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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) "
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