C API Client Project

C API Client Project

Overview

If you want to practice server and API communication in C, a great beginner-to-intermediate project is a small HTTP client that can:

  • connect to a server
  • send an HTTP request
  • receive a response
  • store the data in memory
  • parse JSON data
  • display useful information to the user

This project uses two important libraries:

  • libcurl: for HTTP communication
  • cJSON: for parsing JSON responses

What You Will Learn

By building this project, you will learn:

  • how C programs communicate with web servers
  • how to send HTTP requests using libcurl
  • how to handle response data dynamically
  • how to parse JSON objects in C
  • how to display extracted data in a readable format

Required Libraries

Before compiling this program, make sure the following libraries are installed:

  • libcurl
  • cJSON

On Linux, you may install them using:

sudo apt-get install libcurl4-openssl-dev libcjson-dev

On other systems, use the equivalent package manager or development environment tools.

Example C Program: HTTP API Client

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>
#include "cJSON.h"

/* Structure to store HTTP response data */
struct Response {
    char *data;
    size_t size;
};

/* Callback function called when data is received from the server */
size_t write_callback(void *contents, size_t size, size_t nmemb, void *userp)
{
    size_t total_size = size * nmemb;
    struct Response *response = (struct Response *)userp;

    char *ptr = realloc(response->data, response->size + total_size + 1);

    if (ptr == NULL)
    {
        printf("Memory allocation failed!\n");
        return 0;
    }

    response->data = ptr;

    memcpy(&(response->data[response->size]), contents, total_size);
    response->size += total_size;
    response->data[response->size] = '\0';

    return total_size;
}

int main(void)
{
    CURL *curl;
    CURLcode result;
    struct Response response;

    response.data = malloc(1);
    response.size = 0;

    if (response.data == NULL)
    {
        printf("Failed to allocate initial memory.\n");
        return 1;
    }

    /* Initialize libcurl */
    curl_global_init(CURL_GLOBAL_DEFAULT);
    curl = curl_easy_init();

    if (curl == NULL)
    {
        printf("CURL initialization failed!\n");
        free(response.data);
        return 1;
    }

    /* Target API endpoint */
    curl_easy_setopt(curl, CURLOPT_URL, "https://jsonplaceholder.typicode.com/users/1");

    /* Set callback to receive response data */
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response);

    /* Perform the HTTP request */
    result = curl_easy_perform(curl);

    if (result != CURLE_OK)
    {
        printf("Request failed: %s\n", curl_easy_strerror(result));
        curl_easy_cleanup(curl);
        curl_global_cleanup();
        free(response.data);
        return 1;
    }

    printf("\n========== SERVER RESPONSE ==========");
    printf("\n%s\n", response.data);

    /* Parse JSON response */
    cJSON *json = cJSON_Parse(response.data);

    if (json == NULL)
    {
        const char *error_ptr = cJSON_GetErrorPtr();
        if (error_ptr != NULL)
        {
            printf("JSON parse error before: %s\n", error_ptr);
        }
        printf("Invalid JSON response!\n");
    }
    else
    {
        cJSON *id = cJSON_GetObjectItemCaseSensitive(json, "id");
        cJSON *name = cJSON_GetObjectItemCaseSensitive(json, "name");
        cJSON *username = cJSON_GetObjectItemCaseSensitive(json, "username");
        cJSON *email = cJSON_GetObjectItemCaseSensitive(json, "email");

        printf("\n========== PARSED DATA ==========");

        if (cJSON_IsNumber(id))
            printf("\nID       : %d", id->valueint);

        if (cJSON_IsString(name) && (name->valuestring != NULL))
            printf("\nName     : %s", name->valuestring);

        if (cJSON_IsString(username) && (username->valuestring != NULL))
            printf("\nUsername : %s", username->valuestring);

        if (cJSON_IsString(email) && (email->valuestring != NULL))
            printf("\nEmail    : %s", email->valuestring);

        printf("\n");
        cJSON_Delete(json);
    }

    /* Cleanup */
    curl_easy_cleanup(curl);
    curl_global_cleanup();
    free(response.data);

    return 0;
}

Explanation of the Program

1. Including required headers

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>
#include "cJSON.h"

These headers provide:

  • stdio.h: input and output functions
  • stdlib.h: memory allocation functions like malloc and free
  • string.h: string and memory functions like memcpy
  • curl/curl.h: libcurl functions
  • cJSON.h: JSON parsing functions

2. Response storage structure

struct Response {
    char *data;
    size_t size;
};

This structure stores the response received from the HTTP server. It keeps:

  • the actual response string
  • the total size of the received data

3. write_callback function

size_t write_callback(void *contents, size_t size, size_t nmemb, void *userp)

This callback is called by libcurl whenever new data is received. It appends the received bytes to the response.data buffer. This is important because the server may send data in chunks, not all at once.

4. Sending the request

curl_easy_setopt(curl, CURLOPT_URL, "https://jsonplaceholder.typicode.com/users/1");

This tells libcurl which URL to request.

5. Receiving the response

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response);

These lines connect the callback function and the response structure so that incoming HTTP data is saved properly.

6. Performing the request

result = curl_easy_perform(curl);

This sends the HTTP request and waits for the response.

7. Parsing JSON

cJSON *json = cJSON_Parse(response.data);

The returned response is usually JSON text. This function converts the JSON string into a C structure that can be read easily.

Then the code reads values such as:

cJSON *id = cJSON_GetObjectItemCaseSensitive(json, "id");
cJSON *name = cJSON_GetObjectItemCaseSensitive(json, "name");

These commands extract specific keys from the JSON object.

Example JSON Response

The API may return data like this:

{
  "id": 1,
  "name": "Leanne Graham",
  "username": "Bret",
  "email": "Sincere@april.biz"
}

The program prints:

========== SERVER RESPONSE ==========
{
  "id": 1,
  "name": "Leanne Graham",
  "username": "Bret",
  "email": "Sincere@april.biz"
}

========== PARSED DATA ==========
ID       : 1
Name     : Leanne Graham
Username : Bret
Email    : Sincere@april.biz

How the Program Works

C Program
   │
   │ HTTP GET Request
   ▼
API Server
   │
   │ JSON Response
   ▼
libcurl
   │
   ▼
write_callback()
   │
   ▼
Dynamic memory stores response
   │
   ▼
cJSON parser
   │
   ▼
Extract fields such as id, name, and email
   │
   ▼
Display result to the user

Compile the Program

Use this command:

gcc api_client.c -o api_client -lcurl -lcjson

Then run it:

./api_client

Libraries Used

LibraryPurpose
stdio.hInput and output functions
stdlib.hMemory allocation and standard utilities
string.hString and memory functions
libcurlSending HTTP requests and receiving responses
cJSONParsing JSON data

Possible Improvements for a Bigger Project

A more advanced version of this project could include:

  • GET requests
  • POST requests
  • PUT requests
  • DELETE requests
  • request headers
  • authentication tokens
  • JSON request body creation
  • response status code handling
  • timeout handling
  • retry logic
  • better error messages
  • logging
  • saving response into files
  • database integration

A larger project idea could be:

C API Client
├── GET
├── POST
├── PUT
├── DELETE
├── Headers
├── Authentication
├── JSON request support
├── JSON response parsing
├── Timeout handling
├── Error handling
├── Retry mechanism
├── Logging
├── File saving
└── Database integration

Summary

This C project is a practical and useful introduction to working with APIs. It shows how to connect to a remote service, retrieve data, parse JSON, and present meaningful information to the user. It is a strong foundation for building larger networking applications in C.

This is a good starting point for students who want to learn real-world programming with web APIs and C.

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