ck_sequence(3) | Library Functions Manual | ck_sequence(3) |
NAME
ck_sequence_init
,
ck_sequence_read_begin
,
ck_sequence_read_retry
,
ck_sequence_write_begin
,
ck_sequence_write_end
—
sequence locks
LIBRARY
Concurrency Kit (libck, -lck)
SYNOPSIS
#include
<ck_sequence.h>
ck_sequence_t seqlock =
CK_SEQUENCE_INITIALIZER;
void
ck_sequence_init
(ck_sequence_t
*sq);
unsigned int
ck_sequence_read_begin
(const
ck_sequence_t *sq);
bool
ck_sequence_read_retry
(const
ck_sequence_t *sq,
unsigned int
version);
void
ck_sequence_write_begin
(ck_sequence_t
*sq);
void
ck_sequence_write_end
(ck_sequence_t
*sq);
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
ck_sequence_init
().
Before readers attempt to read data that may be concurrently modified they
must first save the return value of
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
ck_sequence_read_begin
() to
ck_sequence_read_retry
().
If 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
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
ck_sequence_write_end
()
to indicate that the updates have left respective objects in a consistent
state.
EXAMPLE
#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; }
SEE ALSO
ck_brlock(3), ck_bytelock(3), ck_rwlock(3)
Additional information available at http://concurrencykit.org/
July 26, 2013. |