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curl_easy_pause(3) libcurl curl_easy_pause(3)
NAME
curl_easy_pause - pause and unpause a connection
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_pause(CURL *handle, int bitmask);
DESCRIPTION
Using this function, you can explicitly mark a running connection to
get paused, and you can unpause a connection that was previously
paused. Unlike most other libcurl functions, curl_easy_pause(3) can be
used from within callbacks.
A connection can be paused by using this function or by letting the
read or the write callbacks return the proper magic return code
(CURL_READFUNC_PAUSE and CURL_WRITEFUNC_PAUSE). A write callback that
returns pause signals to the library that it could not take care of any
data at all, and that data is then 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 (CURLOPT_PROGRESSFUNCTION(3)).
When this function is called to unpause receiving, the write callback
might get called before this function returns to deliver cached con-
tent. When libcurl delivers such cached data to the write callback, it
is delivered as fast as possible, which may overstep the boundary set
in CURLOPT_MAX_RECV_SPEED_LARGE(3) etc.
The handle argument identifies the transfer you want to pause or
unpause.
A paused transfer is excluded from low speed cancels via the CUR-
LOPT_LOW_SPEED_LIMIT(3) option and unpausing a transfer resets the time
period required for the low speed limit to be met.
The bitmask argument is a set of bits that sets the new state of the
connection. The following bits can be used:
CURLPAUSE_RECV
Pause receiving data. There is no data received on this connec-
tion until this function is called again without this bit set.
Thus, the write callback (CURLOPT_WRITEFUNCTION(3)) is not
called.
CURLPAUSE_SEND
Pause sending data. There is no data sent on this connection
until this function is called again without this bit set. Thus,
the read callback (CURLOPT_READFUNCTION(3)) is not called.
CURLPAUSE_ALL
Convenience define that pauses both directions.
CURLPAUSE_CONT
Convenience define that unpauses both directions.
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, might cause problems or error.
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 flow-
ing, 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
allows 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 is
delivered first.
EXAMPLE
/* pause a transfer in both directions */
curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);
MEMORY USE
When pausing a download transfer by returning the magic return code
from a write callback, the read data is already in libcurl's internal
buffers so it has 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 continues to uncompress the entire
downloaded chunk and it caches 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. consider not using paused receiving if you
allow libcurl to uncompress data automatically.
If the download is done with HTTP/2 or HTTP/3, there is up to a stream
window size worth of data that curl cannot stop but instead needs to
cache while the transfer is paused. This means that if a window size of
64 MB is used, libcurl might end up having to cache 64 MB of data.
AVAILABILITY
Added in 7.18.0.
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 libcurl-errors(3) man page for the full list with descriptions.
SEE ALSO
curl_easy_cleanup(3), curl_easy_reset(3)
libcurl 8.4.0 October 06, 2023 curl_easy_pause(3)
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