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123 lines
3.8 KiB
Markdown
123 lines
3.8 KiB
Markdown
---
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c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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Title: curl_multi_fdset
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Section: 3
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Source: libcurl
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See-also:
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- curl_multi_cleanup (3)
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- curl_multi_init (3)
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- curl_multi_perform (3)
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- curl_multi_timeout (3)
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- curl_multi_wait (3)
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- curl_multi_waitfds (3)
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- select (2)
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Protocol:
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- All
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---
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# NAME
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curl_multi_fdset - extracts file descriptor information from a multi handle
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# SYNOPSIS
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~~~c
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#include <curl/curl.h>
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CURLMcode curl_multi_fdset(CURLM *multi_handle,
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fd_set *read_fd_set,
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fd_set *write_fd_set,
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fd_set *exc_fd_set,
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int *max_fd);
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~~~
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# DESCRIPTION
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This function extracts file descriptor information from a given multi_handle.
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libcurl returns its *fd_set* sets. The application can use these to
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select() on, but be sure to *FD_ZERO* them before calling this function as
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curl_multi_fdset(3) only adds its own descriptors, it does not zero or
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otherwise remove any others. The curl_multi_perform(3) function should
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be called as soon as one of them is ready to be read from or written to.
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The *read_fd_set* argument should point to an object of type **fd_set**
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that on returns specifies the file descriptors to be checked for being ready
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to read.
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The *write_fd_set* argument should point to an object of type **fd_set**
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that on return specifies the file descriptors to be checked for being ready to
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write.
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The *exc_fd_set* argument should point to an object of type **fd_set**
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that on return specifies the file descriptors to be checked for error
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conditions.
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If no file descriptors are set by libcurl, *max_fd* contain -1 when this
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function returns. Otherwise it contains the highest descriptor number libcurl
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set. When libcurl returns -1 in *max_fd*, it is because libcurl currently
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does something that is not possible for your application to monitor with a
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socket and unfortunately you can then not know exactly when the current action
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is completed using select(). You then need to wait a while before you proceed
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and call curl_multi_perform(3) anyway. How long to wait? Unless
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curl_multi_timeout(3) gives you a lower number, we suggest 100
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milliseconds or so, but you may want to test it out in your own particular
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conditions to find a suitable value.
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When doing select(), you should use curl_multi_timeout(3) to figure out
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how long to wait for action. Call curl_multi_perform(3) even if no
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activity has been seen on the **fd_sets** after the timeout expires as
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otherwise internal retries and timeouts may not work as you would think and
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want.
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If one of the sockets used by libcurl happens to be larger than what can be
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set in an **fd_set**, which on POSIX systems means that the file descriptor
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is larger than **FD_SETSIZE**, then libcurl tries to not set it. Setting a
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too large file descriptor in an **fd_set** implies an out of bounds write
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which can cause crashes, or worse. The effect of NOT storing it might possibly
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save you from the crash, but makes your program NOT wait for sockets it should
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wait for...
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# EXAMPLE
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~~~c
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int main(void)
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{
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fd_set fdread;
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fd_set fdwrite;
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fd_set fdexcep;
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int maxfd;
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int rc;
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CURLMcode mc;
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struct timeval timeout = {1, 0};
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CURLM *multi = curl_multi_init();
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do {
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/* call curl_multi_perform() */
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/* get file descriptors from the transfers */
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mc = curl_multi_fdset(multi, &fdread, &fdwrite, &fdexcep, &maxfd);
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if(mc != CURLM_OK) {
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fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc);
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break;
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}
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/* wait for activity on one of the sockets */
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rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
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} while(!mc);
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}
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~~~
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# AVAILABILITY
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Added in 7.9.6
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# RETURN VALUE
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**CURLMcode** type, general libcurl multi interface error code. See
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libcurl-errors(3)
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