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<h1 class="topictitle1">Example: Use condition variables in Pthread programs</h1>
<div><p>This example shows a Pthread program using condition variables
to notify threads of a condition. Notice what mutual exclusion (mutex) locking
protocol is used.</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: ATEST18.QCSRC
The output of this example is as follows:
Enter Testcase - LIBRARY/ATEST18
Create/start threads
Producer: 'Finding' data
Consumer Thread 00000000 00000022: Entered
Consumer Thread 00000000 00000023: Entered
Consumer Thread 00000000 00000022: Wait for data to be produced
Consumer Thread 00000000 00000023: Wait for data to be produced
Producer: Make data shared and notify consumer
Producer: Unlock shared data and flag
Producer: 'Finding' data
Consumer Thread 00000000 00000022: Found data or Notified, CONSUME IT while holding lock
Consumer Thread 00000000 00000022: Wait for data to be produced
Producer: Make data shared and notify consumer
Producer: Unlock shared data and flag
Producer: 'Finding' data
Consumer Thread 00000000 00000023: Found data or Notified, CONSUME IT while holding lock
Consumer Thread 00000000 00000023: Wait for data to be produced
Producer: Make data shared and notify consumer
Producer: Unlock shared data and flag
Producer: 'Finding' data
Consumer Thread 00000000 00000022: Found data or Notified, CONSUME IT while holding lock
Consumer Thread 00000000 00000022: All done
Producer: Make data shared and notify consumer
Producer: Unlock shared data and flag
Wait for the threads to complete, and release their resources
Consumer Thread 00000000 00000023: Found data or Notified, CONSUME IT while holding lock
Consumer Thread 00000000 00000023: All done
Clean up
Main completed
*/
#define _MULTI_THREADED
#include &lt;pthread.h&gt;
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;unistd.h&gt;
#define checkResults(string, val) { \
if (val) { \
printf("Failed with %d at %s", val, string); \
exit(1); \
} \
}
#define NUMTHREADS 2
pthread_mutex_t dataMutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t dataPresentCondition = PTHREAD_COND_INITIALIZER;
int dataPresent=0;
int sharedData=0;
void *theThread(void *parm)
{
int rc;
int retries=2;
printf("Consumer Thread %.8x %.8x: Entered\n", pthread_getthreadid_np());
rc = pthread_mutex_lock(&amp;dataMutex);
checkResults("pthread_mutex_lock()\n", rc);
while (retries--) {
/* The boolean dataPresent value is required for safe use of */
/* condition variables. If no data is present we wait, other */
/* wise we process immediately. */
while (!dataPresent) {
printf("Consumer Thread %.8x %.8x: Wait for data to be produced\n");
rc = pthread_cond_wait(&amp;dataPresentCondition, &amp;dataMutex);
if (rc) {
printf("Consumer Thread %.8x %.8x: condwait failed, rc=%d\n",rc);
pthread_mutex_unlock(&amp;dataMutex);
exit(1);
}
}
printf("Consumer Thread %.8x %.8x: Found data or Notified, "
"CONSUME IT while holding lock\n",
pthread_getthreadid_np());
/* Typically an application should remove the data from being */
/* in the shared structure or Queue, then unlock. Processing */
/* of the data does not necessarily require that the lock is held */
/* Access to shared data goes here */
--sharedData;
/* We consumed the last of the data */
if (sharedData==0) {dataPresent=0;}
/* Repeat holding the lock. pthread_cond_wait releases it atomically */
}
printf("Consumer Thread %.8x %.8x: All done\n",pthread_getthreadid_np());
rc = pthread_mutex_unlock(&amp;dataMutex);
checkResults("pthread_mutex_unlock()\n", rc);
return NULL;
}
int main(int argc, char **argv)
{
pthread_t thread[NUMTHREADS];
int rc=0;
int amountOfData=4;
int i;
printf("Enter Testcase - %s\n", argv[0]);
printf("Create/start threads\n");
for (i=0; i &lt;NUMTHREADS; ++i) {
rc = pthread_create(&amp;thread[i], NULL, theThread, NULL);
checkResults("pthread_create()\n", rc);
}
/* The producer loop */
while (amountOfData--) {
printf("Producer: 'Finding' data\n");
sleep(3);
rc = pthread_mutex_lock(&amp;dataMutex); /* Protect shared data and flag */
checkResults("pthread_mutex_lock()\n", rc);
printf("Producer: Make data shared and notify consumer\n");
++sharedData; /* Add data */
dataPresent=1; /* Set boolean predicate */
rc = pthread_cond_signal(&amp;dataPresentCondition); /* wake up a consumer */
if (rc) {
pthread_mutex_unlock(&amp;dataMutex);
printf("Producer: Failed to wake up consumer, rc=%d\n", rc);
exit(1);
}
printf("Producer: Unlock shared data and flag\n");
rc = pthread_mutex_unlock(&amp;dataMutex);
checkResults("pthread_mutex_lock()\n",rc);
}
printf("Wait for the threads to complete, and release their resources\n");
for (i=0; i &lt;NUMTHREADS; ++i) {
rc = pthread_join(thread[i], NULL);
checkResults("pthread_join()\n", rc);
}
printf("Clean up\n");
rc = pthread_mutex_destroy(&amp;dataMutex);
rc = pthread_cond_destroy(&amp;dataPresentCondition);
printf("Main completed\n");
return 0;
}
</pre>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzahwvarco.htm" title="Condition variables allow threads to wait for certain events or conditions to occur and they notify other threads that are also waiting for the same events or conditions.">Condition variables and threads</a></div>
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