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+.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
+.TH "PTHREAD_RWLOCK_RDLOCK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
+.\" pthread_rwlock_rdlock 
+.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 .