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 17Added-in: 7.9.6 18--- 19 20# NAME 21 22curl_multi_fdset - extract file descriptor information from a multi handle 23 24# SYNOPSIS 25 26~~~c 27#include <curl/curl.h> 28 29CURLMcode curl_multi_fdset(CURLM *multi_handle, 30 fd_set *read_fd_set, 31 fd_set *write_fd_set, 32 fd_set *exc_fd_set, 33 int *max_fd); 34~~~ 35 36# DESCRIPTION 37 38This function extracts file descriptor information from a given multi_handle. 39libcurl returns its *fd_set* sets. The application can use these to 40select() on, but be sure to *FD_ZERO* them before calling this function as 41curl_multi_fdset(3) only adds its own descriptors, it does not zero or 42otherwise remove any others. The curl_multi_perform(3) function should 43be called as soon as one of them is ready to be read from or written to. 44 45The *read_fd_set* argument should point to an object of type **fd_set** 46that on returns specifies the file descriptors to be checked for being ready 47to read. 48 49The *write_fd_set* argument should point to an object of type **fd_set** 50that on return specifies the file descriptors to be checked for being ready to 51write. 52 53The *exc_fd_set* argument should point to an object of type **fd_set** 54that on return specifies the file descriptors to be checked for error 55conditions. 56 57If no file descriptors are set by libcurl, *max_fd* contain -1 when this 58function returns. Otherwise it contains the highest descriptor number libcurl 59set. When libcurl returns -1 in *max_fd*, it is because libcurl currently 60does something that is not possible for your application to monitor with a 61socket and unfortunately you can then not know exactly when the current action 62is completed using select(). You then need to wait a while before you proceed 63and call curl_multi_perform(3) anyway. How long to wait? Unless 64curl_multi_timeout(3) gives you a lower number, we suggest 100 65milliseconds or so, but you may want to test it out in your own particular 66conditions to find a suitable value. 67 68When doing select(), you should use curl_multi_timeout(3) to figure out 69how long to wait for action. Call curl_multi_perform(3) even if no 70activity has been seen on the **fd_sets** after the timeout expires as 71otherwise internal retries and timeouts may not work as you would think and 72want. 73 74If one of the sockets used by libcurl happens to be larger than what can be 75set in an **fd_set**, which on POSIX systems means that the file descriptor 76is larger than **FD_SETSIZE**, then libcurl tries to not set it. Setting a 77too large file descriptor in an **fd_set** implies an out of bounds write 78which can cause crashes, or worse. The effect of NOT storing it might possibly 79save you from the crash, but makes your program NOT wait for sockets it should 80wait for... 81 82# %PROTOCOLS% 83 84# EXAMPLE 85 86~~~c 87int main(void) 88{ 89 fd_set fdread; 90 fd_set fdwrite; 91 fd_set fdexcep; 92 int maxfd; 93 int rc; 94 CURLMcode mc; 95 struct timeval timeout = {1, 0}; 96 97 CURLM *multi = curl_multi_init(); 98 99 do { 100 101 /* call curl_multi_perform() */ 102 103 /* get file descriptors from the transfers */ 104 mc = curl_multi_fdset(multi, &fdread, &fdwrite, &fdexcep, &maxfd); 105 106 if(mc != CURLM_OK) { 107 fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc); 108 break; 109 } 110 111 /* wait for activity on one of the sockets */ 112 rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout); 113 114 } while(!mc); 115} 116~~~ 117 118# %AVAILABILITY% 119 120# RETURN VALUE 121 122**CURLMcode** type, general libcurl multi interface error code. See 123libcurl-errors(3) 124
