curl_multi_poll

Section: libcurl Manual (3)
Updated: November 05, 2020
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NAME

curl_multi_poll - polls 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(3) polls all file descriptors used by the curl easy handles contained in the given multi handle set. It will block 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 will be 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 will be 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(3) 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(3) 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 will instead wait 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(3).

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.

EXAMPLE

CURL *easy_handle;
CURLM *multi_handle;

/* 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) {
    /* wait for activity or timeout */
    mc = curl_multi_poll(multi_handle, NULL, 0, 1000, &numfds);
  }

  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);

RETURN VALUE

CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)

AVAILABILITY

This function was added in libcurl 7.66.0.