curl_multi_fdset(3)						  libcurl Manual					       curl_multi_fdset(3)

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 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  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?  We	suggest  100  mil-
       liseconds 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 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...

RETURN VALUE

       CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)

SEE ALSO

       curl_multi_cleanup(3), curl_multi_init(3), curl_multi_wait(3), curl_multi_timeout(3), curl_multi_perform(3), select(2)

libcurl 7.54.0							 February 03, 2016					       curl_multi_fdset(3)