157 lines
6.7 KiB
HTML
157 lines
6.7 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Example: Use semaphores in Pthread programs to protect shared data" />
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<meta name="abstract" content="This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set." />
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<meta name="description" content="This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set." />
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<meta name="DC.Relation" scheme="URI" content="rzahwsemco.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Language" content="en-us" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Example: Use semaphores in Pthread programs to protect
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shared data</title>
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</head>
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<body id="rzahwe19-e19rx"><a name="rzahwe19-e19rx"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Example: Use semaphores in Pthread programs to protect
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shared data</h1>
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<div><p>This example shows a Pthread program starting several threads that
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protect access to shared data with a semaphore set.</p>
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<div class="section"><div class="note"><span class="notetitle">Note:</span> By using the code examples, you agree to the terms of the <a href="codedisclaimer.htm">Code license and disclaimer information</a>.</div>
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</div>
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<div class="example"> <pre>/*
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Filename: ATEST17.QCSRC
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The output of this example is as follows:
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Enter Testcase - LIBRARY/ATEST17
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Wait on semaphore to prevent access to shared data
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Create/start threads
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Wait a bit until we are 'done' with the shared data
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Thread 00000000 00000020: Entered
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Thread 00000000 0000001f: Entered
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Thread 00000000 0000001e: Entered
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Unlock shared data
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Wait for the threads to complete, and release their resources
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Thread 00000000 0000001f: Start critical section, holding semaphore
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Thread 00000000 0000001f: End critical section, release semaphore
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Thread 00000000 00000020: Start critical section, holding semaphore
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Thread 00000000 00000020: End critical section, release semaphore
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Thread 00000000 0000001e: Start critical section, holding semaphore
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Thread 00000000 0000001e: End critical section, release semaphore
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Clean up
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Main completed
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*/
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#define _MULTI_THREADED
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#include <pthread.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <errno.h>
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#include <unistd.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#include <sys/stat.h>
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#define checkResults(string, val) { \
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if (val) { \
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printf("Failed with %d at %s", val, string); \
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exit(1); \
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} \
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}
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#define NUMTHREADS 3
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int sharedData=0;
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int sharedData2=0;
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/* Simple semaphore used here is actually a set of 1 */
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int semaphoreId=-1;
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/* Simple lock operation. 0=which-semaphore, -1=decrement, 0=noflags */
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struct sembuf lockOperation = { 0, -1, 0};
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/* Simple unlock operation. 0=which-semaphore, 1=increment, 0=noflags */
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struct sembuf unlockOperation = { 0, 1, 0};
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void *theThread(void *parm)
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{
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int rc;
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printf("Thread %.8x %.8x: Entered\n", pthread_getthreadid_np());
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rc = semop(semaphoreId, &lockOperation, 1);
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checkResults("semop(lock)\n",rc);
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/********** Critical Section *******************/
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printf("Thread %.8x %.8x: Start critical section, holding semaphore\n",
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pthread_getthreadid_np());
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/* Access to shared data goes here */
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++sharedData; --sharedData2;
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printf("Thread %.8x %.8x: End critical section, release semaphore\n",
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pthread_getthreadid_np());
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/********** Critical Section *******************/
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rc = semop(semaphoreId, &unlockOperation, 1);
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checkResults("semop(unlock)\n",rc);
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return NULL;
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}
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int main(int argc, char **argv)
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{
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pthread_t thread[NUMTHREADS];
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int rc=0;
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int i;
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printf("Enter Testcase - %s\n", argv[0]);
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/* Create a private semaphore set with 1 semaphore that only I can use */
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semaphoreId = semget(IPC_PRIVATE, 1, S_IRUSR|S_IWUSR);
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if (semaphoreId &lt 0) { printf("semget failed, err=%d\n",errno); exit(1); }
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/* Set the semaphore (#0 in the set) count to 1. Simulate a mutex */
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rc = semctl(semaphoreId, 0, SETVAL, (int)1);
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checkResults("semctl(SETALL)\n", rc);
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printf("Wait on semaphore to prevent access to shared data\n");
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rc = semop(semaphoreId, &lockOperation, 1);
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checkResults("semop(lock)\n",rc);
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printf("Create/start threads\n");
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for (i=0; i <NUMTHREADS; ++i) {
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checkResults("pthread_create()\n", rc);
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}
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printf("Wait a bit until we are 'done' with the shared data\n");
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sleep(3);
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printf("Unlock shared data\n");
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rc = semop(semaphoreId, &unlockOperation, 1);
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checkResults("semop(unlock)\n",rc);
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printf("Wait for the threads to complete, and release their resources\n");
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for (i=0; i <NUMTHREADS; ++i) {
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rc = pthread_join(thread[i], NULL);
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checkResults("pthread_join()\n", rc);
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}
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printf("Clean up\n");
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rc = semctl(semaphoreId, 0, IPC_RMID);
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checkResults("semctl(removeID)\n", rc);
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printf("Main completed\n");
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return 0;
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}
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</pre>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzahwsemco.htm" title="Semaphores (sometimes referred to as counting semaphores) can be used to control access to shared resources. A semaphore can be thought of as an intelligent counter. Every semaphore has a current count, which is greater than or equal to 0.">Semaphores and threads</a></div>
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</div>
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</div>
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</body>
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</html>
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