1--- 2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3SPDX-License-Identifier: curl 4Title: curl_multi_fdset 5Section: 3 6Source: libcurl 7See-also: 8 - curl_multi_cleanup (3) 9 - curl_multi_init (3) 10 - curl_multi_perform (3) 11 - curl_multi_timeout (3) 12 - curl_multi_wait (3) 13 - curl_multi_waitfds (3) 14 - select (2) 15Protocol: 16 - All 17--- 18 19# NAME 20 21curl_multi_fdset - extracts file descriptor information from a multi handle 22 23# SYNOPSIS 24 25~~~c 26#include <curl/curl.h> 27 28CURLMcode curl_multi_fdset(CURLM *multi_handle, 29 fd_set *read_fd_set, 30 fd_set *write_fd_set, 31 fd_set *exc_fd_set, 32 int *max_fd); 33~~~ 34 35# DESCRIPTION 36 37This function extracts file descriptor information from a given multi_handle. 38libcurl returns its *fd_set* sets. The application can use these to 39select() on, but be sure to *FD_ZERO* them before calling this function as 40curl_multi_fdset(3) only adds its own descriptors, it does not zero or 41otherwise remove any others. The curl_multi_perform(3) function should 42be called as soon as one of them is ready to be read from or written to. 43 44The *read_fd_set* argument should point to an object of type **fd_set** 45that on returns specifies the file descriptors to be checked for being ready 46to read. 47 48The *write_fd_set* argument should point to an object of type **fd_set** 49that on return specifies the file descriptors to be checked for being ready to 50write. 51 52The *exc_fd_set* argument should point to an object of type **fd_set** 53that on return specifies the file descriptors to be checked for error 54conditions. 55 56If no file descriptors are set by libcurl, *max_fd* contain -1 when this 57function returns. Otherwise it contains the highest descriptor number libcurl 58set. When libcurl returns -1 in *max_fd*, it is because libcurl currently 59does something that is not possible for your application to monitor with a 60socket and unfortunately you can then not know exactly when the current action 61is completed using select(). You then need to wait a while before you proceed 62and call curl_multi_perform(3) anyway. How long to wait? Unless 63curl_multi_timeout(3) gives you a lower number, we suggest 100 64milliseconds or so, but you may want to test it out in your own particular 65conditions to find a suitable value. 66 67When doing select(), you should use curl_multi_timeout(3) to figure out 68how long to wait for action. Call curl_multi_perform(3) even if no 69activity has been seen on the **fd_sets** after the timeout expires as 70otherwise internal retries and timeouts may not work as you would think and 71want. 72 73If one of the sockets used by libcurl happens to be larger than what can be 74set in an **fd_set**, which on POSIX systems means that the file descriptor 75is larger than **FD_SETSIZE**, then libcurl tries to not set it. Setting a 76too large file descriptor in an **fd_set** implies an out of bounds write 77which can cause crashes, or worse. The effect of NOT storing it might possibly 78save you from the crash, but makes your program NOT wait for sockets it should 79wait for... 80 81# EXAMPLE 82 83~~~c 84int main(void) 85{ 86 fd_set fdread; 87 fd_set fdwrite; 88 fd_set fdexcep; 89 int maxfd; 90 int rc; 91 CURLMcode mc; 92 struct timeval timeout = {1, 0}; 93 94 CURLM *multi = curl_multi_init(); 95 96 do { 97 98 /* call curl_multi_perform() */ 99 100 /* get file descriptors from the transfers */ 101 mc = curl_multi_fdset(multi, &fdread, &fdwrite, &fdexcep, &maxfd); 102 103 if(mc != CURLM_OK) { 104 fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc); 105 break; 106 } 107 108 /* wait for activity on one of the sockets */ 109 rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout); 110 111 } while(!mc); 112} 113~~~ 114 115# AVAILABILITY 116 117Added in 7.9.6 118 119# RETURN VALUE 120 121**CURLMcode** type, general libcurl multi interface error code. See 122libcurl-errors(3) 123
