add cbor_value_ref_{text,byte}_string

add functions to directly obtain a pointer to the data
of a text or byte string, without the need to copy.

otherwise these are analog to the corresponding _copy_ functions.

src/cbor.h

@@ -425,6 +425,8 @@ CBOR_PRIVATE_API CborError _cbor_value_copy_string(const CborValue *value, void
                                                    size_t *buflen, CborValue *next);
 CBOR_PRIVATE_API CborError _cbor_value_dup_string(const CborValue *value, void **buffer,
                                                   size_t *buflen, CborValue *next);
+CBOR_PRIVATE_API CborError _cbor_value_ref_string(const CborValue *value, void **buffer,
+                                                  size_t *buflen, CborValue *next);
 
 CBOR_API CborError cbor_value_calculate_string_length(const CborValue *value, size_t *length);
 
@@ -454,6 +456,19 @@ CBOR_INLINE_API CborError cbor_value_dup_byte_string(const CborValue *value, uin
     return _cbor_value_dup_string(value, (void **)buffer, buflen, next);
 }
 
+CBOR_INLINE_API CborError cbor_value_ref_text_string(const CborValue *value, char **buffer,
+                                                     size_t *buflen, CborValue *next)
+{
+    assert(cbor_value_is_text_string(value));
+    return _cbor_value_ref_string(value, (void **)buffer, buflen, next);
+}
+CBOR_INLINE_API CborError cbor_value_ref_byte_string(const CborValue *value, uint8_t **buffer,
+                                                     size_t *buflen, CborValue *next)
+{
+    assert(cbor_value_is_byte_string(value));
+    return _cbor_value_ref_string(value, (void **)buffer, buflen, next);
+}
+
 CBOR_API CborError cbor_value_text_string_equals(const CborValue *value, const char *string, bool *result);
 
 /* Maps and arrays */

src/cborparser.c

@@ -1217,6 +1217,29 @@ CborError _cbor_value_copy_string(const CborValue *value, void *buffer,
                  copied_all ? CborNoError : CborErrorOutOfMemory;
 }
 
+CborError _cbor_value_ref_string(const CborValue *value, void **ref,
+                                 size_t *len, CborValue *next)
+{
+    CborError err;
+    CborValue tmp;
+
+    cbor_assert(cbor_value_is_length_known(value));
+    if (!next)
+        next = &tmp;
+    *next = *value;
+
+    err = extract_length(next->parser, &next->ptr, len);
+    if (err)
+        return err;
+    if (*len > (size_t)(next->parser->end - next->ptr))
+        return CborErrorUnexpectedEOF;
+
+    *ref = (void *)next->ptr;
+    next->ptr += *len;
+
+    return CborNoError;
+}
+
 /**
  * Compares the entry \a value with the string \a string and stores the result
  * in \a result. If the value is different from \a string \a result will