curl_multi_waitfds(3) Library Functions Manual curl_multi_waitfds(3)
NAME
curl_multi_waitfds - extract file descriptor information from a multi
handle
SYNOPSIS
#include <curl/curl.h>
#include <stdlib.h>
CURLMcode curl_multi_waitfds(CURLM *multi,
struct curl_waitfd *ufds,
unsigned int size,
unsigned int *fd_count);
DESCRIPTION
This function extracts curl_waitfd structures which are similar to
poll(2)'s pollfd structure from a given multi_handle.
These structures can be used for polling on multi_handle file descrip-
tors in a fashion similar to curl_multi_poll(3). The curl_multi_per-
form(3) function should be called as soon as one of them is ready to be
read from or written to.
libcurl fills provided ufds array up to the size. If a number of de-
scriptors used by the multi_handle is greater than the size parameter
then libcurl returns CURLM_OUT_OF_MEMORY error.
If the fd_count argument is not a null pointer, it points to a variable
that on return specifies the number of descriptors used by the
multi_handle to be checked for being ready to read or write.
The client code can pass size equal to zero just to get the number of
the descriptors and allocate appropriate storage for them to be used in
a subsequent function call. In this case, fd_count receives a number
greater than or equal to the number of descriptors.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
#include <stdlib.h>
int main(void)
{
CURLMcode mc;
struct curl_waitfd *ufds;
CURLM *multi = curl_multi_init();
do {
/* call curl_multi_perform() */
/* get the count of file descriptors from the transfers */
unsigned int fd_count = 0;
mc = curl_multi_waitfds(multi, NULL, 0, &fd_count);
if(mc != CURLM_OK) {
fprintf(stderr, "curl_multi_waitfds() failed, code %d.\n", mc);
break;
}
if(!fd_count)
continue; /* no descriptors yet */
/* allocate storage for our descriptors */
ufds = malloc(fd_count * sizeof(struct curl_waitfd));
/* get wait descriptors from the transfers and put them into array. */
mc = curl_multi_waitfds(multi, ufds, fd_count, &fd_count);
if(mc != CURLM_OK) {
fprintf(stderr, "curl_multi_waitfds() failed, code %d.\n", mc);
free(ufds);
break;
}
/* Do polling on descriptors in ufds */
free(ufds);
} while(!mc);
}
AVAILABILITY
Added in curl 8.8.0
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_fdset(3), curl_multi_perform(3), curl_multi_poll(3),
curl_multi_wait(3)
libcurl 2026-07-12 curl_multi_waitfds(3)
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