rw_rdlock(3synch)                                  rw_rdlock(3synch)


       NAME
             rw_rdlock - acquire a reader-writer lock in read mode.

       SYNOPSIS
             cc [options] -Kthread file
             #include <synch.h>
             int rw_rdlock(rwlock_t *lock);

          Parameters
             lock  pointer to the reader-writer lock to be acquired

       DESCRIPTION
             rw_rdlock acquires the reader-writer lock pointed to by lock
             in read mode.

             A reader-writer lock can be held by any number of readers at
             one time, but only one writer at a time can hold the lock.
             Once a writer has requested the lock with rw_wrlock, all
             subsequent requests for the lock in either read or write mode
             are queued.

             If the lock is free, or is currently held by another reader
             and there are no writers waiting, rw_rdlock increments the
             reader count and the caller proceeds.  If a writer holds the
             lock or if any writer is waiting for the lock, the caller
             blocks to wait for the lock.

             lock must previously have been initialized (see rwlock_init).

             From the point of view of the caller, this function is atomic:
             even if interrupted by a signal or forkall [see fork(2)],
             rw_rdlock will not return until it holds the lock.  As a
             consequence, if rw_rdlock is interrupted, an error indication
             such as EINTR is never returned to the user.

          Return Values
             rw_rdlock returns zero for success and an error number for
             failure, as described below.

          Errors
             If any of the following conditions is detected, rw_rdlock
             returns the corresponding value:

             EINVAL   Invalid argument specified




                           Copyright 1994 Novell, Inc.               Page 1

      rw_rdlock(3synch)                                  rw_rdlock(3synch)


            ENOMEM   Insufficient memory

      USAGE
            For consistency, locks acquired with rw_rdlock should be
            released with rw_unlock.

         Warnings
            If a thread exits while holding a reader-writer lock, the lock
            will not be unlocked, and other threads waiting for the lock
            will wait forever.  Similarly, if a process exits while
            holding a USYNC_PROCESS reader-writer lock, the lock will not
            be unlocked, and other processes waiting for the reader-writer
            lock will wait forever.

      REFERENCES
            fork(2), rwlock(3synch), rwlock_destroy(3synch),
            rwlock_init(3synch), rw_tryrdlock(3synch),
            rw_trywrlock(3synch), rw_unlock(3synch), rw_wrlock(3synch),
            synch(3synch)





























                          Copyright 1994 Novell, Inc.               Page 2