curl_easy_pause(3) libcurl Manual curl_easy_pause(3)NAMEcurl_easy_pause - pause and unpause a connection
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
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 couldn't take care of any
data at all, and that data will then be delivered again to the callback
when the writing is later 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 (see curl_easy_setopt(3)'s CURLOPT_PRO‐
GRESSFUNCTION), which gets called at least once per second, even if the
connection is paused.
When this function is called to unpause reading, the chance is high
that you will get your write callback called before this function
The handle argument is of course identifying the handle that operates
on the connection you want to pause or unpause.
The bitmask argument is a set of bits that sets the new state of the
connection. The following bits can be used:
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) won't be
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) won't be called.
Convenience define that pauses both directions.
Convenience define that unpauses both directions
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.
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
This function was added in libcurl 7.18.0. Before this version, there
was no explicit support for pausing transfers.
USAGE WITH THE MULTI-SOCKET INTERFACE
Before libcurl 7.32.0, when a specific handle was unpaused with this
function, there was no particular forced rechecking or similar of the
socket's state, which made the continuation of the transfer get delayed
until next multi-socket call invoke or even longer. Alternatively, the
user could forcibly call for example curl_multi_socket_all(3) - with a
rather hefty performance penalty.
Starting in libcurl 7.32.0, unpausing a transfer will schedule a time‐
out trigger for that handle 1 millisecond into the future, so that a
curl_multi_socket_action( ... CURL_SOCKET_TIMEOUT) can be used immedi‐
ately afterwards to get the transfer going again as desired.
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'll have to keep it in an allocated buffer until the reading 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 very large data amount needing to be allo‐
cated to save the data during the pause. This said, you should probably
consider not using paused reading if you allow libcurl to uncompress
SEE ALSOcurl_easy_cleanup(3), curl_easy_reset(3)libcurl 7.18.0 17 Dec 2007 curl_easy_pause(3)