curl_easy_pause.md (4917B)
1 --- 2 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3 SPDX-License-Identifier: curl 4 Title: curl_easy_pause 5 Section: 3 6 Source: libcurl 7 See-also: 8 - curl_easy_cleanup (3) 9 - curl_easy_reset (3) 10 Protocol: 11 - All 12 Added-in: 7.18.0 13 --- 14 15 # NAME 16 17 curl_easy_pause - pause and unpause a connection 18 19 # SYNOPSIS 20 21 ~~~c 22 #include <curl/curl.h> 23 24 CURLcode curl_easy_pause(CURL *handle, int bitmask ); 25 ~~~ 26 27 # DESCRIPTION 28 29 Using this function, you can explicitly mark a running connection to get 30 paused, and you can unpause a connection that was previously paused. Unlike 31 most other libcurl functions, curl_easy_pause(3) can be used from within 32 callbacks. 33 34 A connection can be paused by using this function or by letting the read or 35 the write callbacks return the proper magic return code 36 (*CURL_READFUNC_PAUSE* and *CURL_WRITEFUNC_PAUSE*). A write callback 37 that returns pause signals to the library that it could not take care of any 38 data at all, and that data is then delivered again to the callback when the 39 transfer is unpaused. 40 41 While it may feel tempting, take care and notice that you cannot call this 42 function from another thread. To unpause, you may for example call it from the 43 progress callback (CURLOPT_PROGRESSFUNCTION(3)). 44 45 When this function is called to unpause receiving, the write callback might 46 get called before this function returns to deliver cached content. When 47 libcurl delivers such cached data to the write callback, it is delivered as 48 fast as possible, which may overstep the boundary set in 49 CURLOPT_MAX_RECV_SPEED_LARGE(3) etc. 50 51 The **handle** argument identifies the transfer you want to pause or 52 unpause. 53 54 A paused transfer is excluded from low speed cancels via the 55 CURLOPT_LOW_SPEED_LIMIT(3) option and unpausing a transfer resets the 56 time period required for the low speed limit to be met. 57 58 The **bitmask** argument is a set of bits that sets the new state of the 59 connection. The following bits can be used: 60 61 ## CURLPAUSE_RECV 62 63 Pause receiving data. There is no data received on this connection until this 64 function is called again without this bit set. Thus, the write callback 65 (CURLOPT_WRITEFUNCTION(3)) is not called. 66 67 ## CURLPAUSE_SEND 68 69 Pause sending data. There is no data sent on this connection until this 70 function is called again without this bit set. Thus, the read callback 71 (CURLOPT_READFUNCTION(3)) is not called. 72 73 ## CURLPAUSE_ALL 74 75 Convenience define that pauses both directions. 76 77 ## CURLPAUSE_CONT 78 79 Convenience define that unpauses both directions. 80 81 # LIMITATIONS 82 83 The pausing of transfers does not work with protocols that work without 84 network connectivity, like FILE://. Trying to pause such a transfer, in any 85 direction, might cause problems or error. 86 87 # MULTIPLEXED 88 89 When a connection is used multiplexed, like for HTTP/2, and one of the 90 transfers over the connection is paused and the others continue flowing, 91 libcurl might end up buffering contents for the paused transfer. It has to do 92 this because it needs to drain the socket for the other transfers and the 93 already announced window size for the paused transfer allows the server to 94 continue sending data up to that window size amount. By default, libcurl 95 announces a 32 megabyte window size, which thus can make libcurl end up 96 buffering 32 megabyte of data for a paused stream. 97 98 When such a paused stream is unpaused again, any buffered data is delivered 99 first. 100 101 # %PROTOCOLS% 102 103 # EXAMPLE 104 105 ~~~c 106 int 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 118 119 When pausing a download transfer by returning the magic return code from a 120 write callback, the read data is already in libcurl's internal buffers so it 121 has to keep it in an allocated buffer until the receiving is again unpaused 122 using this function. 123 124 If the downloaded data is compressed and is asked to get uncompressed 125 automatically on download, libcurl continues to uncompress the entire 126 downloaded chunk and it caches the data uncompressed. This has the side- 127 effect that if you download something that is compressed a lot, it can result 128 in a large data amount needing to be allocated to save the data during the 129 pause. Consider not using paused receiving if you allow libcurl to uncompress 130 data automatically. 131 132 If the download is done with HTTP/2 or HTTP/3, there is up to a stream window 133 size worth of data that curl cannot stop but instead needs to cache while the 134 transfer is paused. This means that if a window size of 64 MB is used, libcurl 135 might end up having to cache 64 MB of data. 136 137 # %AVAILABILITY% 138 139 # RETURN VALUE 140 141 This function returns a CURLcode indicating success or error. 142 143 CURLE_OK (0) means everything was OK, non-zero means an error occurred, see 144 libcurl-errors(3). If CURLOPT_ERRORBUFFER(3) was set with curl_easy_setopt(3) 145 there can be an error message stored in the error buffer when non-zero is 146 returned.