Jansson library

 void add_2array_to_json( json_t* obj, const char* name, const int*
marr, size_t dim1, size_t dim2 )
{
    size_t i, j;
    json_t* jarr1 = json_array();

    for( i=0; i<dim1; ++i ) {
        json_t* jarr2 = json_array();

        for( j=0; j<dim2; ++j ) {
            int val = marr[ i*dim2 + j ];
            json_t* jval = json_integer( val );
            json_array_append_new( jarr2, jval );
        }
        json_array_append_new( jarr1, jarr2 );
    }
    json_object_set_new( obj, name, jarr1 );
    return;
}
void SystemClock_Config(void);
static json_t* jdata;
int main()
{

    char* s;
    int arr1[2][3] = { {1,2,3}, {4,5,6} };
    int arr2[4][4] = { {1,2,3,4}, {5,6,7,8}, {9,10,11,12}, {13,14,15,16} };
    char ss[128];
  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* Configure the system clock */
  SystemClock_Config();

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_DMA_Init();
  MX_USART1_UART_Init();

    json_object_seed(1);
   jdata = json_object();
    add_2array_to_json( jdata, "arr1", &arr1[0][0], 2, 3 );
    add_2array_to_json( jdata, "arr2", &arr2[0][0], 4, 4 );

    s = json_dumps( jdata, 0 );

    HAL_UART_Transmit(&huart1,(uint8_t*)s,strlen(s),1000);
  //  puts( s );
    free( s );
    json_decref( jdata );

    while(1){
      HAL_GPIO_TogglePin(GPIOC,GPIO_PIN_13);
      HAL_Delay(200);
    };
}