curl_easy_pause(3) Library Functions 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. Un-
like 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_READ-
FUNC_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 content.
When libcurl delivers such cached data to the write callback, it is de-
livered as fast as possible, which may overstep the boundary set in CUR-
LOPT_MAX_RECV_SPEED_LARGE(3) etc.
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 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 un-
til 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 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 allows the
server to continue sending data up to that window size amount. By de-
fault, 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 deliv-
ered first.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
/* pause a transfer in both directions */
curl_easy_pause(curl, CURLPAUSE_RECV | CURLPAUSE_SEND);
}
}
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 curl 7.18.0
RETURN VALUE
This function returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3). If CURLOPT_ERRORBUFFER(3) was set with
curl_easy_setopt(3) there can be an error message stored in the error
buffer when non-zero is returned.
SEE ALSO
curl_easy_cleanup(3), curl_easy_reset(3)
libcurl 2026-07-12 curl_easy_pause(3)
Generated by dwww version 1.16 on Sat Oct 3 06:25:43 CEST 2026.