curl_easy_pause - pause and unpause a connection
#include <curl/curl.h>
CURLcode curl_easy_pause(CURL *handle, int bitmask);
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)).
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 will be 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
CURLOPT_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
connection 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.
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.
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.
/* pause a transfer in both directions */
curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);
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.
Added in 7.18.0.
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.
curl_easy_cleanup(3),
curl_easy_reset(3)