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.Dd April 20, 2013
.Dt CK_RING_TRYDEQUEUE_SPMC 3
.Sh NAME
.Nm ck_ring_trydequeue_spmc
.Nd dequeue from bounded FIFO and allow for spurious failure
.Sh LIBRARY
Concurrency Kit (libck, \-lck)
.Sh SYNOPSIS
.In ck_ring.h
.Ft bool
.Fn ck_ring_trydequeue_spmc "ck_ring_t *ring" "ck_ring_buffer_t *buffer" "void *result"
.Sh DESCRIPTION
The
.Fn ck_ring_trydequeue_spmc 3
function attempts to dequeue a pointer from the bounded buffer
pointed to by
.Fa ring
in FIFO fashion. The pointer is stored in the pointer
pointed to by
.Fa result .
The buffer pointed to by
.Fa buffer
must be unique to
.Fa ring
and point to an array of ck_ring_buffer_t of sufficient
length (according to the power-of-2 elements in the buffer).
The decoupling of the ring from the buffer serves
to address use-cases involving multiple address spaces
and DMA, among others.
If you are on non-POSIX platforms or wish for strict
compliance with C, then it is recommended to pass a
pointer of type void ** for
.Fa result .
This function is safe to call without locking for UINT_MAX
concurrent
.Fn ck_ring_dequeue_spmc 3
or
.Fn ck_ring_trydequeue_spmc 3
invocations and up to one concurrent
.Fn ck_ring_enqueue_spmc 3
or
.Fn ck_ring_tryenqueue_spmc 3
invocation. This operation will always complete
in a bounded number of steps. It is
possible for the function to return false even
if
.Fa ring
is non-empty. This
.Sh EXAMPLE
.Bd -literal -offset indent
#include <ck_ring.h>

/* This ring was previously initialized with ck_ring_init. */
ck_ring_t ring;

/* The ring was initialized for 1023 elements. */
ck_ring_buffer_t buffer[1024];

void
dequeue(void)
{
	void *result;

	/* Dequeue from ring until contention is actively observed. */
	while (ck_ring_trydequeue_spmc(&ring, &buffer, &result) == true) {
		/*
		 * Results contains the oldest pointer in ring
		 * since the dequeue operation returned true.
		 */
		operation(result);
	}

	/* An empty ring was encountered, leave. */
	return;
}
.Ed
.Sh RETURN VALUES
The function returns true if the dequeue operation
completely successfully in a bounded number of steps.
The result of the dequeue operation is stored in the
value pointed to by
.Fa result .
Otherwise, the function will return false if the buffer was empty
or if the operation could not be completed in a bounded
number of steps. If the function returns false, then the contents
of
.Fa result
are undefined.
.Sh SEE ALSO
.Xr ck_ring_init 3 ,
.Xr ck_ring_dequeue_spmc 3 ,
.Xr ck_ring_enqueue_spmc 3 ,
.Xr ck_ring_enqueue_spmc_size 3 ,
.Xr ck_ring_dequeue_spsc 3 ,
.Xr ck_ring_enqueue_spsc 3 ,
.Xr ck_ring_enqueue_spsc_size 3 ,
.Xr ck_ring_capacity 3 ,
.Xr ck_ring_size 3
.Pp
Additional information available at http://concurrencykit.org/