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  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  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(). When max_fd returns with -1,
       you need to wait a while and then proceed and call curl_multi_perform anyway. How long  to
       wait?  I  would 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 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
       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_timeout(3), curl_multi_perform(3)