jems

jems: a stream-based JSON serialized for embedded systems

About jems

jems is a compact, stream-based JSON serializer written in pure C for embedded systems.

jems makes it easy generate complex JSON structures, writing the results into a buffer, string or stream. Specifically designed for embedded systems, jems is:

• compact: one source file and one header file
• portable: written in pure C (with C++ compatible headers)
• deterministic: jems uses user-provided data structures and never calls malloc().
• yours to use: jems is covered under the permissive MIT License.

jems derives much of its efficiency and small footprint by a philosophy of trust: Rather than provide rigorous error checking of input parameters, jems instead assumes that you provide valid parameters to function calls.

A Short Example

jems has a "emit characters as you go" philosophy, which allows you to generate huge JSON structures with minimal memory usage. Here is a short example:

#include "jems.h"
#include <stdint.h>
#include <stdio.h>

#define MAX_LEVEL 10  // how deeply nested the JSON structures can get
static jems_level_t jems_levels[MAX_LEVEL];
static jems_t jems;

static void write_char(char ch, uintptr_t arg) {
  fputc(ch, (FILE *)arg);
}

int main(void) {
    // initalize the jems object, using fputc() as the method for writing chars.
    jems_init(&jems, jems_levels, MAX_LEVEL, write_char, (uintptr_t)stdout);

    jems_object_open(&jems);       // start an object.
    jems_string(&jems, "colors");  // first object key is "colors"
    jems_array_open(&jems);        // first object value is an array
    jems_integer(&jems, 1);        // ... with three numbers
    jems_integer(&jems, 2);
    jems_integer(&jems, 3);
    jems_array_close(&jems);      // end of the array
    jems_string(&jems, "valid");  // second object key is "valid"
    jems_true(&jems);             // second object value is true
    jems_object_close(&jems);     // end of the object
}

This program will print

    {"colors":[1,2,3],"valid":true}

on the standard output.