cURL C++ Example

curl_example.cpp

#include <iostream>
#include <string>
#include <curl/curl.h>


static size_t WriteCallback(void *contents, size_t size, size_t nmemb, void *userp)
{
    ((std::string*)userp)->append((char*)contents, size * nmemb);
    return size * nmemb;
}

int main(void)
{
  CURL *curl;
  CURLcode res;
  std::string readBuffer;

  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://www.google.com");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &readBuffer);
    res = curl_easy_perform(curl);
    curl_easy_cleanup(curl);

    std::cout << readBuffer << std::endl;
  }
  return 0;
}

@hasirciogli

1. It looks like you did not include the -lcurl library when you compiled, hence the "unresolved external symbol" errors.
2. https://example.com does not support --http3, yet; you could test this using any HTTP/3 verifier.
   Try using any of the HTTP/3 supported websites, some are mentioned here.
3. I compiled and executed your code using https://quic.tech but received a curl_easy_perform() failed: SSL connect error error.
   Most probably because I don't have the OpenSSL libraries installed, and this is expected.
   But the code itself is executing fine.

#include <curl/curl.h>

int main()
{
	CURL* curl;
	CURLcode res;
	curl = curl_easy_init();
	if (curl)
	{
		curl_easy_setopt(curl, CURLOPT_URL, "https://quic.tech");
		curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, (long)CURL_HTTP_VERSION_3);
		res = curl_easy_perform(curl);
		if (res != CURLE_OK)
			fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
		
		curl_easy_cleanup(curl);
	}
}

Hope that helps :)

I developed a client which is receiving data from server occasionally. But when I receive first data, then program exists. I tried while loop like windows message loop, but it gives me last data.

How can I stay in program so that I can receive more messages from server.

@alghanmi Hi!

First off, thank you for sharing your code example! It was incredibly helpful for getting me started quickly. However, I wanted to point out a couple of discrepancies that I noticed:

1. Missing global initialization:
   The libcurl documentation recommends using curl_global_init() at the beginning of the program to set up the necessary environment for curl operations. See "Global preparation" section:
   - https://curl.se/libcurl/c/libcurl-tutorial.html
2. Callback prototype mismatch:
   The prototype for the callback function seems to be inconsistent with the one documented on the official libcurl site. The prototype for this callback function is described here:
   - https://curl.se/libcurl/c/CURLOPT_WRITEFUNCTION.html

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);

On a related note, I've created a more detailed setup documentation and added a simple example as well as a slightly more complex one that includes POST requests, which some may find useful:
- https://github.com/voidlib/cpp-curl-example

Thanks again for your contribution—it was a great starting point for me!

Best regards,
@voiduin

this was the best explanation

I'm getting this output

$ g++ main.cpp -lcurl

C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0x51): undefined reference to `_imp__curl_easy_init'
C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0x7b): undefined reference to `_imp__curl_easy_setopt'
C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0x98): undefined reference to `_imp__curl_easy_setopt'
C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0xb4): undefined reference to `_imp__curl_easy_setopt'
C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0xc1): undefined reference to `_imp__curl_easy_perform'
C:\Users\Admin\AppData\Local\Temp\ccEeyCDI.o:cleopatra.cpp:(.text+0xd1): undefined reference to `_imp__curl_easy_cleanup'
collect2.exe: error: ld returned 1 exit status