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.TH PTHREAD_RWLOCK_RDLOCK 3C "Mar 23, 2005"
.SH NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock \- lock or attempt to lock
read-write lock object for reading
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>

\fBint\fR \fBpthread_rwlock_rdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR);
.fi

.LP
.nf
\fBint\fR \fBpthread_rwlock_tryrdlock\fR(\fBpthread_rwlock_t\fR \fI*rwlock\fR);
.fi

.SH DESCRIPTION
.LP
The \fBpthread_rwlock_rdlock()\fR function applies a read lock to the
read-write lock referenced by \fIrwlock\fR.  The calling thread acquires the
read lock if a writer does not hold the lock and there are no writers blocked
on the lock.
.sp
.LP
The calling thread does 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 acquires the lock. If the read lock is not acquired, the calling
thread blocks until it can acquire the lock.
.sp
.LP
A thread can hold multiple concurrent read locks on \fIrwlock\fR (that is,
successfully call the \fBpthread_rwlock_rdlock()\fR function \fIn\fR times). If
so, the thread must perform matching unlocks (that is, it must call the
\fBpthread_rwlock_unlock()\fR function \fIn\fR times).
.sp
.LP
The maximum number of concurrent read locks that a thread can hold on one
read-write lock is currently set at 100,000, though this number could change in
a future release. There is no imposed limit on the number of different threads
that can apply a read lock to one read-write lock.
.sp
.LP
The \fBpthread_rwlock_tryrdlock()\fR function applies a read lock like the
\fBpthread_rwlock_rdlock()\fR function, with the exception that the function
fails if the equivalent \fBpthread_rwlock_rdlock()\fR call would have blocked
the calling thread.  In no case will the \fBpthread_rwlock_tryrdlock()\fR
function ever block. It always either acquires the lock or fails and returns
immediately.
.sp
.LP
Results are undefined if any of these functions are called with an
uninitialized read-write lock.
.sp
.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 VALUES
.LP
If successful, the \fBpthread_rwlock_rdlock()\fR function returns \fB0\fR.
Otherwise, an error number is returned to indicate the error.
.sp
.LP
The \fBpthread_rwlock_tryrdlock()\fR function returns \fB0\fR if the lock for
reading on the read-write lock object referenced by \fIrwlock\fR is acquired.
Otherwise an error number  is returned to indicate the error.
.SH ERRORS
.LP
The \fBpthread_rwlock_rdlock()\fR and \fBpthread_rwlock_tryrdlock()\fR
functions will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The read lock could not be acquired because the maximum number of read locks by
the current thread for \fIrwlock\fR has been exceeded.
.RE

.sp
.LP
The \fBpthread_rwlock_rdlock()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEDEADLK\fR\fR
.ad
.RS 11n
The current thread already owns the read-write lock for writing.
.RE

.sp
.LP
The \fBpthread_rwlock_tryrdlock()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBUSY\fR\fR
.ad
.RS 9n
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.
.RE

.SH ATTRIBUTES
.LP
See  \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.LP
\fBpthread_rwlock_init\fR(3C), \fBpthread_rwlock_wrlock\fR(3C),
\fBpthread_rwlockattr_init\fR(3C), \fBpthread_rwlock_unlock\fR(3C),
\fBattributes\fR(5), \fBstandards\fR(5)