Tribblix: manual page: curl_multi

curl_multi_fdset(3) Introduction to Library Functions curl_multi_fdset(3)

NAME

curl_multi_fdset - extract 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.

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
conditions.

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 activity has been seen on the fd_sets after the timeout expires as
otherwise 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
descriptor 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() */

/* 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).