curl_multi_fdset(3) -- Linux man page
NAMEcurl_multi_fdset - extracts file descriptor information from a multi handle
#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);
DESCRIPTIONThis function extracts file descriptor information from a given multi_handle. libcurl returns its fd_set sets. The application can use these to select() on. The curl_multi_perform(3) function should be called as soon as one of them are ready to be read from or written to.
NOTE that once this call is made, you must not remove the sets you point to, as libcurl will need to be able to read them. It needs them after select() calls, to know if certain sockets are readable or writable.
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.
RETURN VALUECURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)