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.Dd July 26, 2013.
.Dt ck_sequence 3
.Sh NAME
.Nm ck_sequence
.Nd sequence locks
.Sh LIBRARY
Concurrency Kit (libck, \-lck)
.Sh SYNOPSIS
.In ck_sequence.h
.Pp
.Dv ck_sequence_t seqlock = CK_SEQUENCE_INITIALIZER;
.Pp
.Ft void
.Fn ck_sequence_init "ck_sequence_t *sq"
.Ft unsigned int
.Fn ck_sequence_read_begin "ck_sequence_t *sq"
.Ft bool
.Fn ck_sequence_read_retry "ck_sequence_t *sq" "unsigned int version"
.Ft void
.Fn ck_sequence_write_begin "ck_sequence_t *sq"
.Ft void
.Fn ck_sequence_write_end "ck_sequence_t *sq"
.Sh DESCRIPTION
It is recommended to use ck_sequence when a small amount of data that cannot be
accessed atomically has to be synchronized with readers in a fashion that does
not block any writer. Readers are able to execute their read-side critical
sections without any atomic operations. A ck_sequence_t must be initialized
before use. It may be initialized using either a static initializer
(CK_SEQUENCE_INITIALIZER) or using
.Fn ck_sequence_init .
Before readers attempt to
read data that may be concurrently modified they must first save the return
value of
.Fn ck_sequence_read_begin .
While or after a reader has completed copying
the data associated with a ck_sequence_t it must pass the earlier return value
of
.Fn ck_sequence_read_begin
to
.Fn ck_sequence_read_retry. If
.Fn ck_sequence_read_retry
returns true then the copy of data may be inconsistent and the read process
must be retried. Writers must rely on their own synchronization primitives.
Once a writer has entered its respective critical section, it must call
.Fn ck_sequence_write_begin
to signal intent to update the data protected
by the ck_sequence_t. Before the writer leaves its critical section it must
execute
.Fn ck_sequence_write_end
to indicate that the updates have left respective objects in a consistent state.
.Sh EXAMPLE
.Bd -literal -offset indent
#include <ck_sequence.h>
#include <stdlib.h>

static struct example {
	int a;
	int b;
	int c;
} global;

static ck_sequence_t seqlock = CK_SEQUENCE_INITIALIZER;

void
reader(void)
{
	struct example copy;
	unsigned int version;

	/*
	 * Attempt a read of the data structure. If the structure
	 * has been modified between ck_sequence_read_begin and
	 * ck_sequence_read_retry then attempt another read since
	 * the data may be in an inconsistent state.
	 */
	do {
		version = ck_sequence_read_begin(&seqlock);
		copy = global;
	} while (ck_sequence_read_retry(&seqlock, version));

	/*
	 * The previous may also be expressed using CK_SEQUENCE_READ.
	 * Generally recommend to only use ck_sequence_read_retry
	 * if you would like to detect a conflicting write at some
	 * higher granularity.
	 */
	CK_SEQUENCE_READ(&seqlock, version) {
		copy = global;
	}

	return;
}

void
writer(void)
{

	for (;;) {
		ck_sequence_write_begin(&seqlock);
		global.a = rand();
		global.b = global.a + global.b;
		global.c = global.b + global.c;
		ck_sequence_write_end(&seqlock);
	}

	return;
}
.Ed
.Sh SEE ALSO
.Xr ck_bytelock 3 ,
.Xr ck_rwlock 3
.Pp
Additional information available at http://concurrencykit.org/