example code using libcurl and json-c to post and parse a return from http://jsonplaceholder.typicode.com

curltest.c

/**
 * example C code using libcurl and json-c
 * to post and return a payload using
 * http://jsonplaceholder.typicode.com
 *
 * License:
 *
 * This code is licensed under MIT license
 * https://opensource.org/licenses/MIT
 *
 * Requirements:
 *
 * json-c - https://github.com/json-c/json-c
 * libcurl - http://curl.haxx.se/libcurl/c
 *
 * Build:
 *
 * cc curltest.c -lcurl -ljson-c -o curltest
 *
 * Run:
 *
 * ./curltest
 * 
 */

/* standard includes */
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>

/* json-c (https://github.com/json-c/json-c) */
#include <json-c/json.h>

/* libcurl (http://curl.haxx.se/libcurl/c) */
#include <curl/curl.h>

/* holder for curl fetch */
struct curl_fetch_st {
    char *payload;
    size_t size;
};

/* callback for curl fetch */
size_t curl_callback (void *contents, size_t size, size_t nmemb, void *userp) {
    size_t realsize = size * nmemb;                             /* calculate buffer size */
    struct curl_fetch_st *p = (struct curl_fetch_st *) userp;   /* cast pointer to fetch struct */

    /* expand buffer using a temporary pointer to avoid memory leaks */
    char * temp = realloc(p->payload, p->size + realsize + 1);

    /* check allocation */
    if (temp == NULL) {
      /* this isn't good */
      fprintf(stderr, "ERROR: Failed to expand buffer in curl_callback");
      /* free buffer */
      free(p->payload);
      /* return */
      return 1;
    }

    /* assign payload */
    p->payload = temp;

    /* copy contents to buffer */
    memcpy(&(p->payload[p->size]), contents, realsize);

    /* set new buffer size */
    p->size += realsize;

    /* ensure null termination */
    p->payload[p->size] = 0;

    /* return size */
    return realsize;
}

/* fetch and return url body via curl */
CURLcode curl_fetch_url(CURL *ch, const char *url, struct curl_fetch_st *fetch) {
    CURLcode rcode;                   /* curl result code */

    /* init payload */
    fetch->payload = (char *) calloc(1, sizeof(fetch->payload));

    /* check payload */
    if (fetch->payload == NULL) {
        /* log error */
        fprintf(stderr, "ERROR: Failed to allocate payload in curl_fetch_url");
        /* return error */
        return CURLE_FAILED_INIT;
    }

    /* init size */
    fetch->size = 0;

    /* set url to fetch */
    curl_easy_setopt(ch, CURLOPT_URL, url);

    /* set calback function */
    curl_easy_setopt(ch, CURLOPT_WRITEFUNCTION, curl_callback);

    /* pass fetch struct pointer */
    curl_easy_setopt(ch, CURLOPT_WRITEDATA, (void *) fetch);

    /* set default user agent */
    curl_easy_setopt(ch, CURLOPT_USERAGENT, "libcurl-agent/1.0");

    /* set timeout */
    curl_easy_setopt(ch, CURLOPT_TIMEOUT, 15);

    /* enable location redirects */
    curl_easy_setopt(ch, CURLOPT_FOLLOWLOCATION, 1);

    /* set maximum allowed redirects */
    curl_easy_setopt(ch, CURLOPT_MAXREDIRS, 1);

    /* fetch the url */
    rcode = curl_easy_perform(ch);

    /* return */
    return rcode;
}

int main(int argc, char *argv[]) {
    CURL *ch;                                               /* curl handle */
    CURLcode rcode;                                         /* curl result code */

    json_object *json;                                      /* json post body */
    enum json_tokener_error jerr = json_tokener_success;    /* json parse error */

    struct curl_fetch_st curl_fetch;                        /* curl fetch struct */
    struct curl_fetch_st *cf = &curl_fetch;                 /* pointer to fetch struct */
    struct curl_slist *headers = NULL;                      /* http headers to send with request */

    /* url to test site */
    char *url = "http://jsonplaceholder.typicode.com/posts/";

    /* init curl handle */
    if ((ch = curl_easy_init()) == NULL) {
        /* log error */
        fprintf(stderr, "ERROR: Failed to create curl handle in fetch_session");
        /* return error */
        return 1;
    }

    /* set content type */
    headers = curl_slist_append(headers, "Accept: application/json");
    headers = curl_slist_append(headers, "Content-Type: application/json");

    /* create json object for post */
    json = json_object_new_object();

    /* build post data */
    json_object_object_add(json, "title", json_object_new_string("testies"));
    json_object_object_add(json, "body", json_object_new_string("testies ... testies ... 1,2,3"));
    json_object_object_add(json, "userId", json_object_new_int(133));

    /* set curl options */
    curl_easy_setopt(ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_easy_setopt(ch, CURLOPT_HTTPHEADER, headers);
    curl_easy_setopt(ch, CURLOPT_POSTFIELDS, json_object_to_json_string(json));

    /* fetch page and capture return code */
    rcode = curl_fetch_url(ch, url, cf);

    /* cleanup curl handle */
    curl_easy_cleanup(ch);

    /* free headers */
    curl_slist_free_all(headers);

    /* free json object */
    json_object_put(json);

    /* check return code */
    if (rcode != CURLE_OK || cf->size < 1) {
        /* log error */
        fprintf(stderr, "ERROR: Failed to fetch url (%s) - curl said: %s",
            url, curl_easy_strerror(rcode));
        /* return error */
        return 1;
    }

    /* check payload */
    if (cf->payload != NULL) {
        /* print result */
        printf("CURL Returned: \n%s\n", cf->payload);
        /* parse return */
        json = json_tokener_parse_verbose(cf->payload, &jerr);
        /* free payload */
        free(cf->payload);
    } else {
        /* error */
        fprintf(stderr, "ERROR: Failed to populate payload");
        /* free payload */
        free(cf->payload);
        /* return */
        return 1;
    }

    /* check error */
    if (jerr != json_tokener_success) {
        /* error */
        fprintf(stderr, "ERROR: Failed to parse json string");
        /* free json object */
        json_object_put(json);
        /* return */
        return 1;
    }

    /* debugging */
    printf("Parsed JSON: %s\n", json_object_to_json_string(json));

    /* free json object before return */
    json_object_put(json);

    /* exit */
    return 0;
}

First of all thanks for sharing this gist. Your code has a memory leak because you aren't freeing the json_object *json at the end of the program. This is confirmed by running valgrind --leak-check=full ./program:

==8196== LEAK SUMMARY:
==8196==    definitely lost: 72 bytes in 1 blocks
==8196==    indirectly lost: 1,101 bytes in 14 blocks
==8196==      possibly lost: 0 bytes in 0 blocks
==8196==    still reachable: 97,892 bytes in 685 blocks
==8196==         suppressed: 0 bytes in 0 blocks

Please update your code and add json_object_put(json) at the very end before the return statement

I would not return -1 as an error code in function returning unsigned int ;)

mark

Hi!
Why do you use &(p->payload[p->size]) in memcpy before assigning the real size to the payload:
p->size += realsize; ?

Thanks. Can I use this in my application? i.e license.

Thanks. Can I use this in my application? i.e license.

Let me apply some updates and give it a license in the comment block.

Ok.

Hi
In line 57, you're basically freeing a NULL pointer. I suggest checking the realloc() result before storing in it in p->payload (line 50).

Thanks @med8bra, in general I prefer to improve readability by reducing unnecessary checks. Freeing a NULL pointer is always safe.

You right, freeing a NULL pointer is safe and useless. But there will be a memory leak anyway since p->payload is lost.

I see, that's a valid point.

Thanks @med8bra, I pushed an update to fix that issue.

Thanks, this helps.
I have added the json object like this.
The requirement for header is
Content-Encoding : gzip.

If I append this into header list, should I perform anything for json object or need to compress json object ?
Please suggest how can I do that?

@mahedi13 cURL does not (to my knowledge) provide a way to automatically compress payloads. You will need to do you own compression of the payload using zlib. You should probably also make sure that the server accepts gzip before sending it gzipped data.

See usage examples for zlib here: http://www.zlib.net/zlib_how.html

@leprechau According to your Json Object how will you handle, if you append Content-Encoding:gzip in header?
Can you share please?