curl_multi_fdset(3) libcurl Manual curl_multi_fdset(3)
NAME
curl_multi_fdset - extracts file descriptor information from a multi
handle
SYNOPSIS
#include <curl/curl.h>
CURLMcode curl_multi_fdset(CURLM *multi_handle,
fd_set *read_fd_set,
fd_set *write_fd_set,
fd_set *exc_fd_set,
int *max_fd);
DESCRIPTION
This function extracts file descriptor information from a given
multi_handle. libcurl returns its fd_set sets. The application can use
these to select() on, but be sure to FD_ZERO them before calling this
function as curl_multi_fdset(3) only adds its own descriptors, it does
not zero or otherwise remove any others. The curl_multi_perform(3)
function should be called as soon as one of them is ready to be read
from or written to.
If the read_fd_set argument is not a null pointer, it points to an ob-
ject of type fd_set that on returns specifies the file descriptors to
be checked for being ready to read.
If the write_fd_set argument is not a null pointer, it points to an ob-
ject of type fd_set that on return specifies the file descriptors to be
checked for being ready to write.
If the exc_fd_set argument is not a null pointer, it points to an ob-
ject of type fd_set that on return specifies the file descriptors to be
checked for error conditions pending.
If no file descriptors are set by libcurl, max_fd will contain -1 when
this function returns. Otherwise it will contain the highest descriptor
number libcurl set. When libcurl returns -1 in max_fd, it is because
libcurl currently does something that is not possible for your applica-
tion to monitor with a socket and unfortunately you can then not know
exactly when the current action is completed using select(). You then
need to wait a while before you proceed and call curl_multi_perform(3)
anyway. How long to wait? Unless curl_multi_timeout(3) gives you a
lower number, we suggest 100 milliseconds or so, but you may want to
test it out in your own particular conditions to find a suitable value.
When doing select(), you should use curl_multi_timeout(3) to figure out
how long to wait for action. Call curl_multi_perform(3) even if no ac-
tivity has been seen on the fd_sets after the timeout expires as other-
wise internal retries and timeouts may not work as you would think and
want.
If one of the sockets used by libcurl happens to be larger than what
can be set in an fd_set, which on POSIX systems means that the file de-
scriptor is larger than FD_SETSIZE, then libcurl will try to not set
it. Setting a too large file descriptor in an fd_set implies an out of
bounds write which can cause crashes, or worse. The effect of NOT stor-
ing it will possibly save you from the crash, but will make your pro-
gram NOT wait for sockets it should wait for...
EXAMPLE
/* get file descriptors from the transfers */
mc = curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);
if(mc != CURLM_OK) {
fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc);
break;
}
/* wait for activity on one of the sockets */
rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);
AVAILABILITY
Added in 7.9.6
RETURN VALUE
CURLMcode type, general libcurl multi interface error code. See
libcurl-errors(3)
SEE ALSO
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_wait(3),
curl_multi_timeout(3), curl_multi_perform(3), select(2)
libcurl 7.81.0 October 31, 2021 curl_multi_fdset(3)
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