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.\" $FreeBSD: release/9.1.0/share/man/man3/pthread_rwlock_rdlock.3 172880 2007-10-22 10:08:01Z ru $
.\"
.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 LIBRARY
.Lb libpthread
.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 .