132 lines
5.7 KiB
Groff
132 lines
5.7 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) 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_easy_pause 3 "17 Dec 2007" "libcurl" "libcurl"
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.SH NAME
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curl_easy_pause - pause and unpause a connection
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.SH SYNOPSIS
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.nf
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#include <curl/curl.h>
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CURLcode curl_easy_pause(CURL *handle, int bitmask );
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.fi
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.SH DESCRIPTION
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Using this function, you can explicitly mark a running connection to get
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paused, and you can unpause a connection that was previously paused. Unlike
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most other libcurl functions, \fIcurl_easy_pause(3)\fP can be used from within
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callbacks.
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A connection can be paused by using this function or by letting the read or
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the write callbacks return the proper magic return code
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(\fICURL_READFUNC_PAUSE\fP and \fICURL_WRITEFUNC_PAUSE\fP). A write callback
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that returns pause signals to the library that it could not take care of any
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data at all, and that data is then delivered again to the callback when the
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transfer is unpaused.
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While it may feel tempting, take care and notice that you cannot call this
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function from another thread. To unpause, you may for example call it from the
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progress callback (\fICURLOPT_PROGRESSFUNCTION(3)\fP).
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When this function is called to unpause receiving, the write callback might
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get called before this function returns to deliver cached content. When
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libcurl delivers such cached data to the write callback, it is delivered as
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fast as possible, which may overstep the boundary set in
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\fICURLOPT_MAX_RECV_SPEED_LARGE(3)\fP etc.
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The \fBhandle\fP argument identifies the transfer you want to pause or
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unpause.
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A paused transfer is excluded from low speed cancels via the
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\fICURLOPT_LOW_SPEED_LIMIT(3)\fP option and unpausing a transfer resets the
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time period required for the low speed limit to be met.
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The \fBbitmask\fP argument is a set of bits that sets the new state of the
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connection. The following bits can be used:
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.IP CURLPAUSE_RECV
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Pause receiving data. There is no data received on this connection until this
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function is called again without this bit set. Thus, the write callback
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(\fICURLOPT_WRITEFUNCTION(3)\fP) is not called.
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.IP CURLPAUSE_SEND
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Pause sending data. There is no data sent on this connection until this
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function is called again without this bit set. Thus, the read callback
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(\fICURLOPT_READFUNCTION(3)\fP) is not called.
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.IP CURLPAUSE_ALL
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Convenience define that pauses both directions.
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.IP CURLPAUSE_CONT
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Convenience define that unpauses both directions.
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.SH LIMITATIONS
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The pausing of transfers does not work with protocols that work without
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network connectivity, like FILE://. Trying to pause such a transfer, in any
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direction, might cause problems or error.
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.SH MULTIPLEXED
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When a connection is used multiplexed, like for HTTP/2, and one of the
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transfers over the connection is paused and the others continue flowing,
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libcurl might end up buffering contents for the paused transfer. It has to do
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this because it needs to drain the socket for the other transfers and the
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already announced window size for the paused transfer allows the server to
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continue sending data up to that window size amount. By default, libcurl
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announces a 32 megabyte window size, which thus can make libcurl end up
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buffering 32 megabyte of data for a paused stream.
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When such a paused stream is unpaused again, any buffered data is delivered
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first.
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.SH EXAMPLE
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.nf
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int main(void)
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{
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CURL *curl = curl_easy_init();
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if(curl) {
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/* pause a transfer in both directions */
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curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);
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}
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}
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.fi
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.SH "MEMORY USE"
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When pausing a download transfer by returning the magic return code from a
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write callback, the read data is already in libcurl's internal buffers so it
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has to keep it in an allocated buffer until the receiving is again unpaused
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using this function.
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If the downloaded data is compressed and is asked to get uncompressed
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automatically on download, libcurl continues to uncompress the entire
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downloaded chunk and it caches the data uncompressed. This has the side-
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effect that if you download something that is compressed a lot, it can result
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in a large data amount needing to be allocated to save the data during the
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pause. consider not using paused receiving if you allow libcurl to uncompress
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data automatically.
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If the download is done with HTTP/2 or HTTP/3, there is up to a stream window
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size worth of data that curl cannot stop but instead needs to cache while the
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transfer is paused. This means that if a window size of 64 MB is used, libcurl
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might end up having to cache 64 MB of data.
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.SH AVAILABILITY
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Added in 7.18.0.
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.SH RETURN VALUE
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CURLE_OK (zero) means that the option was set properly, and a non-zero return
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code means something wrong occurred after the new state was set. See the
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\fIlibcurl-errors(3)\fP man page for the full list with descriptions.
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.SH "SEE ALSO"
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.BR curl_easy_cleanup (3),
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.BR curl_easy_reset (3)
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