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Mjson

C/C++ JSON parser, emitter, JSON-RPC engine for embedded systems

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Install / Use

/learn @cesanta/Mjson
About this skill

Quality Score

0/100

Supported Platforms

Universal

README

mjson - a JSON parser + emitter + JSON-RPC engine

Build Status License: MIT Code Coverage

Features

  • Small, ~1k lines of code, embedded-friendly
  • No dependencies
  • State machine parser, no allocations, no recursion
  • High level API - fetch from JSON directly into C/C++ by jsonpath
  • Low level SAX API
  • Flexible JSON generation API - print to buffer, file, socket, etc
  • JSON-RPC client/server. Connects any microcontroller online via https://vcon.io

Parsing example

const char *s = "{\"a\":1,\"b\":[2,false]}";  // {"a":1,"b":[2,false]}

double val;                                       // Get `a` attribute
if (mjson_get_number(s, strlen(s), "$.a", &val))  // into C variable `val`
  printf("a: %g\n", val);                         // a: 1

const char *buf;  // Get `b` sub-object
int len;          // into C variables `buf,len`
if (mjson_find(s, strlen(s), "$.b", &buf, &len))  // And print it
  printf("%.*s\n", len, buf);                     // [2,false]

int v;                                           // Extract `false`
if (mjson_get_bool(s, strlen(s), "$.b[1]", &v))  // into C variable `v`
  printf("boolean: %d\n", v);                    // boolean: 0

Printing example

// Print into a statically allocated buffer
char buf[100];
mjson_snprintf(buf, sizeof(buf), "{%Q:%d}", "a", 123);
printf("%s\n", buf);  // {"a":123}

// Print into a dynamically allocated string
char *s = mjson_aprintf("{%Q:%g}", "a", 3.1415);
printf("%s\n", s);  // {"a":3.1415}
free(s);            // Don't forget to free an allocated string

JSON-RPC example

In the following example, we initialize JSON-RPC context, and call a couple of JSON-RPC methods: a built-in rpc.list method which lists all registered methods, and our own foo method.

The sender() implementation just prints the reply to the standard output, but in real life it should send a reply to the real remote peer - UART, socket, or whatever else.

#include "mjson.h"

// A custom RPC handler. Many handlers can be registered.
static void foo(struct jsonrpc_request *r) {
  double x;
  mjson_get_number(r->params, r->params_len, "$[1]", &x);
  jsonrpc_return_success(r, "{%Q:%g,%Q:%Q}", "x", x, "ud", r->userdata);
}

// Sender function receives a reply frame and must forward it to the peer.
static int sender(char *frame, int frame_len, void *privdata) {
  printf("%.*s\n", frame_len, frame); // Print the JSON-RPC reply to stdout
  return frame_len;
}

int main(void) {
  jsonrpc_init(NULL, NULL);

  // Call rpc.list
  char request1[] = "{\"id\": 1, \"method\": \"rpc.list\"}";
  jsonrpc_process(request1, strlen(request1), sender, NULL, NULL);

  // Call non-existent method
  char request2[] = "{\"id\": 1, \"method\": \"foo\"}";
  jsonrpc_process(request2, strlen(request2), sender, NULL, NULL);

  // Register our own function
  char request3[] = "{\"id\": 2, \"method\": \"foo\",\"params\":[0,1.23]}";
  jsonrpc_export("foo", foo);
  jsonrpc_process(request3, strlen(request3), sender, NULL, (void *) "hi!");

  return 0;
}

Build options

  • -D MJSON_ENABLE_PRINT=0 disable emitting functionality, default: enabled
  • -D MJSON_MAX_DEPTH=30 define max object depth, default: 20
  • -D MJSON_ENABLE_BASE64=0 disable base64 parsing/printing, default: enabled
  • -D MJSON_ENABLE_RPC=0 disable RPC functionality, default: enabled
  • -D MJSON_DYNBUF_CHUNK=256 sets the allocation granularity of mjson_print_dynamic_buf
  • -D MJSON_ENABLE_PRETTY=0 disable mjson_pretty(), default: enabled
  • -D MJSON_ENABLE_MERGE=0 disable mjson_merge(), default: enabled
  • -D MJSON_ENABLE_NEXT=0 disable mjson_next(), default: enabled
  • -D MJSON_REALLOC=my_realloc redefine realloc() used by mjson_print_dynamic_buf(), default: realloc

Parsing API

mjson_find()

int mjson_find(const char *s, int len, const char *path, const char **tokptr, int *toklen);

In a JSON string s, len, find an element by its JSONPATH path. Save found element in tokptr, toklen. If not found, return JSON_TOK_INVALID. If found, return one of: MJSON_TOK_STRING, MJSON_TOK_NUMBER, MJSON_TOK_TRUE, MJSON_TOK_FALSE, MJSON_TOK_NULL, MJSON_TOK_ARRAY, MJSON_TOK_OBJECT. If a searched key contains ., [ or ] characters, they should be escaped by a backslash.

Example:

// s, len is a JSON string: {"foo": { "bar": [ 1, 2, 3] }, "b.az": true} 
char *p;
int n;
assert(mjson_find(s, len, "$.foo.bar[1]", &p, &n) == MJSON_TOK_NUMBER);
assert(mjson_find(s, len, "$.b\\.az", &p, &n) == MJSON_TOK_TRUE);
assert(mjson_find(s, len, "$", &p, &n) == MJSON_TOK_OBJECT);

mjson_get_number()

int mjson_get_number(const char *s, int len, const char *path, double *v);

In a JSON string s, len, find a number value by its JSONPATH path and store into v. Return 0 if the value was not found, non-0 if found and stored. Example:

// s, len is a JSON string: {"foo": { "bar": [ 1, 2, 3] }, "baz": true} 
double v = 0;
mjson_get_number(s, len, "$.foo.bar[1]", &v);  // v now holds 2

mjson_get_bool()

int mjson_get_bool(const char *s, int len, const char *path, int *v);

In a JSON string s, len, store value of a boolean by its JSONPATH path into a variable v. Return 0 if not found, non-0 otherwise. Example:

// s, len is a JSON string: {"foo": { "bar": [ 1, 2, 3] }, "baz": true} 
bool v = mjson_get_bool(s, len, "$.baz", false);   // Assigns to true

mjson_get_string()

int mjson_get_string(const char *s, int len, const char *path, char *to, int sz);

In a JSON string s, len, find a string by its JSONPATH path and unescape it into a buffer to, sz with terminating \0. If a string is not found, return -1. If a string is found, return the length of unescaped string. Example:

// s, len is a JSON string [ "abc", "de\r\n" ]
char buf[100];
int n = mjson_get_string(s, len, "$[1]", buf, sizeof(buf));  // Assigns to 4

mjson_get_hex()

int mjson_get_hex(const char *s, int len, const char *path, char *to, int sz);

In a JSON string s, len, find a string by its JSONPATH path and hex decode it into a buffer to, sz with terminating \0. If a string is not found, return -1. If a string is found, return the length of decoded string. The hex string should be lowercase, e.g. string Hello is hex-encoded as "48656c6c6f". Example:

// s, len is a JSON string [ "48656c6c6f" ]
char buf[100];
int n = mjson_get_hex(s, len, "$[0]", buf, sizeof(buf));  // Assigns to 5

mjson_get_base64()

int mjson_get_base64(const char *s, int len, const char *path, char *to, int sz);

In a JSON string s, len, find a string by its JSONPATH path and base64 decode it into a buffer to, sz with terminating \0. If a string is not found, return 0. If a string is found, return the length of decoded string. Example:

// s, len is a JSON string [ "MA==" ]
char buf[100];
int n = mjson_get_base64(s, len, "$[0]", buf, sizeof(buf));  // Assigns to 1

mjson()

int mjson(const char *s, int len, mjson_cb_t cb, void *cbdata);

Parse JSON string s, len, calling callback cb for each token. This is a low-level SAX API, intended for fancy stuff like pretty printing, etc.

mjson_next()

int mjson_next(const char *s, int n, int off, int *koff, int *klen, int *voff,
               int *vlen, int *vtype);

Assuming that JSON string s, n contains JSON object or JSON array, return the next key/value pair starting from offset off. key is returned as koff (key offset), klen (key length), value is returned as voff (value offset), vlen (value length), vtype (value type). Pointers could be NULL. Return next offset. When iterating over the array, koff will hold value index inside an array, and klen will be 0. Therefore, if klen holds 0, we're iterating over an array, otherwise over an object. Note: initial offset should be 0.

Usage example:

const char *s = "{\"a\":123,\"b\":[1,2,3,{\"c\":1}],\"d\":null}";
int koff, klen, voff, vlen, vtype, off;

for (off = 0; (off = mjson_next(s, strlen(s), off, &koff, &klen,
																&voff, &vlen, &vtype)) != 0; ) {
	printf("key: %.*s, value: %.*s\n", klen, s + koff, vlen, s + voff);
}

Emitting API

The emitting API is flexible and can print to anything: fixed buffer, dynamic growing buffer, FILE *, network socket, etc etc. The printer function gets the pointer to the buffer to print, and a user-specified data:

typedef int (*mjson_print_fn_t)(const char *buf, int len, void *userdata);

mjson library defines the following built-in printer functions:

struct mjson_fixedbuf {
  char *ptr;
  int size, len;
};
int mjson_print_fixed_buf(const char *ptr, int len, void *userdata);

int mjson_print_file(const char *ptr, int len, void *userdata);
int mjson_print_dynamic_buf(const char *ptr, int len, void *userdata);

If you want to print to something else, for example to a network socket, define your own printing function. If you want to see usage examples for the built-in printing functions, see unit_test.c file.

mjson_printf()

int mjson_vprintf(mjson_print_fn_t, void *, const char *fmt, va_list ap);
int mjson_printf(mjson_print_fn_t, void *, const char *fmt, ...);

Print using printf()-like format string. Supported specifiers are:

  • %Q print quoted escaped string. Expect NUL-terminated char *
  • %.*Q print quoted escaped string. Expect int, char *
  • %s print string as is. Expect NUL-terminated char *
  • %.*s print string as is. Expect int, char *
  • %g, print floating point number,

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cesanta/mjson

Languages

C

Security Score

100/100

Audited on Mar 24, 2026

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