ARES_SET_NOTIFY_PENDING_WRITE_CALLBACK(3) Introduction to Library Functions

NAME

ares_set_pending_write_cb, ares_process_pending_write - Function for
setting a callback which is triggered when there is potential pending
data which needs to be written.

SYNOPSIS

#include <ares.h>

typedef void (*ares_pending_write_cb)(void *data);

void ares_set_pending_write_cb(
ares_channel_t *channel,
ares_pending_write_cb callback,
void *user_data);

void ares_process_pending_write(ares_channel_t *channel);

DESCRIPTION

The ares_set_pending_write_cb(3) function sets a callback function
callback in the given ares channel handle channel that is invoked
whenever there is new pending TCP data to be written. Since TCP is
stream based, if there are multiple queries being enqueued back to
back they can be sent as one large buffer. Normally a send(2) syscall
operation would be triggered for each query.

When setting this callback, an event will be triggered when data is
buffered, but not written. This event is used to wake the caller's
event loop which should call ares_process_pending_write(3) using the
channel associated with the callback. Each time the callback is
triggered must result in a call to ares_process_pending_write(3) from
the caller's event loop otherwise stalls and timeouts may occur. The
callback must not call ares_process_pending_write(3) directly as
otherwise it would invalidate any advantage of this use-case.

This is considered an optimization, especially when using TLS-based
connections which add additional overhead to the data stream. Due to
the asyncronous nature of c-ares, there is no way to identify when a
caller may be finished enqueuing queries via any of the possible
public API calls such as ares_getaddrinfo(3) or
ares_search_dnsrec(3), so this is an enhancement to try to group
query send operations together and will rely on the singaling latency
involved in waking the user's event loop.

If no callback is set, data will be written immediately to the
socket, thus bypassing this optimization.

This option cannot be used with ARES_OPT_EVENT_THREAD passed to
ares_init_options(3) since the user has no event loop. This
optimization is automatically enabled when using the Event Thread as
it sets the callback for its own internal signaling.

AVAILABILITY

This function was first introduced in c-ares version 1.34.0.

SEE ALSO

ares_init_options(3)

13 Aug 2024
ARES_SET_NOTIFY_PENDING_WRITE_CALLBACK(3)