#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);
If the read_fd_set argument is not a null pointer, it points to an object of type fd_set that on returns specifies the file descriptors to be checked for being ready to read.
If the write_fd_set argument is not a null pointer, it points to an object of type fd_set that on return specifies the file descriptors to be checked for being ready to write.
If the exc_fd_set argument is not a null pointer, it points to an object of type fd_set that on return specifies the file descriptors to be checked for error conditions pending.
If no file descriptors are set by libcurl, max_fd will contain -1 when this function returns. Otherwise it will contain the highest descriptor number libcurl set. When libcurl returns -1 in max_fd, it is because libcurl currently does something that isn't possible for your application to monitor with a socket and unfortunately you can then not know exactly when the current action is completed using select(). You then need to wait a while before you proceed and call curl_multi_perform(3) anyway. How long to wait? Unless curl_multi_timeout(3) gives you a lower number, we suggest 100 milliseconds or so, but you may want to test it out in your own particular conditions to find a suitable value.
When doing select(), you should use curl_multi_timeout(3) to figure out how long to wait for action. Call curl_multi_perform(3) 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 want.
If one of the sockets used by libcurl happens to be larger than what can be set in an fd_set, which on POSIX systems means that the file descriptor is larger than FD_SETSIZE, then libcurl will try to not set it. Setting a too large file descriptor in an fd_set implies an out of bounds write which can cause crashes, or worse. The effect of NOT storing it will possibly save you from the crash, but will make your program NOT wait for sockets it should wait for...