curl/docs/libcurl/curl_multi_fdset.3

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.TH curl_multi_fdset 3 "2 Jan 2006" "libcurl 7.16.0" "libcurl Manual"
.SH NAME
curl_multi_fdset - extracts file descriptor information from a multi handle
.SH SYNOPSIS
.nf
#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);
.ad
.SH 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
\fIcurl_multi_fdset(3)\fP only adds its own descriptors, it doesn't zero or
otherwise remove any others. The \fIcurl_multi_perform(3)\fP 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, \fImax_fd\fP will contain -1 when
this function returns. Otherwise it will contain the higher descriptor number
libcurl set. When libcurl returns -1 in \fImax_fd\fP, 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 \fIcurl_multi_perform\fP 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 \fBcurl_multi_timeout\fP to figure out how
long to wait for action. Call \fIcurl_multi_perform\fP 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...
.SH RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See
\fIlibcurl-errors(3)\fP
.SH "SEE ALSO"
.BR curl_multi_cleanup "(3), " curl_multi_init "(3), "
.BR curl_multi_timeout "(3), " curl_multi_perform "(3) "