1curl_multi_fdset(3) libcurl curl_multi_fdset(3)
2
6 curl_multi_fdset - extracts file descriptor information from a multi
7 handle
8
10 #include <curl/curl.h>
11 CURLMcode curl_multi_fdset(CURLM *multi_handle,
13 fd_set *read_fd_set,
14 fd_set *write_fd_set,
15 fd_set *exc_fd_set,
16 int *max_fd);
17
19 This function extracts file descriptor information from a given
20 multi_handle. libcurl returns its fd_set sets. The application can use
21 these to select() on, but be sure to FD_ZERO them before calling this
22 function as curl_multi_fdset(3) only adds its own descriptors, it does
23 not zero or otherwise remove any others. The curl_multi_perform(3)
24 function should be called as soon as one of them is ready to be read
25 from or written to.
26 If the read_fd_set argument is not a null pointer, it points to an ob‐
28 ject of type fd_set that on returns specifies the file descriptors to
29 be checked for being ready to read.
30 If the write_fd_set argument is not a null pointer, it points to an ob‐
32 ject of type fd_set that on return specifies the file descriptors to be
33 checked for being ready to write.
34 If the exc_fd_set argument is not a null pointer, it points to an ob‐
36 ject of type fd_set that on return specifies the file descriptors to be
37 checked for error conditions pending.
38 If no file descriptors are set by libcurl, max_fd will contain -1 when
40 this function returns. Otherwise it will contain the highest descriptor
41 number libcurl set. When libcurl returns -1 in max_fd, it is because
42 libcurl currently does something that is not possible for your applica‐
43 tion to monitor with a socket and unfortunately you can then not know
44 exactly when the current action is completed using select(). You then
45 need to wait a while before you proceed and call curl_multi_perform(3)
46 anyway. How long to wait? Unless curl_multi_timeout(3) gives you a
47 lower number, we suggest 100 milliseconds or so, but you may want to
48 test it out in your own particular conditions to find a suitable value.
49 When doing select(), you should use curl_multi_timeout(3) to figure out
51 how long to wait for action. Call curl_multi_perform(3) even if no ac‐
52 tivity has been seen on the fd_sets after the timeout expires as other‐
53 wise internal retries and timeouts may not work as you would think and
54 want.
55 If one of the sockets used by libcurl happens to be larger than what
57 can be set in an fd_set, which on POSIX systems means that the file de‐
58 scriptor is larger than FD_SETSIZE, then libcurl will try to not set
59 it. Setting a too large file descriptor in an fd_set implies an out of
60 bounds write which can cause crashes, or worse. The effect of NOT stor‐
61 ing it will possibly save you from the crash, but will make your pro‐
62 gram NOT wait for sockets it should wait for...
63
65 /* get file descriptors from the transfers */
66 mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
67 if(mc != CURLM_OK) {
69 fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc);
70 break;
71 }
72 /* wait for activity on one of the sockets */
74 rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
75
77 Added in 7.9.6
78
80 CURLMcode type, general libcurl multi interface error code. See
81 libcurl-errors(3)
82
84 curl_multi_cleanup(3), curl_multi_init(3), curl_multi_wait(3),
85 curl_multi_timeout(3), curl_multi_perform(3), select(2)
86libcurl 8.2.1 April 26, 2023 curl_multi_fdset(3)