.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "PTHREAD_RWLOCK_RDLOCK" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_rwlock_rdlock 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock \- lock a read-write
lock object for reading
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
int pthread_rwlock_rdlock(pthread_rwlock_t\fP \fI*rwlock\fP\fB);
.br
int pthread_rwlock_tryrdlock(pthread_rwlock_t\fP \fI*rwlock\fP\fB);
\fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpthread_rwlock_rdlock\fP() function shall apply a read lock
to the read-write lock referenced by \fIrwlock\fP. The
calling thread acquires the read lock if a writer does not hold the
lock and there are no writers blocked on the lock.
.LP
If the Thread Execution Scheduling option is supported, and the threads
involved in the lock are executing with the scheduling
policies SCHED_FIFO or SCHED_RR, the calling thread shall not acquire
the lock if a writer holds the lock or if writers of higher
or equal priority are blocked on the lock; otherwise, the calling
thread shall acquire the lock. 
.LP
If the Threads Execution Scheduling option is supported, and the threads
involved in the lock are executing with the
SCHED_SPORADIC scheduling policy, the calling thread shall not acquire
the lock if a writer holds the lock or if writers of higher
or equal priority are blocked on the lock; otherwise, the calling
thread shall acquire the lock. 
.LP
If the Thread Execution Scheduling option is not supported, it is
implementation-defined whether the calling thread acquires the
lock when a writer does not hold the lock and there are writers blocked
on the lock. If a writer holds the lock, the calling thread
shall not acquire the read lock. If the read lock is not acquired,
the calling thread shall block until it can acquire the lock.
The calling thread may deadlock if at the time the call is made it
holds a write lock.
.LP
A thread may hold multiple concurrent read locks on \fIrwlock\fP (that
is, successfully call the \fIpthread_rwlock_rdlock\fP()
function \fIn\fP times). If so, the application shall ensure that
the thread performs matching unlocks (that is, it calls the \fIpthread_rwlock_unlock\fP()
function \fIn\fP times).
.LP
The maximum number of simultaneous read locks that an implementation
guarantees can be applied to a read-write lock shall be
implementation-defined. The \fIpthread_rwlock_rdlock\fP() function
may fail if this maximum would be exceeded.
.LP
The \fIpthread_rwlock_tryrdlock\fP() function shall apply a read lock
as in the \fIpthread_rwlock_rdlock\fP() function, with
the exception that the function shall fail if the equivalent \fIpthread_rwlock_rdlock\fP()
call would have blocked the calling
thread. In no case shall the \fIpthread_rwlock_tryrdlock\fP() function
ever block; it always either acquires the lock or fails and
returns immediately.
.LP
Results are undefined if any of these functions are called with an
uninitialized read-write lock.
.LP
If a signal is delivered to a thread waiting for a read-write lock
for reading, upon return from the signal handler the thread
resumes waiting for the read-write lock for reading as if it was not
interrupted.
.SH RETURN VALUE
.LP
If successful, the \fIpthread_rwlock_rdlock\fP() function shall return
zero; otherwise, an error number shall be returned to
indicate the error.
.LP
The \fIpthread_rwlock_tryrdlock\fP() function shall return zero if
the lock for reading on the read-write lock object
referenced by \fIrwlock\fP is acquired. Otherwise, an error number
shall be returned to indicate the error.
.SH ERRORS
.LP
The \fIpthread_rwlock_tryrdlock\fP() function shall fail if:
.TP 7
.B EBUSY
The read-write lock could not be acquired for reading because a writer
holds the lock or a writer with the appropriate priority
was blocked on it.
.sp
.LP
The \fIpthread_rwlock_rdlock\fP() and \fIpthread_rwlock_tryrdlock\fP()
functions may fail if:
.TP 7
.B EINVAL
The value specified by \fIrwlock\fP does not refer to an initialized
read-write lock object.
.TP 7
.B EAGAIN
The read lock could not be acquired because the maximum number of
read locks for \fIrwlock\fP has been exceeded.
.sp
.LP
The \fIpthread_rwlock_rdlock\fP() function may fail if:
.TP 7
.B EDEADLK
The current thread already owns the read-write lock for writing.
.sp
.LP
These functions shall not return an error code of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
Applications using these functions may be subject to priority inversion,
as discussed in the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 3.285, Priority Inversion.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpthread_rwlock_destroy\fP(), \fIpthread_rwlock_timedrdlock\fP(),
\fIpthread_rwlock_timedwrlock\fP(), \fIpthread_rwlock_trywrlock\fP(),
\fIpthread_rwlock_unlock\fP(), \fIpthread_rwlock_wrlock\fP(),
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .