<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation"> <title>pthread_rwlock_trywrlock()--Get Exclusive Write Lock with No Wait</title> <!-- All rights reserved. Licensed Materials Property of IBM --> <!-- US Government Users Restricted Rights --> <!-- Use, duplication or disclosure restricted by --> <!-- GSA ADP Schedule Contract with IBM Corp. --> <!-- Begin Header Records ========================================== --> <!-- NETMG2 SCRIPT A converted by B2H R4.1 (346) (CMS) by HOLTJM at --> <!-- RCHVMW2 on 29 Jan 1999 at 10:01:37 --> <!--File Edited November 2001 --> <!--End Header Records --> <link rel="stylesheet" type="text/css" href="../rzahg/ic.css"> </head> <body> <!-- Java sync-link --> <script language="Javascript" src="../rzahg/synch.js" type="text/javascript"> </script> <a name="Top_Of_Page"></a> <h2>pthread_rwlock_trywrlock()--Get Exclusive Write Lock with No Wait</h2> <div class="box" style="width: 60%;"> <br> Syntax: <pre> #include <pthread.h> int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock); </pre> Service Program Name: QP0WPTHR<br> <!-- iddvc RMBR --> <br> Default Public Authority: *USE <br> <!-- iddvc RMBR --> <br> Threadsafe: Yes<br> <!-- iddvc RMBR --> <br> Signal Safe: Yes<br> <!-- iddvc RMBR --> <br> </div> <p>The <strong>pthread_rwlock_trywrlock</strong>() function attempts to acquire an exclusive write lock on the read/write lock specified by <em>rwlock</em>. If the exclusive write lock cannot be immediately acquired, <strong>pthread_rwlock_timedwrlock_np</strong>() returns the <strong>EBUSY</strong> error.</p> <p>Only one thread can hold an exclusive write lock on a read/write lock object. If any thread holds an exclusive write lock on a read/write lock object, no other threads will be allowed to hold a shared read or exclusive write lock.</p> <p>If there are no threads holding an exclusive write lock or shared read lock on the read/write lock, the calling thread will successfully acquire the exclusive write lock.</p> <p>If the calling thread already holds an exclusive write lock on the read/write lock, another write lock can be successfully acquired by the calling thread. If more than one exclusive write lock is successfully acquired by a thread on a read/write lock object, that thread is required to successfully call <strong>pthread_rwlock_unlock</strong>() a matching number of times.</p> <p>With a large number of readers, and relatively few writers, there is the possibility of writer starvation. If there are threads waiting for an exclusive write lock on the read/write lock and there are threads that currently hold a shared read lock, the subsequent attempts to acquire a shared read lock request will be granted, while attempts to acquire the exclusive write lock will return the <strong>EBUSY</strong> error.</p> <br> <h3>Read/Write Lock Deadlocks</h3> <p>If a thread ends while holding a write lock, the attempt by another thread to acquire a shared read or exclusive write lock will not be successful. In this case, the attempt to acquire the lock will return the <strong>EBUSY</strong> error. If a thread ends while holding a read lock, the system automatically releases the read lock.</p> <br> <h3>Upgrade / Downgrade a Lock</h3> <p>If the calling thread currently holds a shared read lock on the read/write lock object and there are no other threads holding a shared read lock, the exclusive write request will be granted. After the exclusive write lock request is granted, the calling thread holds <strong>both</strong> the shared read, <strong>and</strong> the exclusive write lock for the specified read/write lock object. If the thread calls <strong>pthread_rwlock_unlock</strong>() while holding one or more shared read locks <strong>and</strong> one or more exclusive write locks, the exclusive write locks are unlocked first. If more than one outstanding exclusive write lock was held by the thread, a matching number of successful calls to <strong>pthread_rwlock_unlock</strong>() must be done before all write locks are unlocked. At that time, subsequent calls to <strong>pthread_rwlock_unlock</strong>() will unlock the shared read locks.</p> <p>This behavior can be used to allow your application to upgrade or downgrade one lock type to another. See <a href="concep26.htm#293561">Read/write locks can be upgraded/downgraded</a>.</p> <br> <h3>Authorities and Locks</h3> <p>None.</p> <br> <h3>Parameters</h3> <dl> <dt><strong>rwlock</strong></dt> <dd>(Input) The address of the read/write lock</dd> </dl> <br> <h3>Return Value</h3> <dl> <dt><strong>0</strong></dt> <dd><strong>pthread_rwlock_trywrlock</strong>() was successful.</dd> <dt><strong>value</strong></dt> <dd><strong>pthread_rwlock_trywrlock</strong>() was not successful. <em>value</em> is set to indicate the error condition.</dd> </dl> <br> <h3>Error Conditions</h3> <p>If <strong>pthread_rwlock_trywrlock</strong>() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.</p> <dl> <dt><em>[EINVAL]</em></dt> <dd><p>The value specified for the argument is not correct.</p></dd> <dt><em>[EBUSY]</em></dt> <dd><p>The lock could not be acquired in the timed specified.</p></dd> </dl> <br> <h3>Related Information</h3> <ul> <li>The <<strong>pthread.h</strong>> header file. See <a href="rzah4hed.htm">Header files for Pthread functions</a>.<br><br></li> <li><a href="users_86.htm">pthread_rwlock_init()</a>--Initialize Read/Write Lock<br><br></li> <li><a href="users_87.htm">pthread_rwlock_rdlock()</a>--Get Shared Read Lock<br><br></li> <li><a href="users_88.htm">pthread_rwlock_timedrdlock_np()</a>--Get Shared Read Lock with Time-Out<br><br></li> <li><a href="users_89.htm">pthread_rwlock_timedwrlock_np()</a>--Get Exclusive Write Lock with Time-Out<br><br></li> <li><a href="users_90.htm">pthread_rwlock_tryrdlock()</a>--Get Shared Read Lock with No Wait<br><br></li> <li><a href="users_92.htm">pthread_rwlock_unlock()</a>--Unlock Exclusive Write or Shared Read Lock<br><br></li> <li><a href="users_93.htm">pthread_rwlock_wrlock()</a>--Get Exclusive Write Lock</li> </ul> <br> <h3><a name="373418">Example</a></h3> <p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a> for information pertaining to code examples.</p> <pre>#define _MULTI_THREADED #include <pthread.h> #include <stdio.h> #include "check.h" pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER; void *wrlockThread(void *arg) { int rc; int count=0; printf("%.8x %.8x: Entered thread, getting write lock with timeout\n", pthread_getthreadid_np()); Retry: rc = pthread_rwlock_trywrlock(&rwlock); if (rc == EBUSY) { if (count >= 10) { printf("%.8x %.8x: Retried too many times, failure!\n", pthread_getthreadid_np()); exit(EXIT_FAILURE); } ++count; printf("%.8x %.8x: Go off an do other work, then RETRY...\n", pthread_getthreadid_np()); sleep(1); goto Retry; } checkResults("pthread_rwlock_trywrlock() 1\n", rc); printf("%.8x %.8x: Got the write lock\n", pthread_getthreadid_np()); sleep(2); printf("%.8x %.8x: Unlock the write lock\n", pthread_getthreadid_np()); rc = pthread_rwlock_unlock(&rwlock); checkResults("pthread_rwlock_unlock()\n", rc); printf("%.8x %.8x: Secondary thread complete\n", pthread_getthreadid_np()); return NULL; } int main(int argc, char **argv) { int rc=0; pthread_t thread, thread2; printf("Enter Testcase - %s\n", argv[0]); printf("Main, get the write lock\n"); rc = pthread_rwlock_wrlock(&rwlock); checkResults("pthread_rwlock_wrlock()\n", rc); printf("Main, create the timed write lock threads\n"); rc = pthread_create(&thread, NULL, wrlockThread, NULL); checkResults("pthread_create\n", rc); rc = pthread_create(&thread2, NULL, wrlockThread, NULL); checkResults("pthread_create\n", rc); printf("Main, wait a bit holding this write lock\n"); sleep(1); printf("Main, Now unlock the write lock\n"); rc = pthread_rwlock_unlock(&rwlock); checkResults("pthread_rwlock_unlock()\n", rc); printf("Main, wait for the threads to end\n"); rc = pthread_join(thread, NULL); checkResults("pthread_join\n", rc); rc = pthread_join(thread2, NULL); checkResults("pthread_join\n", rc); rc = pthread_rwlock_destroy(&rwlock); checkResults("pthread_rwlock_destroy()\n", rc); printf("Main completed\n"); return 0; }</pre> <p><strong>Output:</strong></p> <pre>Enter Testcase - QP0WTEST/TPRWLWR1 Main, get the write lock Main, create the timed write lock threads 00000000 0000000d: Entered thread, getting write lock with timeout 00000000 0000000d: Go off an do other work, then RETRY... Main, wait a bit holding this write lock 00000000 0000000e: Entered thread, getting write lock with timeout 00000000 0000000e: Go off an do other work, then RETRY... 00000000 0000000d: Go off an do other work, then RETRY... Main, Now unlock the write lock Main, wait for the threads to end 00000000 0000000e: Got the write lock 00000000 0000000d: Go off an do other work, then RETRY... 00000000 0000000e: Unlock the write lock 00000000 0000000e: Secondary thread complete 00000000 0000000d: Got the write lock 00000000 0000000d: Unlock the write lock 00000000 0000000d: Secondary thread complete Main completed</pre> <hr> API introduced: V4R3 <hr> <center> <table cellpadding="2" cellspacing="2"> <tr align="center"> <td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href="rzah4mst.htm">Pthread APIs</a> | <a href="aplist.htm">APIs by category</a></td> </tr> </table> </center> </body> </html>