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.

EXAMPLE

       int main(void)
       {
         CURL *curl = curl_easy_init();
         if(curl) {
           /* pause a transfer in both directions */
           curl_easy_pause(curl, CURL_READFUNC_PAUSE | CURL_WRITEFUNC_PAUSE);

         }
       }

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 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)