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)

libcurl 7.16.0                                     2 Jan 2006                                curl_multi_fdset(3)