Simple and easy-to-use JSON parser in C
Tags: c, programming, json, opensource
Published: 2023-09-12 12:25:40

This article introduces the JSON parser of the open source C library [Melon](https://github.com/Water-Melon/Melon).

I believe many readers have heard of or even used the cJSON. So this article 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 first take a look at the **cJSON** version:

```c
#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**:

```c
#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;
}
```



## 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:

```c
#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**:

```c
#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;
}
```



## 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

Melon's JSON component provides a functional interface that is easy to read and use, making it easier for developers to maintain projects.

Welcome to try the open source C library Melon.

Github：https://github.com/Water-Melon/Melon