c++ libcurl get request is returning 0 but still printing in the console

Question

I've installed libcurl on ubuntu 20.4 and I've been playing around with it. I decidedly wanted to create a program that would write a file from the internet. So I wrote this.

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

int main() {

  std::ofstream htmlFile("index.html");

  auto curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/");

    curl_easy_setopt(curl, CURLOPT_HTTPGET, 1L);


    htmlFile <<curl_easy_perform(curl);

    

    htmlFile.close();

  } 

  return 0;
}

When I run this file I'm given my apache webserver html code through the console. But when looking at the file I want to write to there's just a zero.

I'm stumped, I've seen so many different ways to do this. I find it strange that the html is logged to the console, but all it does is return 0.

Answer 1

curl_easy_perform returns a CURLcode. where "CURLE_OK (0) means everything was ok". That's what is written to your file.

If you want to capture the text inside the program, you need to set a callback function with CURLOPT_WRITEFUNCTION and also provide somewhere to store the data via CURLOPT_WRITEDATA

Example:

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

size_t write_callback(char* ptr, size_t size, size_t nmemb, void* userdata) {
    std::string& data = *static_cast<std::string*>(userdata);
    size_t len = size * nmemb;

    // append to the string:
    data.append(ptr, len);
    return len;
}

int main() {
    auto curl = curl_easy_init();
    if(curl) {
        std::string data; // we'll store the data in a string

        curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/");
        curl_easy_setopt(curl, CURLOPT_HTTPGET, 1L);

        // provide a pointer to where you'd like to store the data:
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &data);

        // provide a callback function:
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);

        if(curl_easy_perform(curl) == CURLE_OK) {
            std::ofstream htmlFile("index.html");
            if(htmlFile)
                htmlFile << data; // all's well, store the data in the file.
        }

        curl_easy_cleanup(curl);
    }
}