what to read.
For each format,
the function returns a string or a number with the characters read,
or @nil if it cannot read data with the specified format.
(In this latter case,
the function does not read subsequent formats.)
When called without formats,
it uses a default format that reads the next line
(see below).

The available formats are
@description{

@item{@St{n}|
reads a numeral and returns it as a float or an integer,
following the lexical conventions of Lua.
(The numeral may have leading spaces and a sign.)
This format always reads the longest input sequence that
is a valid prefix for a numeral;
if that prefix does not form a valid numeral
(e.g., an empty string, @St{0x}, or @St{3.4e-}),
it is discarded and the function returns @nil.
}

@item{@St{a}|
reads the whole file, starting at the current position.
On end of file, it returns the empty string.
}

@item{@St{l}|
reads the next line skipping the end of line,
returning @nil on end of file.
This is the default format.
}

@item{@St{L}|
reads the next line keeping the end-of-line character (if present),
returning @nil on end of file.
}

@item{@emph{number}|
reads a string with up to this number of bytes,
returning @nil on end of file.
If @id{number} is zero,
it reads nothing and returns an empty string,
or @nil on end of file.
}

}
The formats @St{l} and @St{L} should be used only for text files.

}

@LibEntry{file:seek ([whence [, offset]])|

Sets and gets the file position,
measured from the beginning of the file,
to the position given by @id{offset} plus a base
specified by the string @id{whence}, as follows:
@description{
@item{@St{set}| base is position 0 (beginning of the file);}
@item{@St{cur}| base is current position;}
@item{@St{end}| base is end of file;}
}
In case of success, @id{seek} returns the final file position,
measured in bytes from the beginning of the file.
If @id{seek} fails, it returns @nil,
plus a string describing the error.

The default value for @id{whence} is @T{"cur"},
and for @id{offset} is 0.
Therefore, the call @T{file:seek()} returns the current
file position, without changing it;
the call @T{file:seek("set")} sets the position to the
beginning of the file (and returns 0);
and the call @T{file:seek("end")} sets the position to the
end of the file, and returns its size.

}

@LibEntry{file:setvbuf (mode [, size])|

Sets the buffering mode for an output file.
There are three available modes:
@description{

@item{@St{no}|
no buffering; the result of any output operation appears immediately.
}

@item{@St{full}|
full buffering; output operation is performed only
when the buffer is full or when
you explicitly @T{flush} the file @seeF{io.flush}.
}

@item{@St{line}|
line buffering; output is buffered until a newline is output
or there is any input from some special files
(such as a terminal device).
}

}
For the last two cases, @id{size}
specifies the size of the buffer, in bytes.
The default is an appropriate size.

}

@LibEntry{file:write (@Cdots)|

Writes the value of each of its arguments to @id{file}.
The arguments must be strings or numbers.

In case of success, this function returns @id{file}.
Otherwise it returns @nil plus a string describing the error.

}

}

@sect2{oslib| @title{Operating System Facilities}

This library is implemented through table @defid{os}.


@LibEntry{os.clock ()|

Returns an approximation of the amount in seconds of CPU time
used by the program.

}

@LibEntry{os.date ([format [, time]])|

Returns a string or a table containing date and time,
formatted according to the given string @id{format}.

If the @id{time} argument is present,
this is the time to be formatted
(see the @Lid{os.time} function for a description of this value).
Otherwise, @id{date} formats the current time.

If @id{format} starts with @Char{!},
then the date is formatted in Coordinated Universal Time.
After this optional character,
if @id{format} is the string @St{*t},
then @id{date} returns a table with the following fields:
@id{year}, @id{month} (1@En{}12), @id{day} (1@En{}31),
@id{hour} (0@En{}23), @id{min} (0@En{}59), @id{sec} (0@En{}61),
@id{wday} (weekday, 1@En{}7, Sunday @N{is 1}),
@id{yday} (day of the year, 1@En{}366),
and @id{isdst} (daylight saving flag, a boolean).
This last field may be absent
if the information is not available.

If @id{format} is not @St{*t},
then @id{date} returns the date as a string,
formatted according to the same rules as the @ANSI{strftime}.

When called without arguments,
@id{date} returns a reasonable date and time representation that depends on
the host system and on the current locale.
(More specifically, @T{os.date()} is equivalent to @T{os.date("%c")}.)

On non-POSIX systems,
this function may be not @x{thread safe}
because of its reliance on @CId{gmtime} and @CId{localtime}.

}

@LibEntry{os.difftime (t2, t1)|

Returns the difference, in seconds,
from time @id{t1} to time @id{t2}
(where the times are values returned by @Lid{os.time}).
In @x{POSIX}, @x{Windows}, and some other systems,
this value is exactly @id{t2}@M{-}@id{t1}.

}

@LibEntry{os.execute ([command])|

This function is equivalent to the @ANSI{system}.
It passes @id{command} to be executed by an operating system shell.
Its first result is @true
if the command terminated successfully,
or @nil otherwise.
After this first result
the function returns a string plus a number,
as follows:
@description{

@item{@St{exit}|
the command terminated normally;
the following number is the exit status of the command.
}

@item{@St{signal}|
the command was terminated by a signal;
the following number is the signal that terminated the command.
}

}

When called without a @id{command},
@id{os.execute} returns a boolean that is true if a shell is available.

}

@LibEntry{os.exit ([code [, close]])|

Calls the @ANSI{exit} to terminate the host program.
If @id{code} is @Rw{true},
the returned status is @idx{EXIT_SUCCESS};
if @id{code} is @Rw{false},
the returned status is @idx{EXIT_FAILURE};
if @id{code} is a number,
the returned status is this number.
The default value for @id{code} is @Rw{true}.

If the optional second argument @id{close} is true,
closes the Lua state before exiting.

}

@LibEntry{os.getenv (varname)|

Returns the value of the process environment variable @id{varname},
or @nil if the variable is not defined.

}

@LibEntry{os.remove (filename)|

Deletes the file (or empty directory, on @x{POSIX} systems)
with the given name.
If this function fails, it returns @nil,
plus a string describing the error and the error code.
Otherwise, it returns true.

}

@LibEntry{os.rename (oldname, newname)|

Renames the file or directory named @id{oldname} to @id{newname}.
If this function fails, it returns @nil,
plus a string describing the error and the error code.
Otherwise, it returns true.

}

@LibEntry{os.setlocale (locale [, category])|

Sets the current locale of the program.
@id{locale} is a system-dependent string specifying a locale;
@id{category} is an optional string describing which category to change:
@T{"all"}, @T{"collate"}, @T{"ctype"},
@T{"monetary"}, @T{"numeric"}, or @T{"time"};
the default category is @T{"all"}.
The function returns the name of the new locale,
or @nil if the request cannot be honored.

If @id{locale} is the empty string,
the current locale is set to an implementation-defined native locale.
If @id{locale} is the string @St{C},
the current locale is set to the standard C locale.

When called with @nil as the first argument,
this function only returns the name of the current locale
for the given category.

This function may be not @x{thread safe}
because of its reliance on @CId{setlocale}.

}

@LibEntry{os.time ([table])|

Returns the current time when called without arguments,
or a time representing the local date and time specified by the given table.
This table must have fields @id{year}, @id{month}, and @id{day},
and may have fields
@id{hour} (default is 12),
@id{min} (default is 0),
@id{sec} (default is 0),
and @id{isdst} (default is @nil).
Other fields are ignored.
For a description of these fields, see the @Lid{os.date} function.

The values in these fields do not need to be inside their valid ranges.
For instance, if @id{sec} is -10,
it means -10 seconds from the time specified by the other fields;
if @id{hour} is 1000,
it means +1000 hours from the time specified by the other fields.

The returned value is a number, whose meaning depends on your system.
In @x{POSIX}, @x{Windows}, and some other systems,
this number counts the number
of seconds since some given start time (the @Q{epoch}).
In other systems, the meaning is not specified,
and the number returned by @id{time} can be used only as an argument to
@Lid{os.date} and @Lid{os.difftime}.

}

@LibEntry{os.tmpname ()|

Returns a string with a file name that can
be used for a temporary file.
The file must be explicitly opened before its use
and explicitly removed when no longer needed.

On @x{POSIX} systems,
this function also creates a file with that name,
to avoid security risks.
(Someone else might create the file with wrong permissions
in the time between getting the name and creating the file.)
You still have to open the file to use it
and to remove it (even if you do not use it).

When possible,
you may prefer to use @Lid{io.tmpfile},
which automatically removes the file when the program ends.

}

}

@sect2{debuglib| @title{The Debug Library}

This library provides
the functionality of the @link{debugI|debug interface} to Lua programs.
You should exert care when using this library.
Several of its functions
violate basic assumptions about Lua code
(e.g., that variables local to a function
cannot be accessed from outside;
that userdata metatables cannot be changed by Lua code;
that Lua programs do not crash)
and therefore can compromise otherwise secure code.
Moreover, some functions in this library may be slow.

All functions in this library are provided
inside the @defid{debug} table.
All functions that operate over a thread
have an optional first argument which is the
thread to operate over.
The default is always the current thread.


@LibEntry{debug.debug ()|

Enters an interactive mode with the user,
running each string that the user enters.
Using simple commands and other debug facilities,
the user can inspect global and local variables,
change their values, evaluate expressions, and so on.
A line containing only the word @id{cont} finishes this function,
so that the caller continues its execution.

Note that commands for @id{debug.debug} are not lexically nested
within any function and so have no direct access to local variables.

}

@LibEntry{debug.gethook ([thread])|

Returns the current hook settings of the thread, as three values:
the current hook function, the current hook mask,
and the current hook count
(as set by the @Lid{debug.sethook} function).

}

@LibEntry{debug.getinfo ([thread,] f [, what])|

Returns a table with information about a function.
You can give the function directly
or you can give a number as the value of @id{f},
which means the function running at level @id{f} of the call stack
of the given thread:
@N{level 0} is the current function (@id{getinfo} itself);
@N{level 1} is the function that called @id{getinfo}
(except for tail calls, which do not count on the stack);
and so on.
If @id{f} is a number larger than the number of active functions,
then @id{getinfo} returns @nil.

The returned table can contain all the fields returned by @Lid{lua_getinfo},
with the string @id{what} describing which fields to fill in.
The default for @id{what} is to get all information available,
except the table of valid lines.
If present,
the option @Char{f}
adds a field named @id{func} with the function itself.
If present,
the option @Char{L}
adds a field named @id{activelines} with the table of
valid lines.

For instance, the expression @T{debug.getinfo(1,"n").name} returns
a name for the current function,
if a reasonable name can be found,
and the expression @T{debug.getinfo(print)}
returns a table with all available information
about the @Lid{print} function.

}

@LibEntry{debug.getlocal ([thread,] f, local)|

This function returns the name and the value of the local variable
with index @id{local} of the function at level @id{f} of the stack.
This function accesses not only explicit local variables,
but also parameters, temporaries, etc.

The first parameter or local variable has @N{index 1}, and so on,
following the order that they are declared in the code,
counting only the variables that are active
in the current scope of the function.
Negative indices refer to vararg parameters;
@num{-1} is the first vararg parameter.
The function returns @nil if there is no variable with the given index,
and raises an error when called with a level out of range.
(You can call @Lid{debug.getinfo} to check whether the level is valid.)

Variable names starting with @Char{(} (open parenthesis) @C{)}
represent variables with no known names
(internal variables such as loop control variables,
and variables from chunks saved without debug information).

The parameter @id{f} may also be a function.
In that case, @id{getlocal} returns only the name of function parameters.

}

@LibEntry{debug.getmetatable (value)|

Returns the metatable of the given @id{value}
or @nil if it does not have a metatable.

}

@LibEntry{debug.getregistry ()|

Returns the registry table @see{registry}.

}

@LibEntry{debug.getupvalue (f, up)|

This function returns the name and the value of the upvalue
with index @id{up} of the function @id{f}.
The function returns @nil if there is no upvalue with the given index.

Variable names starting with @Char{(} (open parenthesis) @C{)}
represent variables with no known names
(variables from chunks saved without debug information).

}

@LibEntry{debug.getuservalue (u)|

Returns the Lua value associated to @id{u}.
If @id{u} is not a full userdata,
returns @nil.

}

@LibEntry{debug.sethook ([thread,] hook, mask [, count])|

Sets the given function as a hook.
The string @id{mask} and the number @id{count} describe
when the hook will be called.
The string mask may have any combination of the following characters,
with the given meaning:
@description{
@item{@Char{c}| the hook is called every time Lua calls a function;}
@item{@Char{r}| the hook is called every time Lua returns from a function;}
@item{@Char{l}| the hook is called every time Lua enters a new line of code.}
}
Moreover,
with a @id{count} different from zero,
the hook is called also after every @id{count} instructions.

When called without arguments,
@Lid{debug.sethook} turns off the hook.

When the hook is called, its first parameter is a string
describing the event that has triggered its call:
@T{"call"} (or @T{"tail call"}),
@T{"return"},
@T{"line"}, and @T{"count"}.
For line events,
the hook also gets the new line number as its second parameter.
Inside a hook,
you can call @id{getinfo} with @N{level 2} to get more information about
the running function
(@N{level 0} is the @id{getinfo} function,
and @N{level 1} is the hook function).

}

@LibEntry{debug.setlocal ([thread,] level, local, value)|

This function assigns the value @id{value} to the local variable
with index @id{local} of the function at level @id{level} of the stack.
The function returns @nil if there is no local
variable with the given index,
and raises an error when called with a @id{level} out of range.
(You can call @id{getinfo} to check whether the level is valid.)
Otherwise, it returns the name of the local variable.

See @Lid{debug.getlocal} for more information about
variable indices and names.

}

@LibEntry{debug.setmetatable (value, table)|

Sets the metatable for the given @id{value} to the given @id{table}
(which can be @nil).
Returns @id{value}.

}

@LibEntry{debug.setupvalue (f, up, value)|

This function assigns the value @id{value} to the upvalue
with index @id{up} of the function @id{f}.
The function returns @nil if there is no upvalue
with the given index.
Otherwise, it returns the name of the upvalue.

}

@LibEntry{debug.setuservalue (udata, value)|

Sets the given @id{value} as
the Lua value associated to the given @id{udata}.
@id{udata} must be a full userdata.

Returns @id{udata}.

}

@LibEntry{debug.traceback ([thread,] [message [, level]])|

If @id{message} is present but is neither a string nor @nil,
this function returns @id{message} without further processing.
Otherwise,
it returns a string with a traceback of the call stack.
The optional @id{message} string is appended
at the beginning of the traceback.
An optional @id{level} number tells at which level
to start the traceback
(default is 1, the function calling @id{traceback}).

}

@LibEntry{debug.upvalueid (f, n)|

Returns a unique identifier (as a light userdata)
for the upvalue numbered @id{n}
from the given function.

These unique identifiers allow a program to check whether different
closures share upvalues.
Lua closures that share an upvalue
(that is, that access a same external local variable)
will return identical ids for those upvalue indices.

}

@LibEntry{debug.upvaluejoin (f1, n1, f2, n2)|

Make the @id{n1}-th upvalue of the Lua closure @id{f1}
refer to the @id{n2}-th upvalue of the Lua closure @id{f2}.

}

}

}


@C{-------------------------------------------------------------------------}
@sect1{lua-sa| @title{Lua Standalone}

Although Lua has been designed as an extension language,
to be embedded in a host @N{C program},
it is also frequently used as a standalone language.
An interpreter for Lua as a standalone language,
called simply @id{lua},
is provided with the standard distribution.
The @x{standalone interpreter} includes
all standard libraries, including the debug library.
Its usage is:
@verbatim{
lua [options] [script [args]]
}
The options are:
@description{
@item{@T{-e @rep{stat}}| executes string @rep{stat};}
@item{@T{-l @rep{mod}}| @Q{requires} @rep{mod};}
@item{@T{-i}| enters interactive mode after running @rep{script};}
@item{@T{-v}| prints version information;}
@item{@T{-E}| ignores environment variables;}
@item{@T{--}| stops handling options;}
@item{@T{-}| executes @id{stdin} as a file and stops handling options.}
}
After handling its options, @id{lua} runs the given @emph{script}.
When called without arguments,
@id{lua} behaves as @T{lua -v -i}
when the standard input (@id{stdin}) is a terminal,
and as @T{lua -} otherwise.

When called without option @T{-E},
the interpreter checks for an environment variable @defid{LUA_INIT_5_3}
(or @defid{LUA_INIT} if the versioned name is not defined)
before running any argument.
If the variable content has the format @T{@At@rep{filename}},
then @id{lua} executes the file.
Otherwise, @id{lua} executes the string itself.

When called with option @T{-E},
besides ignoring @id{LUA_INIT},
Lua also ignores
the values of @id{LUA_PATH} and @id{LUA_CPATH},
setting the values of
@Lid{package.path} and @Lid{package.cpath}
with the default paths defined in @id{luaconf.h}.

All options are handled in order, except @T{-i} and @T{-E}.
For instance, an invocation like
@verbatim{
$ lua -e'a=1' -e 'print(a)' script.lua
}
will first set @id{a} to 1, then print the value of @id{a},
and finally run the file @id{script.lua} with no arguments.
(Here @T{$} is the shell prompt. Your prompt may be different.)

Before running any code,
@id{lua} collects all command-line arguments
in a global table called @id{arg}.
The script name goes to index 0,
the first argument after the script name goes to index 1,
and so on.
Any arguments before the script name
(that is, the interpreter name plus its options)
go to negative indices.
For instance, in the call
@verbatim{
$ lua -la b.lua t1 t2
}
the table is like this:
@verbatim{
arg = { [-2] = "lua", [-1] = "-la",
        [0] = "b.lua",
        [1] = "t1", [2] = "t2" }
}
If there is no script in the call,
the interpreter name goes to index 0,
followed by the other arguments.
For instance, the call
@verbatim{
$ lua -e "print(arg[1])"
}
will print @St{-e}.
If there is a script,
the script is called with parameters
@T{arg[1]}, @Cdots, @T{arg[#arg]}.
(Like all chunks in Lua,
the script is compiled as a vararg function.)

In interactive mode,
Lua repeatedly prompts and waits for a line.
After reading a line,
Lua first try to interpret the line as an expression.
If it succeeds, it prints its value.
Otherwise, it interprets the line as a statement.
If you write an incomplete statement,
the interpreter waits for its completion
by issuing a different prompt.

If the global variable @defid{_PROMPT} contains a string,
then its value is used as the prompt.
Similarly, if the global variable @defid{_PROMPT2} contains a string,
its value is used as the secondary prompt
(issued during incomplete statements).

In case of unprotected errors in the script,
the interpreter reports the error to the standard error stream.
If the error object is not a string but
has a metamethod @idx{__tostring},
the interpreter calls this metamethod to produce the final message.
Otherwise, the interpreter converts the error object to a string
and adds a stack traceback to it.

When finishing normally,
the interpreter closes its main Lua state
@seeF{lua_close}.
The script can avoid this step by
calling @Lid{os.exit} to terminate.

To allow the use of Lua as a
script interpreter in Unix systems,
the standalone interpreter skips
the first line of a chunk if it starts with @T{#}.
Therefore, Lua scripts can be made into executable programs
by using @T{chmod +x} and @N{the @T{#!}} form,
as in
@verbatim{
#!/usr/local/bin/lua
}
(Of course,
the location of the Lua interpreter may be different in your machine.
If @id{lua} is in your @id{PATH},
then
@verbatim{
#!/usr/bin/env lua
}
is a more portable solution.)

}


@sect1{incompat| @title{Incompatibilities with the Previous Version}

Here we list the incompatibilities that you may find when moving a program
from @N{Lua 5.2} to @N{Lua 5.3}.
You can avoid some incompatibilities by compiling Lua with
appropriate options (see file @id{luaconf.h}).
However,
all these compatibility options will be removed in the future.

Lua versions can always change the C API in ways that
do not imply source-code changes in a program,
such as the numeric values for constants
or the implementation of functions as macros.
Therefore,
you should not assume that binaries are compatible between
different Lua versions.
Always recompile clients of the Lua API when
using a new version.

Similarly, Lua versions can always change the internal representation
of precompiled chunks;
precompiled chunks are not compatible between different Lua versions.

The standard paths in the official distribution may
change between versions.

@sect2{@title{Changes in the Language}
@itemize{

@item{
The main difference between @N{Lua 5.2} and @N{Lua 5.3} is the
introduction of an integer subtype for numbers.
Although this change should not affect @Q{normal} computations,
some computations
(mainly those that involve some kind of overflow)
can give different results.

You can fix these differences by forcing a number to be a float
(in @N{Lua 5.2} all numbers were float),
in particular writing constants with an ending @T{.0}
or using @T{x = x + 0.0} to convert a variable.
(This recommendation is only for a quick fix
for an occasional incompatibility;
it is not a general guideline for good programming.
For good programming,
use floats where you need floats
and integers where you need integers.)
}

@item{
The conversion of a float to a string now adds a @T{.0} suffix
to the result if it looks like an integer.
(For instance, the float 2.0 will be printed as @T{2.0},
not as @T{2}.)
You should always use an explicit format
when you need a specific format for numbers.

(Formally this is not an incompatibility,
because Lua does not specify how numbers are formatted as strings,
but some programs assumed a specific format.)
}

@item{
The generational mode for the garbage collector was removed.
(It was an experimental feature in @N{Lua 5.2}.)
}

}

}

@sect2{@title{Changes in the Libraries}
@itemize{

@item{
The @id{bit32} library has been deprecated.
It is easy to require a compatible external library or,
better yet, to replace its functions with appropriate bitwise operations.
(Keep in mind that @id{bit32} operates on 32-bit integers,
while the bitwise operators in @N{Lua 5.3} operate on Lua integers,
which by default have @N{64 bits}.)
}

@item{
The Table library now respects metamethods
for setting and getting elements.
}

@item{
The @Lid{ipairs} iterator now respects metamethods and
its @idx{__ipairs} metamethod has been deprecated.
}


@item{
Option names in @Lid{io.read} do not have a starting @Char{*} anymore.
For compatibility, Lua will continue to accept (and ignore) this character.
}

@item{
The following functions were deprecated in the mathematical library:
@id{atan2}, @id{cosh}, @id{sinh}, @id{tanh}, @id{pow},
@id{frexp}, and @id{ldexp}.
You can replace @T{math.pow(x,y)} with @T{x^y};
you can replace @id{math.atan2} with @id{math.atan},
which now accepts one or two parameters;
you can replace @T{math.ldexp(x,exp)} with @T{x * 2.0^exp}.
For the other operations,
you can either use an external library or
implement them in Lua.
}

@item{
The searcher for C loaders used by @Lid{require}
changed the way it handles versioned names.
Now, the version should come after the module name
(as is usual in most other tools).
For compatibility, that searcher still tries the old format
if it cannot find an open function according to the new style.
(@N{Lua 5.2} already worked that way,
but it did not document the change.)
}

@item{
The call @T{collectgarbage("count")} now returns only one result.
(You can compute that second result from the fractional part
of the first result.)
}

}

}

@sect2{@title{Changes in the API}

@itemize{

@item{
Continuation functions now receive as parameters what they needed
to get through @id{lua_getctx},
so @id{lua_getctx} has been removed.
Adapt your code accordingly.
}

@item{
Function @Lid{lua_dump} has an extra parameter, @id{strip}.
Use 0 as the value of this parameter to get the old behavior.
}

@item{
Functions to inject/project unsigned integers
(@id{lua_pushunsigned}, @id{lua_tounsigned}, @id{lua_tounsignedx},
@id{luaL_checkunsigned}, @id{luaL_optunsigned})
were deprecated.
Use their signed equivalents with a type cast.
}

@item{
Macros to project non-default integer types
(@id{luaL_checkint}, @id{luaL_optint}, @id{luaL_checklong}, @id{luaL_optlong})
were deprecated.
Use their equivalent over @Lid{lua_Integer} with a type cast
(or, when possible, use @Lid{lua_Integer} in your code).
}

}

}

}


@C{[===============================================================}

@sect1{BNF| @title{The Complete Syntax of Lua}

Here is the complete syntax of Lua in extended BNF.
As usual in extended BNF,
@bnfNter{{A}} means 0 or more @bnfNter{A}s,
and @bnfNter{[A]} means an optional @bnfNter{A}.
(For operator precedences, see @See{prec};
for a description of the terminals
@bnfNter{Name}, @bnfNter{Numeral},
and @bnfNter{LiteralString}, see @See{lexical}.)
@index{grammar}

@Produc{

@producname{chunk}@producbody{block}

@producname{block}@producbody{@bnfrep{stat} @bnfopt{retstat}}

@producname{stat}@producbody{
	@bnfter{;}
@OrNL	varlist @bnfter{=} explist
@OrNL	functioncall
@OrNL	label
@OrNL   @Rw{break}
@OrNL   @Rw{goto} Name
@OrNL	@Rw{do} block @Rw{end}
@OrNL	@Rw{while} exp @Rw{do} block @Rw{end}
@OrNL	@Rw{repeat} block @Rw{until} exp
@OrNL	@Rw{if} exp @Rw{then} block
	@bnfrep{@Rw{elseif} exp @Rw{then} block}
	@bnfopt{@Rw{else} block} @Rw{end}
@OrNL	@Rw{for} @bnfNter{Name} @bnfter{=} exp @bnfter{,} exp @bnfopt{@bnfter{,} exp}
	@Rw{do} block @Rw{end}
@OrNL   @Rw{for} namelist @Rw{in} explist @Rw{do} block @Rw{end}
@OrNL	@Rw{function} funcname funcbody
@OrNL	@Rw{local} @Rw{function} @bnfNter{Name} funcbody
@OrNL	@Rw{local} namelist @bnfopt{@bnfter{=} explist}
}

@producname{retstat}@producbody{@Rw{return}
                                   @bnfopt{explist} @bnfopt{@bnfter{;}}}

@producname{label}@producbody{@bnfter{::} Name @bnfter{::}}

@producname{funcname}@producbody{@bnfNter{Name} @bnfrep{@bnfter{.} @bnfNter{Name}}
                              @bnfopt{@bnfter{:} @bnfNter{Name}}}

@producname{varlist}@producbody{var @bnfrep{@bnfter{,} var}}

@producname{var}@producbody{
	@bnfNter{Name}
@Or	prefixexp @bnfter{[} exp @bnfter{]}
@Or	prefixexp @bnfter{.} @bnfNter{Name}
}

@producname{namelist}@producbody{@bnfNter{Name} @bnfrep{@bnfter{,} @bnfNter{Name}}}


@producname{explist}@producbody{exp @bnfrep{@bnfter{,} exp}}

@producname{exp}@producbody{
	@Rw{nil}
@Or	@Rw{false}
@Or	@Rw{true}
@Or	@bnfNter{Numeral}
@Or	@bnfNter{LiteralString}
@Or	@bnfter{...}
@Or	functiondef
@OrNL	prefixexp
@Or	tableconstructor
@Or	exp binop exp
@Or	unop exp
}

@producname{prefixexp}@producbody{var @Or functioncall @Or @bnfter{(} exp @bnfter{)}}

@producname{functioncall}@producbody{
	prefixexp args
@Or	prefixexp @bnfter{:} @bnfNter{Name} args
}

@producname{args}@producbody{
	@bnfter{(} @bnfopt{explist} @bnfter{)}
@Or	tableconstructor
@Or	@bnfNter{LiteralString}
}

@producname{functiondef}@producbody{@Rw{function} funcbody}

@producname{funcbody}@producbody{@bnfter{(} @bnfopt{parlist} @bnfter{)} block @Rw{end}}

@producname{parlist}@producbody{namelist @bnfopt{@bnfter{,} @bnfter{...}}
  @Or @bnfter{...}}

@producname{tableconstructor}@producbody{@bnfter{@Open} @bnfopt{fieldlist} @bnfter{@Close}}

@producname{fieldlist}@producbody{field @bnfrep{fieldsep field} @bnfopt{fieldsep}}

@producname{field}@producbody{@bnfter{[} exp @bnfter{]} @bnfter{=} exp @Or @bnfNter{Name} @bnfter{=} exp @Or exp}

@producname{fieldsep}@producbody{@bnfter{,} @Or @bnfter{;}}

@producname{binop}@producbody{
  @bnfter{+} @Or @bnfter{-} @Or @bnfter{*} @Or @bnfter{/} @Or @bnfter{//}
    @Or @bnfter{^} @Or @bnfter{%}
        @OrNL
  @bnfter{&} @Or @bnfter{~} @Or @bnfter{|} @Or @bnfter{>>} @Or @bnfter{<<}
    @Or @bnfter{..}
        @OrNL
  @bnfter{<} @Or @bnfter{<=} @Or @bnfter{>} @Or @bnfter{>=}
    @Or @bnfter{==} @Or @bnfter{~=}
        @OrNL
  @Rw{and} @Or @Rw{or}}

@producname{unop}@producbody{@bnfter{-} @Or @Rw{not} @Or @bnfter{#} @Or
  @bnfter{~}}

}

}

@C{]===============================================================}

}
@C{)]-------------------------------------------------------------------------}