curl_multi_waitfds - extracts file descriptors from multi handle

Name

curl_multi_waitfds - extracts 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 descriptors in a fashion similar to curl_multi_poll. The curl_multi_perform 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 descriptors 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 returns 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.

Protocols

All

Example

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.
    * Note that a better approach can be used to minimize allocations and
    * deallocations, if needed, like pre-allocated or grow-only array.
    */
    ufds = (struct curl_waitfd*)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, NULL);
 
    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 8.8.0

Return value

CURLMcode type, general libcurl multi interface error code. See libcurl-errors