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103 lines
4.7 KiB
Groff
103 lines
4.7 KiB
Groff
.\" **************************************************************************
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.\" * _ _ ____ _
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.\" * Project ___| | | | _ \| |
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.\" * / __| | | | |_) | |
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.\" * | (__| |_| | _ <| |___
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.\" * \___|\___/|_| \_\_____|
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.\" *
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.\" * Copyright (C) 1998 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al.
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.\" *
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.\" * This software is licensed as described in the file COPYING, which
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.\" * you should have received as part of this distribution. The terms
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.\" * are also available at https://curl.se/docs/copyright.html.
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.\" *
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.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
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.\" * copies of the Software, and permit persons to whom the Software is
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.\" * furnished to do so, under the terms of the COPYING file.
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.\" *
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.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
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.\" * KIND, either express or implied.
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.\" *
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.\" * SPDX-License-Identifier: curl
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.\" *
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.\" **************************************************************************
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.TH curl_multi_fdset 3 "2 Jan 2006" "libcurl 7.16.0" "libcurl Manual"
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.SH NAME
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curl_multi_fdset - extracts file descriptor information from a multi handle
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.SH SYNOPSIS
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.nf
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#include <curl/curl.h>
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CURLMcode curl_multi_fdset(CURLM *multi_handle,
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fd_set *read_fd_set,
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fd_set *write_fd_set,
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fd_set *exc_fd_set,
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int *max_fd);
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.ad
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.SH DESCRIPTION
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This function extracts file descriptor information from a given multi_handle.
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libcurl returns its \fIfd_set\fP sets. The application can use these to
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select() on, but be sure to \fIFD_ZERO\fP them before calling this function as
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\fIcurl_multi_fdset(3)\fP only adds its own descriptors, it does not zero or
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otherwise remove any others. The \fIcurl_multi_perform(3)\fP function should
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be called as soon as one of them is ready to be read from or written to.
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If the \fIread_fd_set\fP argument is not a null pointer, it points to an
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object of type \fBfd_set\fP that on returns specifies the file descriptors to
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be checked for being ready to read.
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If the \fIwrite_fd_set\fP argument is not a null pointer, it points to an
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object of type \fBfd_set\fP that on return specifies the file descriptors to
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be checked for being ready to write.
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If the \fIexc_fd_set\fP argument is not a null pointer, it points to an object
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of type \fBfd_set\fP that on return specifies the file descriptors to be
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checked for error conditions pending.
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If no file descriptors are set by libcurl, \fImax_fd\fP will contain -1 when
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this function returns. Otherwise it will contain the highest descriptor number
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libcurl set. When libcurl returns -1 in \fImax_fd\fP, it is because libcurl
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currently does something that is not possible for your application to monitor
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with a socket and unfortunately you can then not know exactly when the current
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action is completed using select(). You then need to wait a while before you
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proceed and call \fIcurl_multi_perform(3)\fP anyway. How long to wait? Unless
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\fIcurl_multi_timeout(3)\fP gives you a lower number, we suggest 100
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milliseconds or so, but you may want to test it out in your own particular
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conditions to find a suitable value.
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When doing select(), you should use \fIcurl_multi_timeout(3)\fP to figure out
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how long to wait for action. Call \fIcurl_multi_perform(3)\fP even if no
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activity has been seen on the \fBfd_sets\fP after the timeout expires as
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otherwise internal retries and timeouts may not work as you would think and
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want.
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If one of the sockets used by libcurl happens to be larger than what can be
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set in an \fBfd_set\fP, which on POSIX systems means that the file descriptor
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is larger than \fBFD_SETSIZE\fP, then libcurl will try to not set it. Setting
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a too large file descriptor in an \fBfd_set\fP implies an out of bounds write
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which can cause crashes, or worse. The effect of NOT storing it will possibly
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save you from the crash, but will make your program NOT wait for sockets it
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should wait for...
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.SH EXAMPLE
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.nf
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/* get file descriptors from the transfers */
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mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
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if(mc != CURLM_OK) {
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fprintf(stderr, "curl_multi_fdset() failed, code %d.\\n", mc);
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break;
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}
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/* wait for activity on one of the sockets */
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rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
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.fi
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.SH AVAILABILITY
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Added in 7.9.6
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.SH RETURN VALUE
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\fBCURLMcode\fP type, general libcurl multi interface error code. See
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\fIlibcurl-errors(3)\fP
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.SH "SEE ALSO"
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.BR curl_multi_cleanup "(3), " curl_multi_init "(3), "
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.BR curl_multi_wait "(3), "
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.BR curl_multi_timeout "(3), " curl_multi_perform "(3), " select "(2) "
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