man curl_multi_fdset (Fonctions bibliothèques) - extracts file descriptor information from a multi handle
NAME
curl_multi_fdset - extracts file descriptor information from a multi handle
SYNOPSIS
#include <curl/curl.h>
CURLMcode curl_multi_fdset(CURLM *multi_handle, fd_set *read_fd_set, fd_set *write_fd_set, fd_set *exc_fd_set, int *max_fd);
DESCRIPTION
This function extracts file descriptor information from a given multi_handle. libcurl returns its fd_set sets. The application can use these to select() on, but be sure to FD_ZERO them before calling this function as curl_multi_fdset(3) only adds its own descriptors it doesn't zero or otherwise remove any other. The curl_multi_perform(3) function should be called as soon as one of them are ready to be read from or written to.
If no file descriptors are set by libcurl, max_fd will contain -1 when this function returns. Otherwise it will contain the higher descriptor number libcurl set.
You should also be aware that when doing select(), you should consider using a rather small (single-digit number of seconds) timeout and call curl_multi_perform regularly - even if no activity has been seen on the fd_sets - as otherwise libcurl-internal retries and timeouts may not work as you'd think and want.
RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)