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- .\" * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
- .\" *
- .\" * This software is licensed as described in the file COPYING, which
- .\" * you should have received as part of this distribution. The terms
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- .TH curl_multi_fdset 3 "2 Jan 2006" "libcurl" "libcurl"
- .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 \fIfd_set\fP sets. The application can use these to
- select() on, but be sure to \fIFD_ZERO\fP them before calling this function as
- \fIcurl_multi_fdset(3)\fP only adds its own descriptors, it does not 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 the \fIread_fd_set\fP argument is not a null pointer, it points to an
- object of type \fBfd_set\fP that on returns specifies the file descriptors to
- be checked for being ready to read.
- If the \fIwrite_fd_set\fP argument is not a null pointer, it points to an
- object of type \fBfd_set\fP that on return specifies the file descriptors to
- be checked for being ready to write.
- If the \fIexc_fd_set\fP argument is not a null pointer, it points to an object
- of type \fBfd_set\fP that on return specifies the file descriptors to be
- checked for error conditions pending.
- If no file descriptors are set by libcurl, \fImax_fd\fP will contain -1 when
- this function returns. Otherwise it will contain the highest descriptor number
- libcurl set. When libcurl returns -1 in \fImax_fd\fP, it is because libcurl
- currently does something that is not 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(). You then need to wait a while before you
- proceed and call \fIcurl_multi_perform(3)\fP anyway. How long to wait? Unless
- \fIcurl_multi_timeout(3)\fP gives you a lower number, we suggest 100
- milliseconds or so, but you may want to test it out in your own particular
- conditions to find a suitable value.
- When doing select(), you should use \fIcurl_multi_timeout(3)\fP to figure out
- how long to wait for action. Call \fIcurl_multi_perform(3)\fP even if no
- activity has been seen on the \fBfd_sets\fP after the timeout expires as
- otherwise internal retries and timeouts may not work as you would think and
- want.
- If one of the sockets used by libcurl happens to be larger than what can be
- set in an \fBfd_set\fP, which on POSIX systems means that the file descriptor
- is larger than \fBFD_SETSIZE\fP, then libcurl will try to not set it. Setting
- a too large file descriptor in an \fBfd_set\fP 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 EXAMPLE
- .nf
- /* get file descriptors from the transfers */
- mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
- if(mc != CURLM_OK) {
- fprintf(stderr, "curl_multi_fdset() failed, code %d.\\n", mc);
- break;
- }
- /* wait for activity on one of the sockets */
- rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
- .fi
- .SH AVAILABILITY
- Added in 7.9.6
- .SH RETURN VALUE
- \fBCURLMcode\fP 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_wait "(3), "
- .BR curl_multi_timeout "(3), " curl_multi_perform "(3), " select "(2) "
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