Small and complete JSON parser
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.

JSON Parser

This is a simple JSON parser, written in C11. It's suitable for embedded use. In particular, it's small and compact, yet fully featured and easy to use. It requires the caller to preallocate memory and returns an AST (basically a tree structure of JSON values). It also supports a minimum of error reporting.

The main focus is on correctness, small code size, low dependencies, and ease of use.

The relevant specifications are RFC 8259 and RFC 7493.

How to use

Add json.c and json.h to your build system. Call json_parse_destructive() or json_parse(). The arguments and semantics are documented in the header file.

The optional json_out.h/json_out.c files are trivial helpers to write JSON data.

The optional json_helpers.h/json_helpers.c provide trivial helpers to traverse the data structure the parser returns.

Hints for embedded use

  • It uses floats (double C type) and strtod(). If floats are not available on the target architecture, you can replace all occurences of "double" with "int" or "long", and "strtod" with "strtol" (or use your own string to integer conversion).
  • strtod() is very bloated on some embedded libcs. If you do not require correct float conversion, this simple strtod() function can be easily adapted:
  • It uses assert() in at least one place. Some embedded libcs will pull in stdio for it, so you may want to comment all of its uses.
  • In addition, it uses some simple string functions, which should not pose any trouble.
  • The only required C11 feature that is not available in C99 is the use of the _Alignof operator. If you do not have a C11 compiler, replace it with 4 or 8 (depending on "double" alignment requirements), or 1 if your target architecture supports fast unaligned accesses. Adapting to C89 will be harder.
  • json_out.c (not required by the parser) uses stdio's vsnprintf() to format numbers. A sufficiently non-bloated libc for embedded use is here: (This does not correctly format floats as of this writing.)

Design choices

The main parsing function, json_parse_destructive(), mutates the input text (i.e. it writes to the memory pointed to by the text argument). This is an attempt to save memory. Parsed string values do not need to be allocated from the provided memory region.

Arrays are represented as linked lists. This is done because it's hard to allocate a linear array. You would either have to parse the JSON in multiple passes to preallocate the array with the correct number of elements, or have to use a complete memory manager that efficiently supports realloc().

Test program

This is a simple test of the parser. It uses the Meson build system:


  • The test program under is probably not exhaustive enough. Ensure 100% code coverage and fuzz the parser. Also, verify 100% JSON correctness.
  • The json_helpers.h functions are very primitive. There need to be functions which can report errors in a human readable way. (Consider you use JSON for a network API. You want to provide good error information to API users if e.g. a JSON object key is missing or has the wrong type.)


ISC (permissive, similar to BSD and MIT)