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.\" Copyright (c) 2004 Michael Telahun Makonnen
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.\" $FreeBSD: release/9.1.0/share/man/man3/pthread_rwlock_timedrdlock.3 172880 2007-10-22 10:08:01Z ru $
.\"
.Dd January 16, 2004
.Dt PTHREAD_RWLOCK_TIMEDRDLOCK 3
.Os
.Sh NAME
.Nm pthread_rwlock_timedrdlock
.Nd "acquire a read-write lock for reading or give up after a specified period"
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_rwlock_timedrdlock "pthread_rwlock_t *rwlock" "const struct timespec *abs_timeout"
.Sh DESCRIPTION
This function acquires a read lock on the read-write lock
.Fa rwlock .
However, if the lock cannot be
acquired without waiting for another thread to
unlock the lock,
this wait shall be terminated when
.Fa abs_timeout
expires.
.Pp
A thread may hold multiple concurrent read locks.
The
.Xr pthread_rwlock_unlock 3
function must be called once for each lock acquired.
.Pp
If the thread should be interrupted by a signal,
the
.Fn pthread_rwlock_timedrdlock
function will be automatically restarted after the thread returns from
the signal handler.
.Pp
The calling thread may deadlock if
at the time the call is made it holds a write lock on
.Fa rwlock .
The results are undefined if this function is called with
an uninitialized read-write lock.
.Sh IMPLEMENTATION NOTES
To prevent writer starvation, writers are favored over readers.
.Sh RETURN VALUES
If successful, the
.Fn pthread_rwlock_timedrdlock
function will return zero.
Otherwise, an error number will be returned to indicate the error.
.Pp
This function shall not return an error code of
.Er EINTR .
.Sh ERRORS
The
.Fn pthread_rwlock_timedrdlock
function will fail if:
.Bl -tag -width Er
.It Bq Er ETIMEDOUT
The lock could not be acquired before the specified timeout expired.
.El
.Pp
The
.Fn pthread_rwlock_timedrdlock
function may fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The read lock could not be
acquired because the maximum number of read locks for
.Fa rwlock
would be exceeded.
.It Bq Er EDEADLK
The calling thread already holds a write lock on
.Fa rwlock .
.It Bq Er EINVAL
The value specified by
.Fa rwlock
does not refer to an initialized read-write lock object,
or the
.Fa abs_timeout
nanosecond value is less than zero or
greater than or equal to 1 billion.
.El
.Sh SEE ALSO
.Xr pthread_rwlock_init 3 ,
.Xr pthread_rwlock_timedwrlock 3 ,
.Xr pthread_rwlock_unlock 3
.Sh STANDARDS
The
.Fn pthread_rwlock_timedrdlock
function is expected to conform to
.St -p1003.1-96 .
.Sh HISTORY
The
.Fn pthread_rwlock_timedrdlock
function first appeared in
.Fx 5.2 .