Using Libcurl in C/C++ Application

Client URL, or just curl, is a command-line tool for transferring data using various network protocols. It is commonly used by developers to test various applications build on top of HTTP.

That said, curl itself is just a wrapper around libcurl. The library is written in C and has well documented API. In this post, I will demonstrate how you can use libcurl in order to make HTTP requests from your C/C++ applications.

Before we begin, make sure you have C compiler installed. I will be using gcc. Other compilers will work too, but you will have to modify the provided Makefile. You will also need to have curl and libcurl installed. On Linux and OSX, if you can use curl command in your CLI you should be good to go.

Let's start by creating a simple Makefile:

Makefile

default: build
	
build: clean
	gcc -Wall -o curl -l curl main.c util.c 

clean:
	rm -rf curl 

test: build
	./curl https://freegeoip.app/json/ 

Now, moving to the application. It is a simple CLI tool that takes an URL as an argument, makes HTTP Get request, and prints the response.

Here is the code:

main.c

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

int main(int argc, char *argv[]) {
  if( argc != 2 ) {
    printf("usage: try './curl [url]' to make a get request.\n");
    return 1;
  }

  CURL *curl_handle;
  CURLcode res;

  struct MemoryStruct chunk;
  chunk.memory = malloc(1);  
  chunk.size = 0;
 
  curl_handle = curl_easy_init();
  if(curl_handle) {
    curl_easy_setopt(curl_handle, CURLOPT_URL, argv[1]);
    curl_easy_setopt(curl_handle, CURLOPT_FOLLOWLOCATION, 1L);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
    curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
    curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, "libcurl-agent/1.0");
    
    res = curl_easy_perform(curl_handle);
    
    if(res != CURLE_OK) {
      fprintf(stderr, "error: %s\n", curl_easy_strerror(res));
    } else {
      printf("Size: %lu\n", (unsigned long)chunk.size);
      printf("Data: %s\n", chunk.memory);
    }
    curl_easy_cleanup(curl_handle);
    free(chunk.memory);
  }
  return 0;
}

util.h

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#ifndef UTIL_H
#define UTIL_H

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct MemoryStruct {
  char *memory;
  size_t size;
};

size_t WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp);

#endif

util.c

#include "util.h"

size_t WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp) {
  size_t realsize = size * nmemb;
  struct MemoryStruct *mem = (struct MemoryStruct *)userp;
 
  char *ptr = realloc(mem->memory, mem->size + realsize + 1);
  if(ptr == NULL) {
    printf("error: not enough memory\n");
    return 0;
  }
 
  mem->memory = ptr;
  memcpy(&(mem->memory[mem->size]), contents, realsize);
  mem->size += realsize;
  mem->memory[mem->size] = 0;
 
  return realsize;
}

The most interesting function here is curl_easy_setopt. It sets various options on the instance of curl client (in my example curl_handle). Note, that by setting CURLOPT_WRITEFUNCTION and CURLOPT_WRITEDATA we have configured the curl_handle to use custom logic and location for writing the response data.

Similarly, you can set custom headers, or HTTP Post payload. For the list of all the available options refer to the curl docs.