curl_easy_pause(3) libcurl Manual 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.
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 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 (CURLOPT_PROGRESSFUNCTION(3)), 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 re-
turns.
The handle argument identifies the transfer you want to pause or un-
pause.
A paused transfer is excluded from low speed cancels via the CUR-
LOPT_LOW_SPEED_LIMIT(3) option and unpausing a transfer will reset 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 will be no data received on this
connection until this function is called again without this bit
set. Thus, the write callback (CURLOPT_WRITEFUNCTION(3)) will
not be called.
CURLPAUSE_SEND
Pause sending data. There will be no data sent on this connec-
tion until this function is called again without this bit set.
Thus, the read callback (CURLOPT_READFUNCTION(3)) will not be
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, will cause problems in the worst case or an error in the
best case.
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
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.
EXAMPLE
/* pause a transfer in both directions */
curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);
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 en-
tire 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 au-
tomatically.
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 7.81.0 November 26, 2021 curl_easy_pause(3)
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