curl/docs/libcurl/curl_easy_pause.3

.\" **************************************************************************
.\" *                                  _   _ ____  _
.\" *  Project                     ___| | | |  _ \| |
.\" *                             / __| | | | |_) | |
.\" *                            | (__| |_| |  _ <| |___
.\" *                             \___|\___/|_| \_\_____|
.\" *
.\" * Copyright (C) 1998 - 2021, 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
.\" * are also available at https://curl.se/docs/copyright.html.
.\" *
.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
.\" * copies of the Software, and permit persons to whom the Software is
.\" * furnished to do so, under the terms of the COPYING file.
.\" *
.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
.\" * KIND, either express or implied.
.\" *
.\" **************************************************************************
.TH curl_easy_pause 3 "17 Dec 2007" "libcurl 7.18.0" "libcurl Manual"
.SH NAME
curl_easy_pause - pause and unpause a connection
.SH SYNOPSIS
.nf
.B #include <curl/curl.h>

.BI "CURLcode curl_easy_pause(CURL *"handle ", int "bitmask ");"
.fi
.SH DESCRIPTION
Using this function, you can explicitly mark a running connection to get
paused, and you can unpause a connection that was previously paused.

A connection can be paused by using this function or by letting the read or
the write callbacks return the proper magic return code
(\fICURL_READFUNC_PAUSE\fP and \fICURL_WRITEFUNC_PAUSE\fP). A write callback
that returns pause signals to the library that it could not take care of any
data at all, and that data will then be delivered again to the callback when
the transfer is unpaused.

While it may feel tempting, take care and notice that you cannot call this
function from another thread. To unpause, you may for example call it from the
progress callback (\fICURLOPT_PROGRESSFUNCTION(3)\fP), which gets called at
least once per second, even if the connection is paused.

When this function is called to unpause receiving, the chance is high that you
will get your write callback called before this function returns.

The \fBhandle\fP argument identifies the transfer you want to pause or
unpause.

A paused transfer is excluded from low speed cancels via the
\fICURLOPT_LOW_SPEED_LIMIT(3)\fP option and unpausing a transfer will reset
the time period required for the low speed limit to be met.

The \fBbitmask\fP argument is a set of bits that sets the new state of the
connection. The following bits can be used:
.IP CURLPAUSE_RECV
Pause receiving data. There will be no data received on this connection until
this function is called again without this bit set. Thus, the write callback
(\fICURLOPT_WRITEFUNCTION(3)\fP) will not be called.
.IP CURLPAUSE_SEND
Pause sending data. There will be no data sent on this connection until this
function is called again without this bit set. Thus, the read callback
(\fICURLOPT_READFUNCTION(3)\fP) will not be called.
.IP CURLPAUSE_ALL
Convenience define that pauses both directions.
.IP CURLPAUSE_CONT
Convenience define that unpauses both directions.
.SH LIMITATIONS
The pausing of transfers does not work with protocols that work without
network connectivity, like FILE://. Trying to pause such a transfer, in any
direction, will cause problems in the worst case or an error in the best case.
.SH MULTIPLEXED
When a connection is used multiplexed, like for HTTP/2, and one of the
transfers over the connection is paused and the others continue flowing,
libcurl might end up buffering contents for the paused transfer. It has to do
this because it needs to drain the socket for the other transfers and the
already announced window size for the paused transfer will allow the server to
continue sending data up to that window size amount. By default, libcurl
announces a 32 megabyte window size, which thus can make libcurl end up
buffering 32 megabyte of data for a paused stream.

When such a paused stream is unpaused again, any buffered data will be
delivered first.
.SH EXAMPLE
.nf
/* pause a transfer in both directions */
curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);
.fi
.SH "MEMORY USE"
When pausing a read by returning the magic return code from a write callback,
the read data is already in libcurl's internal buffers so it will have to keep
it in an allocated buffer until the receiving is again unpaused using this
function.

If the downloaded data is compressed and is asked to get uncompressed
automatically on download, libcurl will continue to uncompress the entire
downloaded chunk and it will cache the data uncompressed. This has the side-
effect that if you download something that is compressed a lot, it can result
in a large data amount needing to be allocated to save the data during the
pause. This said, you should probably consider not using paused receiving if
you allow libcurl to uncompress data automatically.
.SH AVAILABILITY
Added in 7.18.0.
.SH RETURN VALUE
CURLE_OK (zero) means that the option was set properly, and a non-zero return
code means something wrong occurred after the new state was set. See the
\fIlibcurl-errors(3)\fP man page for the full list with descriptions.
.SH "SEE ALSO"
.BR curl_easy_cleanup "(3), " curl_easy_reset "(3)"