curl_multi_fdset - extracts file descriptor information from a multi
CURLMcode curl_multi_fdset(CURLM *multi_handle,
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 others. The curl_multi_perform(3)
function should be called as soon as one of them is ready to be read
from or written to.
To be sure to have up-to-date results, you should call
curl_multi_perform until it does not return CURLM_CALL_MULTI_PERFORM
prior to calling curl_multi_fdset. This will make sure that libcurl
has updated the handles’ socket states.
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.
When doing select(), you should use curl_multi_timeout to figure out
how long to wait for action. Call curl_multi_perform even if no
activity has been seen on the fd_sets after the timeout expires as
otherwise internal retries and timeouts may not work as you’d think and
CURLMcode type, general libcurl multi interface error code. See
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_timeout(3),