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 doesnt 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.
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 isnt 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? We
suggest 100 milliseconds at least, 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 youd 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