[apple/libpthread.git] / man / pthread_rwlock_rdlock.3

.\" Copyright (c) 1998 Alex Nash
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd August 4, 1998
.Dt PTHREAD_RWLOCK_RDLOCK 3
.Os
.Sh NAME
.Nm pthread_rwlock_rdlock ,
.Nm pthread_rwlock_tryrdlock
.Nd acquire a read/write lock for reading
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
.Sh DESCRIPTION
The
.Fn pthread_rwlock_rdlock
function acquires a read lock on
.Fa lock
provided that
.Fa lock
is not presently held for writing and no writer threads are
presently blocked on the lock.
If the read lock cannot be
immediately acquired, the calling thread blocks until it can
acquire the lock.
.Pp
The
.Fn pthread_rwlock_tryrdlock
function performs the same action, but does not block if the lock
cannot be immediately obtained (i.e., the lock is held for writing
or there are waiting writers).
.Pp
A thread may hold multiple concurrent read locks.
If so,
.Fn pthread_rwlock_unlock
must be called once for each lock obtained.
.Pp
The results of acquiring a read lock while the calling thread holds
a write lock are undefined.
.Sh IMPLEMENTATION NOTES
To prevent writer starvation, writers are favored over readers.
.Sh RETURN VALUES
If successful, the
.Fn pthread_rwlock_rdlock
and
.Fn pthread_rwlock_tryrdlock
functions will return zero.
Otherwise an error number will be returned
to indicate the error.
.Sh ERRORS
The
.Fn pthread_rwlock_tryrdlock
function will fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The lock could not be acquired because a writer holds the lock or
was blocked on it.
.El
.Pp
The
.Fn pthread_rwlock_rdlock
and
.Fn pthread_rwlock_tryrdlock
functions may fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The lock could not be acquired because the maximum number of read locks
against
.Fa lock
has been exceeded.
.It Bq Er EDEADLK
The current thread already owns
.Fa lock
for writing.
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.It Bq Er ENOMEM
Insufficient memory exists to initialize the lock (applies to
statically initialized locks only).
.El
.Sh SEE ALSO
.Xr pthread_rwlock_init 3 ,
.Xr pthread_rwlock_trywrlock 3 ,
.Xr pthread_rwlock_unlock 3 ,
.Xr pthread_rwlock_wrlock 3
.Sh STANDARDS
The
.Fn pthread_rwlock_rdlock
and
.Fn pthread_rwlock_tryrdlock
functions are expected to conform to
.St -susv2 .
.Sh HISTORY
The
.Fn pthread_rwlock_rdlock
function first appeared in
.Fx 3.0 .
Commit	Line	Data
f1a1da6c A	1	.\" Copyright (c) 1998 Alex Nash
	2	.\" All rights reserved.
	3	.\"
	4	.\" Redistribution and use in source and binary forms, with or without
	5	.\" modification, are permitted provided that the following conditions
	6	.\" are met:
	7	.\" 1. Redistributions of source code must retain the above copyright
	8	.\" notice, this list of conditions and the following disclaimer.
	9	.\" 2. Redistributions in binary form must reproduce the above copyright
	10	.\" notice, this list of conditions and the following disclaimer in the
	11	.\" documentation and/or other materials provided with the distribution.
	12	.\"
	13	.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
	14	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	15	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	16	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
	17	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	18	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	19	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	20	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	21	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	22	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	23	.\" SUCH DAMAGE.
	24	.\"
2546420a	25	.\" $FreeBSD$
f1a1da6c A	26	.\"
	27	.Dd August 4, 1998
	28	.Dt PTHREAD_RWLOCK_RDLOCK 3
	29	.Os
	30	.Sh NAME
	31	.Nm pthread_rwlock_rdlock ,
	32	.Nm pthread_rwlock_tryrdlock
	33	.Nd acquire a read/write lock for reading
	34	.Sh SYNOPSIS
	35	.In pthread.h
	36	.Ft int
2546420a	37	.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
f1a1da6c	38	.Ft int
2546420a	39	.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
f1a1da6c A	40	.Sh DESCRIPTION
	41	The
	42	.Fn pthread_rwlock_rdlock
	43	function acquires a read lock on
2546420a	44	.Fa lock
f1a1da6c	45	provided that
2546420a	46	.Fa lock
f1a1da6c	47	is not presently held for writing and no writer threads are
2546420a A	48	presently blocked on the lock.
2546420a A	49	If the read lock cannot be
f1a1da6c A	50	immediately acquired, the calling thread blocks until it can
	51	acquire the lock.
	52	.Pp
	53	The
	54	.Fn pthread_rwlock_tryrdlock
	55	function performs the same action, but does not block if the lock
	56	cannot be immediately obtained (i.e., the lock is held for writing
	57	or there are waiting writers).
	58	.Pp
2546420a A	59	A thread may hold multiple concurrent read locks.
2546420a A	60	If so,
f1a1da6c A	61	.Fn pthread_rwlock_unlock
	62	must be called once for each lock obtained.
	63	.Pp
	64	The results of acquiring a read lock while the calling thread holds
	65	a write lock are undefined.
	66	.Sh IMPLEMENTATION NOTES
	67	To prevent writer starvation, writers are favored over readers.
	68	.Sh RETURN VALUES
	69	If successful, the
	70	.Fn pthread_rwlock_rdlock
	71	and
	72	.Fn pthread_rwlock_tryrdlock
	73	functions will return zero.
2546420a A	74	Otherwise an error number will be returned
2546420a A	75	to indicate the error.
f1a1da6c A	76	.Sh ERRORS
	77	The
	78	.Fn pthread_rwlock_tryrdlock
	79	function will fail if:
	80	.Bl -tag -width Er
	81	.It Bq Er EBUSY
2546420a	82	The lock could not be acquired because a writer holds the lock or
f1a1da6c A	83	was blocked on it.
	84	.El
	85	.Pp
	86	The
	87	.Fn pthread_rwlock_rdlock
	88	and
	89	.Fn pthread_rwlock_tryrdlock
	90	functions may fail if:
	91	.Bl -tag -width Er
	92	.It Bq Er EAGAIN
2546420a	93	The lock could not be acquired because the maximum number of read locks
f1a1da6c A	94	against
	95	.Fa lock
	96	has been exceeded.
	97	.It Bq Er EDEADLK
	98	The current thread already owns
2546420a	99	.Fa lock
f1a1da6c A	100	for writing.
	101	.It Bq Er EINVAL
	102	The value specified by
2546420a	103	.Fa lock
f1a1da6c A	104	is invalid.
	105	.It Bq Er ENOMEM
	106	Insufficient memory exists to initialize the lock (applies to
	107	statically initialized locks only).
	108	.El
2546420a A	109	.Sh SEE ALSO
	110	.Xr pthread_rwlock_init 3 ,
	111	.Xr pthread_rwlock_trywrlock 3 ,
	112	.Xr pthread_rwlock_unlock 3 ,
	113	.Xr pthread_rwlock_wrlock 3
	114	.Sh STANDARDS
	115	The
	116	.Fn pthread_rwlock_rdlock
	117	and
	118	.Fn pthread_rwlock_tryrdlock
	119	functions are expected to conform to
	120	.St -susv2 .
f1a1da6c A	121	.Sh HISTORY
	122	The
	123	.Fn pthread_rwlock_rdlock
	124	function first appeared in
	125	.Fx 3.0 .