curl_multi_poll - polls on all easy handles in a multi handle

Name

curl_multi_poll - poll on all easy handles in a multi handle

Synopsis

#include <curl/curl.h>
 
CURLMcode curl_multi_poll(CURLM *multi_handle,
                          struct curl_waitfd extra_fds[],
                          unsigned int extra_nfds,
                          int timeout_ms,
                          int *numfds);

Description

curl_multi_poll polls all file descriptors used by the curl easy handles contained in the given multi handle set. It blocks until activity is detected on at least one of the handles or timeout_ms has passed. Alternatively, if the multi handle has a pending internal timeout that has a shorter expiry time than timeout_ms, that shorter time is used instead to make sure timeout accuracy is reasonably kept.

The calling application may pass additional curl_waitfd structures which are similar to poll(2)'s pollfd structure to be waited on in the same call.

On completion, if numfds is non-NULL, it gets populated with the total number of file descriptors on which interesting events occurred. This number can include both libcurl internal descriptors as well as descriptors provided in extra_fds.

The curl_multi_wakeup function can be used from another thread to wake up this function and return faster. This is one of the details that makes this function different than curl_multi_wait which cannot be woken up this way.

If no extra file descriptors are provided and libcurl has no file descriptor to offer to wait for, this function instead waits during timeout_ms milliseconds (or shorter if an internal timer indicates so). This is the other detail that makes this function different than curl_multi_wait.

This function is encouraged to be used instead of select(3) when using the multi interface to allow applications to easier circumvent the common problem with 1024 maximum file descriptors.

Curl_waitfd

struct curl_waitfd {
  curl_socket_t fd;
  short events;
  short revents;
};

CURL_WAIT_POLLIN

Bit flag to curl_waitfd.events indicating the socket should poll on read events such as new data received.

CURL_WAIT_POLLPRI

Bit flag to curl_waitfd.events indicating the socket should poll on high priority read events such as out of band data.

CURL_WAIT_POLLOUT

Bit flag to curl_waitfd.events indicating the socket should poll on write events such as the socket being clear to write without blocking.

Protocols

This functionality affects all supported protocols

Example

extern void handle_fd(int);
 
int main(void)
{
  CURL *easy_handle;
  CURLM *multi_handle;
  int still_running = 0;
  int myfd; /* this is our own file descriptor */
 
  /* add the individual easy handle */
  curl_multi_add_handle(multi_handle, easy_handle);
 
  do {
    CURLMcode mc;
    int numfds;
 
    mc = curl_multi_perform(multi_handle, &still_running);
 
    if(mc == CURLM_OK) {
      struct curl_waitfd myown;
      myown.fd = myfd;
      myown.events = CURL_WAIT_POLLIN; /* wait for input */
      myown.revents = 0; /* clear it */
 
      /* wait for activity on curl's descriptors or on our own,
         or timeout */
      mc = curl_multi_poll(multi_handle, &myown, 1, 1000, &numfds);
 
      if(myown.revents) {
        /* did our descriptor receive an event? */
        handle_fd(myfd);
      }
    }
 
    if(mc != CURLM_OK) {
      fprintf(stderr, "curl_multi failed, code %d.\n", mc);
      break;
    }
 
  } while(still_running);
 
  curl_multi_remove_handle(multi_handle, easy_handle);
}

Availability

Added in curl 7.66.0

Return value

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