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 applica-
       tion 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 oth-
       erwise 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 stor-
       ing it will possibly save you from the crash, but will make  your  pro-
       gram 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)