Tribblix: manual page: curl_easy

curl_easy_pause(3) Introduction to Library Functions 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
content. 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
CURLOPT_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
connection 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
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 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.

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.