.\" $OpenBSD: pthread_rwlock_rdlock.3,v 1.11 2013/06/05 03:44:50 tedu Exp $
.\" Copyright (c) 1998 Alex Nash
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.\" $FreeBSD: pthread_rwlock_rdlock.3,v 1.2 1999/08/28 00:03:09 peter Exp $
.\"
.Dd $Mdocdate: June 5 2013 $
.Dt PTHREAD_RWLOCK_RDLOCK 3
.Os
.Sh NAME
.Nm pthread_rwlock_rdlock ,
.Nm pthread_rwlock_timedrdlock ,
.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_timedrdlock "pthread_rwlock_t *lock" "const struct timespec *abstime"
.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_timedrdlock
function performs the same action,
but will not wait beyond
.Fa abstime
to obtain the lock before returning.
.Pp
The
.Fn pthread_rwlock_tryrdlock
function performs the same action as
.Fn pthread_rwlock_rdlock ,
but does not block if the lock cannot be immediately obtained
(i.e., the lock is held for writing or there are writers waiting).
.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 ,
.Fn pthread_rwlock_timedrdlock ,
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_timedrdlock
function will fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The time specified by
.Fa abstime
was reached before the lock could be obtained.
.El
.Pp
The
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_timedrdlock ,
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 ,
.Fn pthread_rwlock_timedrdlock ,
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
and
.Ox 2.5 .
The
.Fn pthread_rwlock_timedrdlock
function first appeared in
.Ox 4.8 .