1--- 2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3SPDX-License-Identifier: curl 4Title: curl_easy_pause 5Section: 3 6Source: libcurl 7See-also: 8 - curl_easy_cleanup (3) 9 - curl_easy_reset (3) 10Protocol: 11 - All 12Added-in: 7.18.0 13--- 14 15# NAME 16 17curl_easy_pause - pause and unpause a connection 18 19# SYNOPSIS 20 21~~~c 22#include <curl/curl.h> 23 24CURLcode curl_easy_pause(CURL *handle, int bitmask ); 25~~~ 26 27# DESCRIPTION 28 29Using this function, you can explicitly mark a running connection to get 30paused, and you can unpause a connection that was previously paused. Unlike 31most other libcurl functions, curl_easy_pause(3) can be used from within 32callbacks. 33 34A connection can be paused by using this function or by letting the read or 35the write callbacks return the proper magic return code 36(*CURL_READFUNC_PAUSE* and *CURL_WRITEFUNC_PAUSE*). A write callback 37that returns pause signals to the library that it could not take care of any 38data at all, and that data is then delivered again to the callback when the 39transfer is unpaused. 40 41While it may feel tempting, take care and notice that you cannot call this 42function from another thread. To unpause, you may for example call it from the 43progress callback (CURLOPT_PROGRESSFUNCTION(3)). 44 45When this function is called to unpause receiving, the write callback might 46get called before this function returns to deliver cached content. When 47libcurl delivers such cached data to the write callback, it is delivered as 48fast as possible, which may overstep the boundary set in 49CURLOPT_MAX_RECV_SPEED_LARGE(3) etc. 50 51The **handle** argument identifies the transfer you want to pause or 52unpause. 53 54A paused transfer is excluded from low speed cancels via the 55CURLOPT_LOW_SPEED_LIMIT(3) option and unpausing a transfer resets the 56time period required for the low speed limit to be met. 57 58The **bitmask** argument is a set of bits that sets the new state of the 59connection. The following bits can be used: 60 61## CURLPAUSE_RECV 62 63Pause receiving data. There is no data received on this connection until this 64function is called again without this bit set. Thus, the write callback 65(CURLOPT_WRITEFUNCTION(3)) is not called. 66 67## CURLPAUSE_SEND 68 69Pause sending data. There is no data sent on this connection until this 70function is called again without this bit set. Thus, the read callback 71(CURLOPT_READFUNCTION(3)) is not called. 72 73## CURLPAUSE_ALL 74 75Convenience define that pauses both directions. 76 77## CURLPAUSE_CONT 78 79Convenience define that unpauses both directions. 80 81# LIMITATIONS 82 83The pausing of transfers does not work with protocols that work without 84network connectivity, like FILE://. Trying to pause such a transfer, in any 85direction, might cause problems or error. 86 87# MULTIPLEXED 88 89When a connection is used multiplexed, like for HTTP/2, and one of the 90transfers over the connection is paused and the others continue flowing, 91libcurl might end up buffering contents for the paused transfer. It has to do 92this because it needs to drain the socket for the other transfers and the 93already announced window size for the paused transfer allows the server to 94continue sending data up to that window size amount. By default, libcurl 95announces a 32 megabyte window size, which thus can make libcurl end up 96buffering 32 megabyte of data for a paused stream. 97 98When such a paused stream is unpaused again, any buffered data is delivered 99first. 100 101# %PROTOCOLS% 102 103# EXAMPLE 104 105~~~c 106int main(void) 107{ 108 CURL *curl = curl_easy_init(); 109 if(curl) { 110 /* pause a transfer in both directions */ 111 curl_easy_pause(curl, CURLPAUSE_RECV | CURLPAUSE_SEND); 112 113 } 114} 115~~~ 116 117# MEMORY USE 118 119When pausing a download transfer by returning the magic return code from a 120write callback, the read data is already in libcurl's internal buffers so it 121has to keep it in an allocated buffer until the receiving is again unpaused 122using this function. 123 124If the downloaded data is compressed and is asked to get uncompressed 125automatically on download, libcurl continues to uncompress the entire 126downloaded chunk and it caches the data uncompressed. This has the side- 127effect that if you download something that is compressed a lot, it can result 128in a large data amount needing to be allocated to save the data during the 129pause. Consider not using paused receiving if you allow libcurl to uncompress 130data automatically. 131 132If the download is done with HTTP/2 or HTTP/3, there is up to a stream window 133size worth of data that curl cannot stop but instead needs to cache while the 134transfer is paused. This means that if a window size of 64 MB is used, libcurl 135might end up having to cache 64 MB of data. 136 137# %AVAILABILITY% 138 139# RETURN VALUE 140 141CURLE_OK (zero) means that the option was set properly, and a non-zero return 142code means something wrong occurred after the new state was set. See the 143libcurl-errors(3) man page for the full list with descriptions. 144
