.\" $NetBSD: pthread_rwlock.3,v 1.5 2012/11/12 23:34:50 uwe Exp $
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.\" Copyright (c) 1998 Alex Nash
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ----------------------------------------------------------------------------
.Dd July 8, 2010
.Dt PTHREAD_RWLOCK 3
.Os
.Sh NAME
.Nm pthread_rwlock
.Nd read/write lock interface
.Sh LIBRARY
.Lb libpthread
.\" ----------------------------------------------------------------------------
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_rwlock_init "pthread_rwlock_t * restrict lock" \
"const pthread_rwlockattr_t * restrict attr"
.Vt pthread_rwlock_t lock No = Dv PTHREAD_RWLOCK_INITIALIZER;
.Ft int
.Fn pthread_rwlock_destroy "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_rdlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_timedrdlock "pthread_rwlock_t * restrict lock" \
"const struct timespec * restrict abstime"
.Ft int
.Fn pthread_rwlock_tryrdlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_wrlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_timedwrlock "pthread_rwlock_t * restrict lock" \
"const struct timespec * restrict abstime"
.Ft int
.Fn pthread_rwlock_trywrlock "pthread_rwlock_t *lock"
.Ft int
.Fn pthread_rwlock_unlock "pthread_rwlock_t *lock"
.\" ----------------------------------------------------------------------------
.Sh DESCRIPTION
The
.Fn pthread_rwlock_init
function is used to initialize a read/write lock, with attributes
specified by
.Fa attr .
If
.Fa attr
is
.Dv NULL ,
the default read/write lock attributes are used.
.Pp
The results of calling
.Fn pthread_rwlock_init
with an already initialized lock are undefined.
.Pp
The macro
.Dv PTHREAD_RWLOCK_INITIALIZER
can be used to initialize a read/write lock when the allocation can be done
statically, no error checking is required, and the default attributes are
appropriate.
The behavior is similar to calling
.Fn pthread_rwlock_init
with
.Fa attr
specified as
.Dv NULL .
.Pp
.\" -----
The
.Fn pthread_rwlock_destroy
function is used to destroy a read/write lock previously created with
.Fn pthread_rwlock_init .
.Pp
.\" -----
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
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 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.
.Pp
.\" -----
The
.Fn pthread_rwlock_wrlock
function blocks until a write lock can be acquired against
.Fa lock .
.Pp
The
.Fn pthread_rwlock_timedwrlock
performs the same action, but will not wait beyond
.Fa abstime
to obtain the lock before returning.
.Pp
The
.Fn pthread_rwlock_trywrlock
function performs the same action as
.Fn pthread_rwlock_wrlock ,
but does not block if the lock cannot be immediately obtained.
.Pp
The results are undefined if the calling thread already holds the
lock at the time the call is made.
.Pp
.\" -----
The
.Fn pthread_rwlock_unlock
function is used to release the read/write lock previously obtained by
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_wrlock ,
.Fn pthread_rwlock_tryrdlock ,
or
.Fn pthread_rwlock_trywrlock .
.\" ----------------------------------------------------------------------------
.Sh RETURN VALUES
If successful, the
.Fn pthread_rwlock_init
function will return zero.
Otherwise an error number will be returned to indicate the error.
.Pp
If successful, the
.Fn pthread_rwlock_destroy ,
.Fn pthread_rwlock_rdlock ,
.Fn pthread_rwlock_timedrdlock ,
.Fn pthread_rwlock_tryrdlock ,
.Fn pthread_rwlock_wrlock ,
.Fn pthread_rwlock_timedwrlock ,
.Fn pthread_rwlock_trywrlock ,
and
.Fn pthread_rwlock_unlock
will return zero.
Otherwise an error number will be returned to indicate the error.
.Pp
The results are undefined if
.Fa lock
is not held by the calling thread.
.\" ----------------------------------------------------------------------------
.Sh ERRORS
The
.Fn pthread_rwlock_init
function may fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The system lacks the resources to initialize another read-write lock.
.It Bq Er EBUSY
The system has detected an attempt to re-initialize the object
referenced by
.Fa lock ,
a previously initialized but not yet destroyed read/write lock.
.It Bq Er EINVAL
The value specified by
.Fa attr
is invalid.
.It Bq Er ENOMEM
Insufficient memory exists to initialize the read-write lock.
.El
.Pp
.\" -----
The
.Fn pthread_rwlock_destroy
function may fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The system has detected an attempt to destroy the object referenced by
.Fa lock
while it is locked.
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.Pp
.\" -----
The
.Fn pthread_rwlock_tryrdlock
function shall 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 shall 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.
.El
.Pp
.\" -----
The
.Fn pthread_rwlock_trywrlock
function shall fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The calling thread is not able to acquire the lock without blocking.
.El
.Pp
The
.Fn pthread_rwlock_timedrdlock
function shall 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_wrlock ,
.Fn pthread_rwlock_timedwrlock ,
and
.Fn pthread_rwlock_trywrlock
functions may fail if:
.Bl -tag -width Er
.It Bq Er EDEADLK
The calling thread already owns the read/write lock (for reading
or writing).
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.Pp
.\" -----
The
.Fn pthread_rwlock_unlock
function may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.It Bq Er EPERM
The current thread does not own the read/write lock.
.El
.\" ----------------------------------------------------------------------------
.Sh SEE ALSO
.Xr pthread 3 ,
.Xr pthread_barrier 3 ,
.Xr pthread_cond 3 ,
.Xr pthread_mutex 3 ,
.Xr pthread_rwlockattr 3 ,
.Xr pthread_spin 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2001 .
.\" ----------------------------------------------------------------------------
.Sh BUGS
The
.Dv PTHREAD_PROCESS_SHARED
attribute is not supported.