curl_multi_fdset(3) Library Functions Manual curl_multi_fdset(3)
NAME
curl_multi_fdset - extract file descriptor information from a multi han-
dle
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) func-
tion should be called as soon as one of them is ready to be read from or
written to.
The read_fd_set argument should point to an object of type fd_set that
on returns specifies the file descriptors to be checked for being ready
to read.
The write_fd_set argument should point to an object of type fd_set that
on return specifies the file descriptors to be checked for being ready
to write.
The exc_fd_set argument should point to an object of type fd_set that on
return specifies the file descriptors to be checked for error condi-
tions.
If no file descriptors are set by libcurl, max_fd contain -1 when this
function returns. Otherwise it contains 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 application 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 descrip-
tor is larger than FD_SETSIZE, then libcurl tries 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 storing it might
possibly save you from the crash, but makes your program NOT wait for
sockets it should wait for...
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
fd_set fdread;
fd_set fdwrite;
fd_set fdexcep;
int maxfd;
int rc;
CURLMcode mc;
struct timeval timeout = {1, 0};
CURLM *multi = curl_multi_init();
do {
/* call curl_multi_perform() */
FD_ZERO(&fdread);
FD_ZERO(&fdwrite);
FD_ZERO(&fdexcep);
/* get file descriptors from the transfers */
mc = curl_multi_fdset(multi, &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);
} while(!mc);
}
AVAILABILITY
Added in curl 7.9.6
RETURN VALUE
This function returns a CURLMcode indicating success or error.
CURLM_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
curl_multi_cleanup(3), curl_multi_init(3), curl_multi_perform(3),
curl_multi_timeout(3), curl_multi_wait(3), curl_multi_waitfds(3), se-
lect(2)
libcurl 2026-07-12 curl_multi_fdset(3)
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