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Added a simple custom allocator system #110

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39a3d91
Added json_allocator struct and function pointers
Dec 25, 2025
39a02dc
Define default allocator (malloc) and a create allocator helper function
Dec 25, 2025
0dd366d
Use the new allocator scheme for json_parse_ex()
Dec 25, 2025
6a543a8
Use the new allocator scheme for json_extract_value_ex()
Dec 25, 2025
bcdc102
Updated comment in json_parse_error_allocator_failed
Dec 25, 2025
ccabbbb
Added json_write_pretty_ex() using the new allocator scheme
Dec 25, 2025
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178 changes: 133 additions & 45 deletions json.h
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Original file line number Diff line number Diff line change
Expand Up @@ -70,6 +70,7 @@ extern "C" {

struct json_value_s;
struct json_parse_result_s;
struct json_allocator_s;

enum json_parse_flags_e {
json_parse_flags_default = 0,
Expand Down Expand Up @@ -145,56 +146,98 @@ enum json_parse_flags_e {
json_parse_flags_allow_multi_line_strings)
};

/* a Define for the allocate memory function pointer. */
#define JSON_ALLOC_FUNC(name) void *name(const size_t size, void *user_data)
/* a Function pointer for allocating a block of memory */
typedef JSON_ALLOC_FUNC(json_alloc_func_t);

/* a Define for the free memory function pointer. */
#define JSON_FREE_FUNC(name) void name(void *base, void *user_data)
/* a Function pointer for freeing a block of memory */
typedef JSON_FREE_FUNC(json_free_func_t);

/* Parse a JSON text file, returning a pointer to the root of the JSON
* structure. json_parse performs 1 call to malloc for the entire encoding.
* Returns 0 if an error occurred (malformed JSON input, or malloc failed). */
* structure. json_parse performs 1 call to the allocator for the entire encoding.
* Returns 0 if an error occurred (malformed JSON input, or allocation failed).
* Uses the default allocator (malloc). If needed, you can use json_parse_ex()
* to pass along your custom memory allocator. */
json_weak struct json_value_s *json_parse(const void *src, size_t src_size);

/* Parse a JSON text file, returning a pointer to the root of the JSON
* structure. json_parse performs 1 call to alloc_func_ptr for the entire
* encoding. Returns 0 if an error occurred (malformed JSON input, or malloc
* encoding. Returns 0 if an error occurred (malformed JSON input, or allocation
* failed). If an error occurred, the result struct (if not NULL) will explain
* the type of error, and the location in the input it occurred. If
* alloc_func_ptr is null then malloc is used. */
* the type of error, and the location in the input it occurred.
* If allocator is null, the default allocator (malloc) is used instead. */
json_weak 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_allocator_s *allocator,
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@sheredom sheredom Jan 4, 2026

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What gives me pause with this is that this can imply that we would call free in this function, which we definitely will not. I think I'd rather keep the old API and just explicitly add another argument for free_func_ptr on the write functions that need it.

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@f1nalspace f1nalspace Jan 7, 2026 •
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I added that which makes the most sense for me. Also i really hate such long prototypes defined directly in an argument. If you could at least make a typedef for that, then it would be much cleaner. But you can do whatever you want. I wrap your library anyway, which i do for all external libraries i use.

struct json_parse_result_s *result);

/* Extracts a value and all the data that makes it up into a newly created
* value. json_extract_value performs 1 call to malloc for the entire encoding.
*/
* value. json_extract_value performs 1 call to the allocator for the entire encoding.
* Uses the default allocator (malloc). If needed, you can use json_extract_value_ex()
* to pass along your custom memory allocator. */
json_weak struct json_value_s *
json_extract_value(const struct json_value_s *value);

/* Extracts a value and all the data that makes it up into a newly created
* value. json_extract_value performs 1 call to alloc_func_ptr for the entire
* encoding. If alloc_func_ptr is null then malloc is used. */
* value. json_extract_value performs 1 call to the allocator for the entire encoding.
* If allocator is null then the default allocator (malloc) is used. */
json_weak struct json_value_s *
json_extract_value_ex(const struct json_value_s *value,
void *(*alloc_func_ptr)(void *, size_t), void *user_data);
struct json_allocator_s *allocator);

/* Write out a minified JSON utf-8 string. This string is an encoding of the
* minimal string characters required to still encode the same data.
* json_write_minified performs 1 call to malloc for the entire encoding. Return
* 0 if an error occurred (malformed JSON input, or malloc failed). The out_size
* parameter is optional as the utf-8 string is null terminated. */
* json_write_minified_ex performs 1 call to the allocator for the entire encoding. Return
* 0 if an error occurred (malformed JSON input, or allocation failed). The out_size
* parameter is optional as the utf-8 string is null terminated.
* Uses the default allocator (malloc). If needed, you can use json_write_minified_ex()
* to pass along your custom memory allocator. */
json_weak void *json_write_minified(const struct json_value_s *value,
size_t *out_size);

/* Write out a minified JSON utf-8 string. This string is an encoding of the
* minimal string characters required to still encode the same data.
* json_write_minified_ex performs 1 call to the allocator for the entire encoding. Return
* 0 if an error occurred (malformed JSON input, or allocation failed). The out_size
* parameter is optional as the utf-8 string is null terminated.
* If allocator is null then the default allocator (malloc) is used. */
json_weak void *json_write_minified_ex(const struct json_value_s *value,
struct json_allocator_s *allocator,
size_t *out_size);

/* Write out a pretty JSON utf-8 string. This string is encoded such that the
* resultant JSON is pretty in that it is easily human readable. The indent and
* newline parameters allow a user to specify what kind of indentation and
* newline they want (two spaces / three spaces / tabs? \r, \n, \r\n ?). Both
* indent and newline can be NULL, indent defaults to two spaces (" "), and
* newline defaults to linux newlines ('\n' as the newline character).
* json_write_pretty performs 1 call to malloc for the entire encoding. Return 0
* if an error occurred (malformed JSON input, or malloc failed). The out_size
* parameter is optional as the utf-8 string is null terminated. */
* json_write_pretty_ex performs 1 call to the allocator for the entire encoding. Return 0
* if an error occurred (malformed JSON input, or allocation failed). The out_size
* parameter is optional as the utf-8 string is null terminated.
* Uses the default allocator (malloc). If needed, you can use json_write_pretty_ex()
* to pass along your custom memory allocator. */
json_weak void *json_write_pretty(const struct json_value_s *value,
const char *indent, const char *newline,
size_t *out_size);

/* Write out a pretty JSON utf-8 string. This string is encoded such that the
* resultant JSON is pretty in that it is easily human readable. The indent and
* newline parameters allow a user to specify what kind of indentation and
* newline they want (two spaces / three spaces / tabs? \r, \n, \r\n ?). Both
* indent and newline can be NULL, indent defaults to two spaces (" "), and
* newline defaults to linux newlines ('\n' as the newline character).
* json_write_pretty_ex performs 1 call to the allocator for the entire encoding. Return 0
* if an error occurred (malformed JSON input, or allocation failed). The out_size
* parameter is optional as the utf-8 string is null terminated.
* If allocator is null then the default allocator (malloc) is used. */
json_weak void *json_write_pretty_ex(const struct json_value_s *value,
const char *indent, const char *newline,
struct json_allocator_s *allocator,
size_t *out_size);

/* Reinterpret a JSON value as a string. Returns null is the value was not a
* string. */
json_weak struct json_string_s *
Expand Down Expand Up @@ -338,6 +381,18 @@ typedef struct json_value_ex_s {

} json_value_ex_t;

/* a JSON Allocator function table. */
typedef struct json_allocator_s {
/* Allocate function pointer. */
json_alloc_func_t *alloc;
/* Free function pointer. */
json_free_func_t *free;
/* Custom user data pointer. */
void *user_data;
/* Padding to align struct. */
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@sheredom sheredom Jan 4, 2026

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Why do we need this padding?

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@f1nalspace f1nalspace Jan 7, 2026

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Alignment padding is always a good idea.
Sure you can leave that out, but i always do this. Its a habit, i am a low-level programmer and i care about CPU cache alignment a lot.

uintptr_t padding;
} json_allocator_t;

/* a parsing error code. */
enum json_parse_error_e {
/* no error occurred (huzzah!). */
Expand Down Expand Up @@ -368,7 +423,7 @@ enum json_parse_error_e {
/* string was malformed! */
json_parse_error_invalid_string,

/* a call to malloc, or a user provider allocator, failed. */
/* a call to default allocator (malloc), or a user provider allocator, failed. */
json_parse_error_allocator_failed,

/* the JSON input had unexpected trailing characters that weren't part of the.
Expand Down Expand Up @@ -473,7 +528,28 @@ typedef struct json_parse_result_s {
#define JSON_MAX_RECURSION 1000
#endif

json_weak JSON_ALLOC_FUNC(json_default_alloc) {
if (0 == size) {
return json_null;
}
return malloc(size);
}

json_weak JSON_FREE_FUNC(json_default_free) {
if (json_null == base) {
return;
}
free(base);
}

static const struct json_allocator_s json_default_allocator = {
.alloc = json_default_alloc,
.free = json_default_free,
.user_data = json_null,
};

struct json_parse_state_s {
struct json_allocator_s allocator;
const char *src;
size_t size;
size_t offset;
Expand All @@ -489,6 +565,13 @@ struct json_parse_state_s {
size_t recursion;
};

json_weak struct json_allocator_s json_create_allocator(struct json_allocator_s *custom_allocator) {
if (json_null == custom_allocator || json_null == custom_allocator->alloc || json_null == custom_allocator->free) {
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@sheredom sheredom Jan 4, 2026

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Not sure about this - so if you mess up one of the allocators it just falls back to the default allocator?

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@f1nalspace f1nalspace Jan 7, 2026

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Correct, you don't want an allocator that is half-baked / mixed. Either you specify it or not.

return json_default_allocator;
}
return *custom_allocator;
}

json_weak int json_hexadecimal_digit(const char c);
int json_hexadecimal_digit(const char c) {
if ('0' <= c && c <= '9') {
Expand Down Expand Up @@ -2102,8 +2185,7 @@ void json_parse_value(struct json_parse_state_s *state, int is_global_object,

struct json_value_s *
json_parse_ex(const void *src, size_t src_size, size_t flags_bitset,
void *(*alloc_func_ptr)(void *user_data, size_t size),
void *user_data, struct json_parse_result_s *result) {
struct json_allocator_s *allocator, struct json_parse_result_s *result) {
struct json_parse_state_s state;
void *allocation;
struct json_value_s *value;
Expand Down Expand Up @@ -2132,6 +2214,7 @@ json_parse_ex(const void *src, size_t src_size, size_t flags_bitset,
state.data_size = 0;
state.flags_bitset = flags_bitset;
state.recursion = 0;
state.allocator = json_create_allocator(allocator);

input_error = json_get_value_size(
&state, (int)(json_parse_flags_allow_global_object & state.flags_bitset));
Expand Down Expand Up @@ -2164,14 +2247,10 @@ json_parse_ex(const void *src, size_t src_size, size_t flags_bitset,
/* the JSON values). */
total_size = state.dom_size + state.data_size;

if (json_null == alloc_func_ptr) {
allocation = malloc(total_size);
} else {
allocation = alloc_func_ptr(user_data, total_size);
}
allocation = state.allocator.alloc(total_size, state.allocator.user_data);

if (json_null == allocation) {
/* malloc failed! */
/* allocation failed! */
if (result) {
result->error = json_parse_error_allocator_failed;
result->error_offset = 0;
Expand Down Expand Up @@ -2214,8 +2293,7 @@ json_parse_ex(const void *src, size_t src_size, size_t flags_bitset,
}

struct json_value_s *json_parse(const void *src, size_t src_size) {
return json_parse_ex(src, src_size, json_parse_flags_default, json_null,
json_null, json_null);
return json_parse_ex(src, src_size, json_parse_flags_default, json_null, json_null);
}

struct json_extract_result_s {
Expand All @@ -2224,7 +2302,7 @@ struct json_extract_result_s {
};

struct json_value_s *json_extract_value(const struct json_value_s *value) {
return json_extract_value_ex(value, json_null, json_null);
return json_extract_value_ex(value, json_null);
}

json_weak struct json_extract_result_s
Expand Down Expand Up @@ -2336,6 +2414,7 @@ json_extract_get_value_size(const struct json_value_s *const value) {
}

struct json_extract_state_s {
struct json_allocator_s allocator;
char *dom;
char *data;
};
Expand Down Expand Up @@ -2445,9 +2524,7 @@ void json_extract_copy_value(struct json_extract_state_s *const state,
}

struct json_value_s *json_extract_value_ex(const struct json_value_s *value,
void *(*alloc_func_ptr)(void *,
size_t),
void *user_data) {
struct json_allocator_s *allocator) {
void *allocation;
struct json_extract_result_s result;
struct json_extract_state_s state;
Expand All @@ -2458,14 +2535,13 @@ struct json_value_s *json_extract_value_ex(const struct json_value_s *value,
return json_null;
}

state.allocator = json_create_allocator(allocator);

result = json_extract_get_value_size(value);
total_size = result.dom_size + result.data_size;

if (json_null == alloc_func_ptr) {
allocation = malloc(total_size);
} else {
allocation = alloc_func_ptr(user_data, total_size);
}

allocation = state.allocator.alloc(total_size, state.allocator.user_data);

state.dom = (char *)allocation;
state.data = state.dom + result.dom_size;
Expand Down Expand Up @@ -3094,6 +3170,10 @@ char *json_write_minified_value(const struct json_value_s *value, char *data) {
}

void *json_write_minified(const struct json_value_s *value, size_t *out_size) {
return json_write_minified_ex(value, json_null, out_size);
}

void *json_write_minified_ex(const struct json_value_s *value, struct json_allocator_s *allocator, size_t *out_size) {
size_t size = 0;
char *data = json_null;
char *data_end = json_null;
Expand All @@ -3102,25 +3182,27 @@ void *json_write_minified(const struct json_value_s *value, size_t *out_size) {
return json_null;
}

struct json_allocator_s active_allocator = json_create_allocator(allocator);

if (json_write_minified_get_value_size(value, &size)) {
/* value was malformed! */
return json_null;
}

size += 1; /* for the '\0' null terminating character. */

data = (char *)malloc(size);
data = (char *)active_allocator.alloc(size, active_allocator.user_data);

if (json_null == data) {
/* malloc failed! */
/* allocation failed! */
return json_null;
}

data_end = json_write_minified_value(value, data);

if (json_null == data_end) {
/* bad chi occurred! */
free(data);
active_allocator.free(data, active_allocator.user_data);
return json_null;
}

Expand Down Expand Up @@ -3435,8 +3517,12 @@ char *json_write_pretty_value(const struct json_value_s *value, size_t depth,
}
}

void *json_write_pretty(const struct json_value_s *value, const char *indent,
const char *newline, size_t *out_size) {
void *json_write_pretty(const struct json_value_s *value, const char *indent, const char *newline, size_t *out_size) {
return json_write_pretty_ex(value, indent, newline, json_null, out_size);
}

void *json_write_pretty_ex(const struct json_value_s *value, const char *indent,
const char *newline, struct json_allocator_s *allocator, size_t *out_size) {
size_t size = 0;
size_t indent_size = 0;
size_t newline_size = 0;
Expand All @@ -3447,6 +3533,8 @@ void *json_write_pretty(const struct json_value_s *value, const char *indent,
return json_null;
}

struct json_allocator_s active_allocator = json_create_allocator(allocator);

if (json_null == indent) {
indent = " "; /* default to two spaces. */
}
Expand All @@ -3471,18 +3559,18 @@ void *json_write_pretty(const struct json_value_s *value, const char *indent,

size += 1; /* for the '\0' null terminating character. */

data = (char *)malloc(size);
data = (char *)active_allocator.alloc(size, active_allocator.user_data);

if (json_null == data) {
/* malloc failed! */
/* allocation failed! */
return json_null;
}

data_end = json_write_pretty_value(value, 0, indent, newline, data);

if (json_null == data_end) {
/* bad chi occurred! */
free(data);
active_allocator.free(data, active_allocator.user_data);
return json_null;
}

Expand Down