json-build is a zero-allocation JSON serializer written in ANSI C. Its tokenizer counterpart can be found at jsmn-find.

• compatible with C89
• no dependencies
• no dynamic memory allocation

Download json-build.h, include it, done.

#include "json-build.h"

...
jsonb b;
char buf[1024];

jsonb_init(&b);
jsonb_object(&b, buf, sizeof(buf));
{
    jsonb_key(&b, buf, sizeof(buf), "foo", strlen("foo"));
    jsonb_array(&b, buf, sizeof(buf));
    {
        jsonb_number(&b, buf, sizeof(buf), 1);
        jsonb_string(&b, buf, sizeof(buf), "hi", 2);
        jsonb_bool(&b, buf, sizeof(buf), 0);
        jsonb_null(&b, buf, sizeof(buf));
        jsonb_array_pop(&b, buf, sizeof(buf));
    }
    jsonb_object_pop(&b, buf, sizeof(buf));
}
printf("JSON: %s", buf); // JSON: {"foo":[1,"hi",false,null]}

Since json-build is a single-header, header-only library, for more complex use cases you might need to define additional macros. #define JSONB_STATIChides all json-build API symbols by making them static. Also, if you want to include json-build.h for multiple C files, to avoid duplication of symbols you may define JSONB_HEADER macro.

/* In every .c file that uses json-build include only declarations: */
#define JSONB_HEADER
#include "json-build.h"

/* Additionally, create one json-build.c file for json-build implementation: */
#include "json-build.h"

• jsonb_init() - initialize a jsonb handle
• jsonb_reset() - reset the buffer's position tracker for streaming purposes
• jsonb_object() - push an object to the builder stack
• jsonb_object_pop() - pop an object from the builder stack
• jsonb_key() - push an object key field to the builder stack
• jsonb_array() - push an array to the builder stack
• jsonb_array_pop() - pop an array from the builder stack
• jsonb_token() - push a raw token to the builder stack
• jsonb_bool() - push a boolean token to the builder stack
• jsonb_null() - push a null token to the builder stack
• jsonb_string() - push a string token to the builder stack
• jsonb_number() - push a number token to the builder stack

The following are the possible return codes for the builder functions:

• JSONB_OK - operation was a success, user can proceed with the next operation
• JSONB_END - operation was a success, JSON is complete and expects no more operations
• JSONB_ERROR_NOMEM - buffer is not large enough
• JSONB_ERROR_INPUT - user action don't match expected next token
• JSONB_ERROR_STACK - user action would lead to out of boundaries access, increase JSONB_MAX_DEPTH!

Its worth mentioning that all JSONB_ERROR_ prefixed codes are negative.

If you get JSONB_ERROR_NOMEM you can either:

1. re-allocate a larger buffer and call the builder function once more
2. call jsonb_reset() to reset the buffer's position tracker and call the builder function once more (useful for streaming with a fixed sized buffer!)

This software is distributed under MIT license, so feel free to integrate it in your commercial products.