Simple and Easy-To-Use JSON Parser in C

Explore the JSON parser of the open-source C library, Melon, and compare its features with the well-known cJSON. Understand the differences and similarities.

Niklaus Schen

Oct. 26, 23 · Presentation

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This article introduces the JSON parser of the open-source C library Melon.

I believe many readers have heard of or even used the cJSON. It is a very famous open-source project. In this article, we will compare cJSON and Melon’s JSON component.

Let’s take a look together below.

Encode

Suppose we want to build the following JSON:

    JSON
   
 

   {
    "name": "Awesome 4K",
    "resolutions": [
        {
            "width": 1280,
            "height": 720
        },
        {
            "width": 1920,
            "height": 1080
        },
        {
            "width": 3840,
            "height": 2160
        }
    ]
}
  

So, let’s take a look at the cJSON version at first:

    JSON
   
 

   #include <stdio.h>
#include <cjson/cJSON.h>

//NOTE: Returns a heap allocated string, you are required to free it after use.
char *create_monitor_with_helpers(void)
{
    const unsigned int resolution_numbers[3][2] = {
        {1280, 720},
        {1920, 1080},
        {3840, 2160}
    };
    char *string = NULL;
    cJSON *resolutions = NULL;
    size_t index = 0;

    cJSON *monitor = cJSON_CreateObject();

    if (cJSON_AddStringToObject(monitor, "name", "Awesome 4K") == NULL)
    {
        goto end;
    }

    resolutions = cJSON_AddArrayToObject(monitor, "resolutions");
    if (resolutions == NULL)
    {
        goto end;
    }

    for (index = 0; index < (sizeof(resolution_numbers) / (2 * sizeof(int))); ++index)
    {
        cJSON *resolution = cJSON_CreateObject();

        if (cJSON_AddNumberToObject(resolution, "width", resolution_numbers[index][0]) == NULL)
        {
            goto end;
        }

        if (cJSON_AddNumberToObject(resolution, "height", resolution_numbers[index][1]) == NULL)
        {
            goto end;
        }

        cJSON_AddItemToArray(resolutions, resolution);
    }

    string = cJSON_Print(monitor);
    if (string == NULL)
    {
        fprintf(stderr, "Failed to print monitor.\n");
    }

end:
    cJSON_Delete(monitor);
    return string;
}

int main(void)
{
    char *p;
    p = create_monitor_with_helpers();
    printf("%s\n", p);
    return 0;
}
  

Next, let’s take a look at the Melon’s:

    JSON
   
 

   #include <stdio.h>
#include "mln_json.h"
#include "mln_log.h"

static mln_string_t *generate(void)
{
    mln_json_t j;
    mln_string_t *ret;

    mln_json_init(&j);

    mln_json_generate(&j, "{s:s,s:[{s:d,s:d},{s:d,s:d},{s:d,s:d}]}", \
        "name", "Awesome 4K", "resolutions", "width", 1280, "height", 720, \
        "width", 1920, "height", 1080, "width", 3840, "height", 2160);
    ret = mln_json_encode(&j);

    mln_json_destroy(&j);

    return ret;
}

int main(void)
{
    mln_string_t *p;
    p = generate();
    mln_log(none, "%S\n", p);
    return 0;
}
  

The latter code is very short, and you can intuitively see the JSON format that will be generated.

Decode

We have the following JSON:

    JSON
   
 

   {
    "name": "Awesome 4K",
    "resolutions": [
        {
            "width": 1280,
            "height": 720
        }
    ]
}

 
  

Let's take a look at the decoding, cJSON first:

    JSON
   
 

   #include <stdio.h>
#include <cjson/cJSON.h>

int supports_full_hd(const char * const monitor)
{
    const cJSON *resolution = NULL;
    const cJSON *resolutions = NULL;
    cJSON *monitor_json = cJSON_Parse(monitor);
    if (monitor_json == NULL)
        return -1;

    resolutions = cJSON_GetObjectItemCaseSensitive(monitor_json, "resolutions");
    cJSON_ArrayForEach(resolution, resolutions)
    {
        cJSON *width = cJSON_GetObjectItemCaseSensitive(resolution, "width");
        return width->valuedouble;
    }

    cJSON_Delete(monitor_json);
    return -1;
}

int main(void)
{
    char p[] = "{\"name\":\"Awesome 4K\",\"resolutions\":[{\"width\":1280,\"height\":720}]}";
    int i = supports_full_hd(p);
    printf("%d\n", i);
    return 0;
}
  

Next is the Melon's:

    JSON
   
 

   #include <stdio.h>
#include "mln_json.h"
#include "mln_log.h"

static int handler(mln_json_t *j, void *data)
{
    return (int)mln_json_number_data_get(j);
}

static int parse(mln_string_t *p)
{
    mln_json_t j;
    mln_string_t exp = mln_string("resolutions.0.width");
    mln_json_decode(p, &j);
    return mln_json_parse(&j, &exp, handler, NULL);
}

int main(void)
{
    mln_string_t p = mln_string("{\"name\":\"Awesome 4K\",\"resolutions\":[{\"width\":1280,\"height\":720}]}");
    int i = parse(&p);
    mln_log(none, "%d\n", i);
    return 0;
}
  

This time, there is not much difference in the number of lines between the two pieces of code. But in the latter implementation, a red-black tree is used as the storage data structure of the object type. Therefore, when facing objects with many keys, the search efficiency will be very stable.

Write at the End

Melon's JSON component mainly provides the following four functions to facilitate users to encode and decode JSON:

mln_json_decode decodes JSON strings into JSON structure nodes
mln_json_parse obtains the corresponding JSON sub-node from the decoded JSON structure based on the given expression
mln_json_generate builds a JSON structure based on the given format information
mln_json_encode generates a JSON string based on the generated JSON structure

Based on these four functions, JSON can be easily encoded and decoded, making it easier for developers to use and code maintenance.

Thanks for reading.

JSON Open source Parser (programming language)

Published at DZone with permission of Niklaus Schen. See the original article here.

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