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🗄️ json.h

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A simple single header solution to parsing JSON in C and C++.

JSON is parsed into a read-only, single allocation buffer.

The current supported compilers are gcc, clang and msvc.

The current supported platforms are Windows, mac OS and Linux.

Usage

Just #include "json.h" in your code!

json_parse

Parse a json string into a DOM.

struct json_value_s *json_parse(
    const void *src,
    size_t src_size);
  • src - a utf-8 json string to parse.
  • src_size - the size of src in bytes.

Returns a struct json_value_s* pointing the root of the json DOM.

struct json_value_s

The main struct for interacting with a parsed JSON Document Object Model (DOM) is the struct json_value_s.

struct json_value_s {
  void *payload;
  size_t type;
};
  • payload - a pointer to the contents of the value.
  • type - the type of struct payload points to, one of json_type_e. Note: if type is json_type_true, json_type_false, or json_type_null, payload will be NULL.

json_parse_ex

Extended parse a json string into a DOM.

struct json_value_s *json_parse_ex(
    const void *src,
    size_t src_size,
    size_t flags_bitset,
    void*(*alloc_func_ptr)(void *, size_t),
    void *user_data,
    struct json_parse_result_s *result);
  • src - a utf-8 json string to parse.
  • src_size - the size of src in bytes.
  • flags_bitset - extra parsing flags, a bitset of flags specified in enum json_parse_flags_e.
  • alloc_func_ptr - a callback function to use for doing the single allocation. If NULL, malloc() is used.
  • user_data - user data to be passed as the first argument to alloc_func_ptr.
  • result - the result of the parsing. If a parsing error occurred this will contain what type of error, and where in the source it occurred. Can be NULL.

Returns a struct json_value_s* pointing the root of the json DOM.

enum json_parse_flags_e

The extra parsing flags that can be specified to json_parse_ex() are as follows:

enum json_parse_flags_e {
  json_parse_flags_default = 0,
  json_parse_flags_allow_trailing_comma = 0x1,
  json_parse_flags_allow_unquoted_keys = 0x2,
  json_parse_flags_allow_global_object = 0x4,
  json_parse_flags_allow_equals_in_object = 0x8,
  json_parse_flags_allow_no_commas = 0x10,
  json_parse_flags_allow_c_style_comments = 0x20,
  json_parse_flags_deprecated = 0x40,
  json_parse_flags_allow_location_information = 0x80,
  json_parse_flags_allow_single_quoted_strings = 0x100,
  json_parse_flags_allow_hexadecimal_numbers = 0x200,
  json_parse_flags_allow_leading_plus_sign = 0x400,
  json_parse_flags_allow_leading_or_trailing_decimal_point = 0x800,
  json_parse_flags_allow_inf_and_nan = 0x1000,
  json_parse_flags_allow_multi_line_strings = 0x2000,
  json_parse_flags_allow_simplified_json =
      (json_parse_flags_allow_trailing_comma |
       json_parse_flags_allow_unquoted_keys |
       json_parse_flags_allow_global_object |
       json_parse_flags_allow_equals_in_object |
       json_parse_flags_allow_no_commas),
  json_parse_flags_allow_json5 =
      (json_parse_flags_allow_trailing_comma |
       json_parse_flags_allow_unquoted_keys |
       json_parse_flags_allow_c_style_comments |
       json_parse_flags_allow_single_quoted_strings |
       json_parse_flags_allow_hexadecimal_numbers |
       json_parse_flags_allow_leading_plus_sign |
       json_parse_flags_allow_leading_or_trailing_decimal_point |
       json_parse_flags_allow_inf_and_nan |
       json_parse_flags_allow_multi_line_strings)
};
  • json_parse_flags_default - the default, no special behaviour is enabled.
  • json_parse_flags_allow_trailing_comma - allow trailing commas in objects and arrays. For example, both [true,] and {"a" : null,} would be allowed with this option on.
  • json_parse_flags_allow_unquoted_keys - allow unquoted keys for objects. For example, {a : null} would be allowed with this option on.
  • json_parse_flags_allow_global_object - allow a global unbracketed object. For example, a : null, b : true, c : {} would be allowed with this option on.
  • json_parse_flags_allow_equals_in_object - allow objects to use '=' as well as ':' between key/value pairs. For example, {"a" = null, "b" : true} would be allowed with this option on.
  • json_parse_flags_allow_no_commas - allow that objects don't have to have comma separators between key/value pairs. For example, {"a" : null "b" : true} would be allowed with this option on.
  • json_parse_flags_allow_c_style_comments - allow c-style comments (// or /* */) to be ignored in the input JSON file.
  • json_parse_flags_deprecated - a deprecated option.
  • json_parse_flags_allow_location_information - allow location information to be tracked for where values are in the input JSON. Useful for alerting users to errors with precise location information pertaining to the original source. When this option is enabled, all json_value_s*'s can be casted to json_value_ex_s*, and the json_string_s* of json_object_element_s*'s name member can be casted to json_string_ex_s* to retrieve specific locations on all the values and keys. Note this option will increase the memory budget required for the DOM used to record the JSON.
  • json_parse_flags_allow_single_quoted_strings - allows strings to be in 'single quotes'.
  • json_parse_flags_allow_hexadecimal_numbers - allows hexadecimal numbers to be used 0x42.
  • json_parse_flags_allow_leading_plus_sign - allows a leading '+' sign on numbers +42.
  • json_parse_flags_allow_leading_or_trailing_decimal_point - allows decimal points to be lead or trailed by 0 digits .42 or 42..
  • json_parse_flags_allow_inf_and_nan - allows using infinity and NaN identifiers Infinity or NaN.
  • json_parse_flags_allow_multi_line_strings - allows strings to span multiple lines.
  • json_parse_flags_allow_simplified_json - allow simplified JSON to be parsed. Simplified JSON is an enabling of a set of other parsing options. See the Bitsquid blog introducing this here.
  • json_parse_flags_allow_json5 - allow JSON5 to be parsed. JSON5 is an enabling of a set of other parsing options. See the website defining this extension here.

Examples

Parsing with json_parse

Lets say we had the JSON string '{"a" : true, "b" : [false, null, "foo"]}'. To get to each part of the parsed JSON we'd do:

const char json[] = "{\"a\" : true, \"b\" : [false, null, \"foo\"]}";
struct json_value_s* root = json_parse(json, strlen(json));
assert(root->type == json_type_object);

struct json_object_s* object = (struct json_object_s*)root->payload;
assert(object->length == 2);

struct json_object_element_s* a = object->start;

struct json_string_s* a_name = a->name;
assert(0 == strcmp(a_name->string, "a"));
assert(a_name->string_size == strlen("a"));

struct json_value_s* a_value = a->value;
assert(a_value->type == json_type_true);
assert(a_value->payload == NULL);

struct json_object_element_s* b = a->next;
assert(b->next == NULL);

struct json_string_s* b_name = b->name;
assert(0 == strcmp(b_name->string, "b"));
assert(b_name->string_size == strlen("b"));

struct json_value_s* b_value = b->value;
assert(b_value->type == json_type_array);

struct json_array_s* array = (struct json_array_s*)b_value->payload;
assert(array->length == 3);

struct json_array_element_s* b_1st = array->start;

struct json_value_s* b_1st_value = b_1st->value;
assert(b_1st_value->type == json_type_false);
assert(b_1st_value->payload == NULL);

struct json_array_element_s* b_2nd = b_1st->next;

struct json_value_s* b_2nd_value = b_2nd->value;
assert(b_2nd_value->type == json_type_null);
assert(b_2nd_value->payload == NULL);

struct json_array_element_s* b_3rd = b_2nd->next;
assert(b_3rd->next == NULL);

struct json_value_s* b_3rd_value = b_3rd->value;
assert(b_3rd_value->type == json_type_string);

struct json_string_s* string = (struct json_string_s*)b_3rd_value->payload;
assert(0 == strcmp(string->string, "foo"));
assert(string->string_size == strlen("foo"));

/* Don't forget to free the one allocation! */
free(root);

Iterator Helpers

There are some functions that serve no purpose other than to make it nicer to iterate through the produced JSON DOM:

  • json_value_as_string - returns a value as a string, or null if it wasn't a string.
  • json_value_as_number - returns a value as a number, or null if it wasn't a number.
  • json_value_as_object - returns a value as an object, or null if it wasn't an object.
  • json_value_as_array - returns a value as an array, or null if it wasn't an array.
  • json_value_is_true - returns non-zero is a value was true, zero otherwise.
  • json_value_is_false - returns non-zero is a value was false, zero otherwise.
  • json_value_is_null - returns non-zero is a value was null, zero otherwise.

Lets look at the same example from above but using these helper iterators instead:

const char json[] = "{\"a\" : true, \"b\" : [false, null, \"foo\"]}";
struct json_value_s* root = json_parse(json, strlen(json));

struct json_object_s* object = json_value_as_object(root);
assert(object != NULL);
assert(object->length == 2);

struct json_object_element_s* a = object->start;

struct json_string_s* a_name = a->name;
assert(0 == strcmp(a_name->string, "a"));
assert(a_name->string_size == strlen("a"));

struct json_value_s* a_value = a->value;
assert(json_value_is_true(a_value));

struct json_object_element_s* b = a->next;
assert(b->next == NULL);

struct json_string_s* b_name = b->name;
assert(0 == strcmp(b_name->string, "b"));
assert(b_name->string_size == strlen("b"));

struct json_array_s* array = json_value_as_array(b->value);
assert(array->length == 3);

struct json_array_element_s* b_1st = array->start;

struct json_value_s* b_1st_value = b_1st->value;
assert(json_value_is_false(b_1st_value));

struct json_array_element_s* b_2nd = b_1st->next;

struct json_value_s* b_2nd_value = b_2nd->value;
assert(json_value_is_null(b_2nd_value));

struct json_array_element_s* b_3rd = b_2nd->next;
assert(b_3rd->next == NULL);

struct json_string_s* string = json_value_as_string(b_3rd->value);
assert(string != NULL);
assert(0 == strcmp(string->string, "foo"));
assert(string->string_size == strlen("foo"));

/* Don't forget to free the one allocation! */
free(root);

As you can see it makes iterating through the DOM a little more pleasant.

Extracting a Value from a DOM

If you want to extract a value from a DOM into a new allocation then json_extract_value and json_extract_value_ex are you friends. These functions let you take any value and its subtree from a DOM and clone it into a new allocation - either a single malloc or a user-provided allocation region.

const char json[] = "{\"foo\" : { \"bar\" : [123, false, null, true], \"haz\" : \"haha\" }}";
struct json_value_s* root = json_parse(json, strlen(json));
assert(root);

struct json_value_s* foo = json_value_as_object(root)->start->value;
assert(foo);

struct json_value_s* extracted = json_extract_value(foo);

/* We can free root now because we've got a new allocation for extracted! */
free(root);

assert(json_value_as_object(extracted));

/* Don't forget to free the one allocation! */
free(extracted);

Design

The json_parse function calls malloc once, and then slices up this single allocation to support all the weird and wonderful JSON structures you can imagine!

The structure of the data is always the JSON structs first (which encode the structure of the original JSON), followed by the data.

Todo

License

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or distribute this software, either in source code form or as a compiled binary, for any purpose, commercial or non-commercial, and by any means.

In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain. We make this dedication for the benefit of the public at large and to the detriment of our heirs and successors. We intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to http://unlicense.org/