How to use curl in C and parse JSON response

Question

How to use curl in C and parse JSON response

3.8k Views Asked by Andrés Sanchez At 21 October 2024 at 04:50

The code below compiles and prints the response.

My question is: being the response a string which represents an object, how can I cast "res" into a string or directly into a JSON object?

#include <stdio.h>
#include <curl/curl.h>
#include <json-c/json.h>

int main(int argc, char **argv) {
    CURL *curl;
    CURLcode res;
    curl = curl_easy_init();

    if(curl) {
        curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "GET");
        curl_easy_setopt(curl, CURLOPT_URL, "http://localhost:8080/system/genpass");
        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
        curl_easy_setopt(curl, CURLOPT_DEFAULT_PROTOCOL, "https");
        struct curl_slist *headers = NULL;
        headers = curl_slist_append(headers, "length: 20");
        headers = curl_slist_append(headers, "numbers: true");
        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
        res = curl_easy_perform(curl);
        printf("%u",res);
    }

    curl_easy_cleanup(curl);
}

Original Q&A

There are 1 best solutions below

**Gerhardh** · Accepted Answer

You cannot simply cast the return code of a function into a string. They are not related in any way.

Instead you need to handle the received data directly. For this purpose you need to register a callback function that can handle the data. This is descriped in cURL manual.

Example:

typedef struct {
    unsigned char *buffer;
    size_t len;
    size_t buflen;
} get_request;

#define CHUNK_SIZE 2048

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
    size_t realsize = size * nmemb; 
    get_request *req = (get_request *) userdata;

    printf("receive chunk of %zu bytes\n", realsize);

    while (req->buflen < req->len + realsize + 1)
    {
        req->buffer = realloc(req->buffer, req->buflen + CHUNK_SIZE);
        req->buflen += CHUNK_SIZE;
    }
    memcpy(&req->buffer[req->len], ptr, realsize);
    req->len += realsize;
    req->buffer[req->len] = 0;

    return realsize;
}


int main(int argc, char **argv) {
    CURL *curl;
    CURLcode res;
    curl = curl_easy_init();

    get_request req = {.buffer = NULL, .len = 0, .buflen = 0};

    if (curl) {
        curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "GET");
        curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
        curl_easy_setopt(curl, CURLOPT_DEFAULT_PROTOCOL, "https");
        struct curl_slist *headers = NULL;
        headers = curl_slist_append(headers, "length: 20");
        headers = curl_slist_append(headers, "numbers: true");
        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

        req.buffer = malloc(CHUNK_SIZE);
        req.buflen = CHUNK_SIZE;

        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&req);

        res = curl_easy_perform(curl);
        printf("Result = %u\n",res);

        printf("Total received bytes: %zu\n", req.len);
        printf("Received data:/n%s\n", req.buffer);
        free(req.buffer);        
    }

    curl_easy_cleanup(curl);
}

Output:

receive chunk of 1256 bytes
Result = 0
Total received bytes: 1256
Received data:
<!doctype html>
<html>
<head>
    <title>Example Domain</title>

    <meta charset="utf-8" />
    <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <style type="text/css">
    body {
        background-color: #f0f0f2;
        margin: 0;
        padding: 0;
        font-family: -apple-system, system-ui, BlinkMacSystemFont, "Segoe UI", "Open Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;

    }
    div {
        width: 600px;
        margin: 5em auto;
        padding: 2em;
        background-color: #fdfdff;
        border-radius: 0.5em;
        box-shadow: 2px 3px 7px 2px rgba(0,0,0,0.02);
    }
    a:link, a:visited {
        color: #38488f;
        text-decoration: none;
    }
    @media (max-width: 700px) {
        div {
            margin: 0 auto;
            width: auto;
        }
    }
    </style>    
</head>

<body>
<div>
    <h1>Example Domain</h1>
    <p>This domain is for use in illustrative examples in documents. You may use this
    domain in literature without prior coordination or asking for permission.</p>
    <p><a href="https://www.iana.org/domains/example">More information...</a></p>
</div>
</body>
</html>

After the GET request is completed, you can access the data in req->buffer and parse for JSON objects or any other data. Error checking (for realloc etc.) needs to be added.

I used an approach that is able to handle any length of content in any number of chunks received. If you know the length of your expected content, you might go for it with a fixed size buffer.

As you can see in my example, the user data structure still exists after the download is complete and the content can be derived after curl_easy_perform returns.

How to use curl in C and parse JSON response

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