The curl_multi_poll function in conjunction with curl_multi_add_handle - for some reason it never waits for an event and immediately returns:

Simple example:

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


int main()
{
    curl_global_init(CURL_GLOBAL_ALL);


    CURLM* CURLM_ = curl_multi_init();
    CURL* CURL_ = curl_easy_init();

    curl_easy_setopt(CURL_, CURLOPT_URL, "https://stackoverflow.com");

    int num_desc_events;


    curl_multi_add_handle(CURLM_, CURL_); //If this line is deleted, then curl_multi_poll enters waits mode.


    if (CURLMcode_ != CURLM_OK)
    {
    std::cout << "curl_multi_add_handle_status:" << CURLMcode_ << std::endl;
    }



    while (1)
    {
        std::cout << "curl_multi_poll_start" << std::endl;

        curl_multi_poll(CURLM_, NULL, 0, 100000, &num_desc_events);

        if (CURLMcode_ != CURLM_OK)
        {
        std::cout << "curl_multi_poll_status:" << CURLMcode_ << std::endl;
        }

        std::cout << "curl_multi_poll_awakened" << std::endl;
        std::cout << "num_desc_events:" << num_desc_events << std::endl;
  
    }



    curl_multi_cleanup(CURLM_);
    curl_global_cleanup();
}

As you can see, this is a very simple code, but it works very strangely or even I would say that it does not work.

From the description of the curl_multi_poll function, it follows that it waits FOREVER until an event occurs on the mult descriptor or a set timeout.

That is, when there is a line with the curl_multi_add_handle function in the code, the curl_multi_poll function does not enter standby mode.

And if the line of code with the curl_multi_add_handle function is removed, then curl_multi_poll works correctly and enters standby mode until the first event, in this case indefinitely or until timeout.

CodePudding user response：

The code doesn't call curl_multi_perform() so it doesn't actually do anything and whatever libcurl wants to do, it still wants to do...