<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html lang="en-us" xml:lang="en-us"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="security" content="public" /> <meta name="Robots" content="index,follow" /> <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))' /> <meta name="DC.Type" content="reference" /> <meta name="DC.Title" content="Example: Use semaphores in Pthread programs to protect shared data" /> <meta name="abstract" content="This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set." /> <meta name="description" content="This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set." /> <meta name="DC.Relation" scheme="URI" content="rzahwsemco.htm" /> <meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" /> <meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" /> <meta name="DC.Format" content="XHTML" /> <meta name="DC.Identifier" content="rzahwe19-e19rx" /> <meta name="DC.Language" content="en-us" /> <!-- 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. --> <link rel="stylesheet" type="text/css" href="./ibmdita.css" /> <link rel="stylesheet" type="text/css" href="./ic.css" /> <title>Example: Use semaphores in Pthread programs to protect shared data</title> </head> <body id="rzahwe19-e19rx"><a name="rzahwe19-e19rx"><!-- --></a> <!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script> <h1 class="topictitle1">Example: Use semaphores in Pthread programs to protect shared data</h1> <div><p>This example shows a Pthread program starting several threads that protect access to shared data with a semaphore set.</p> <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> </div> <div class="example"> <pre>/* Filename: ATEST17.QCSRC The output of this example is as follows: Enter Testcase - LIBRARY/ATEST17 Wait on semaphore to prevent access to shared data Create/start threads Wait a bit until we are 'done' with the shared data Thread 00000000 00000020: Entered Thread 00000000 0000001f: Entered Thread 00000000 0000001e: Entered Unlock shared data Wait for the threads to complete, and release their resources Thread 00000000 0000001f: Start critical section, holding semaphore Thread 00000000 0000001f: End critical section, release semaphore Thread 00000000 00000020: Start critical section, holding semaphore Thread 00000000 00000020: End critical section, release semaphore Thread 00000000 0000001e: Start critical section, holding semaphore Thread 00000000 0000001e: End critical section, release semaphore Clean up Main completed */ #define _MULTI_THREADED #include <pthread.h> #include <stdio.h> #include <stdlib.h> #include <errno.h> #include <unistd.h> #include <sys/ipc.h> #include <sys/sem.h> #include <sys/stat.h> #define checkResults(string, val) { \ if (val) { \ printf("Failed with %d at %s", val, string); \ exit(1); \ } \ } #define NUMTHREADS 3 int sharedData=0; int sharedData2=0; /* Simple semaphore used here is actually a set of 1 */ int semaphoreId=-1; /* Simple lock operation. 0=which-semaphore, -1=decrement, 0=noflags */ struct sembuf lockOperation = { 0, -1, 0}; /* Simple unlock operation. 0=which-semaphore, 1=increment, 0=noflags */ struct sembuf unlockOperation = { 0, 1, 0}; void *theThread(void *parm) { int rc; printf("Thread %.8x %.8x: Entered\n", pthread_getthreadid_np()); rc = semop(semaphoreId, &lockOperation, 1); checkResults("semop(lock)\n",rc); /********** Critical Section *******************/ printf("Thread %.8x %.8x: Start critical section, holding semaphore\n", pthread_getthreadid_np()); /* Access to shared data goes here */ ++sharedData; --sharedData2; printf("Thread %.8x %.8x: End critical section, release semaphore\n", pthread_getthreadid_np()); /********** Critical Section *******************/ rc = semop(semaphoreId, &unlockOperation, 1); checkResults("semop(unlock)\n",rc); return NULL; } int main(int argc, char **argv) { pthread_t thread[NUMTHREADS]; int rc=0; int i; printf("Enter Testcase - %s\n", argv[0]); /* Create a private semaphore set with 1 semaphore that only I can use */ semaphoreId = semget(IPC_PRIVATE, 1, S_IRUSR|S_IWUSR); if (semaphoreId &lt 0) { printf("semget failed, err=%d\n",errno); exit(1); } /* Set the semaphore (#0 in the set) count to 1. Simulate a mutex */ rc = semctl(semaphoreId, 0, SETVAL, (int)1); checkResults("semctl(SETALL)\n", rc); printf("Wait on semaphore to prevent access to shared data\n"); rc = semop(semaphoreId, &lockOperation, 1); checkResults("semop(lock)\n",rc); printf("Create/start threads\n"); for (i=0; i <NUMTHREADS; ++i) { checkResults("pthread_create()\n", rc); } printf("Wait a bit until we are 'done' with the shared data\n"); sleep(3); printf("Unlock shared data\n"); rc = semop(semaphoreId, &unlockOperation, 1); checkResults("semop(unlock)\n",rc); printf("Wait for the threads to complete, and release their resources\n"); for (i=0; i <NUMTHREADS; ++i) { rc = pthread_join(thread[i], NULL); checkResults("pthread_join()\n", rc); } printf("Clean up\n"); rc = semctl(semaphoreId, 0, IPC_RMID); checkResults("semctl(removeID)\n", rc); printf("Main completed\n"); return 0; } </pre> </div> </div> <div> <div class="familylinks"> <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> </div> </div> </body> </html>