例: mutexes の使用 (Pthread プログラム)

この例では、相互排他 (mutex) を使って共用データへのアクセスを保護するいくつかのスレッドを開始する、Pthread プログラムを示しています。

注: コード例を使用すると、コードに関するライセンス情報および特記事項の条件に同意したものとみなされます。

/*
Filename: ATEST16.QCSRC
The output of this example is as follows:
 Enter Testcase - LIBRARY/ATEST16
 Hold Mutex to prevent access to shared data
 Create/start threads
 Wait a bit until we are 'done' with the shared data
 Thread 00000000 00000019: Entered
 Thread 00000000 0000001a: Entered
 Thread 00000000 0000001b: Entered
 Unlock shared data
 Wait for the threads to complete, and release their resources
 Thread 00000000 00000019: Start critical section, holding lock
 Thread 00000000 00000019: End critical section, release lock
 Thread 00000000 0000001a: Start critical section, holding lock
 Thread 00000000 0000001a: End critical section, release lock
 Thread 00000000 0000001b: Start critical section, holding lock
 Thread 00000000 0000001b: End critical section, release lock
 Clean up
 Main completed
*/
#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define checkResults(string, val) {             \
 if (val) {                                     \
   printf("Failed with %d at %s", val, string); \
   exit(1);                                     \
 }                                              \
}
 
#define                 NUMTHREADS   3
pthread_mutex_t         mutex = PTHREAD_MUTEX_INITIALIZER;
int                     sharedData=0;
int                     sharedData2=0;
 
void *theThread(void *parm)
{
   int   rc;
   printf("Thread %.8x %.8x: Entered\n", pthread_getthreadid_np());
   rc = pthread_mutex_lock(&mutex);
   checkResults("pthread_mutex_lock()\n", rc);
   /********** Critical Section *******************/
   printf("Thread %.8x %.8x: Start critical section, holding lock\n",
          pthread_getthreadid_np());
   /* Access to shared data goes here */
   ++sharedData; --sharedData2;
   printf("Thread %.8x %.8x: End critical section, release lock\n",
          pthread_getthreadid_np());
   /********** Critical Section *******************/
   rc = pthread_mutex_unlock(&mutex);
   checkResults("pthread_mutex_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]);
 
  printf("Hold Mutex to prevent access to shared data\n");
  rc = pthread_mutex_lock(&mutex);
  checkResults("pthread_mutex_lock()\n", rc);
 
  printf("Create/start threads\n");
  for (i=0; i<NUMTHREADS; ++i) {
 rc = pthread_create(&thread[i], NULL, theThread, NULL);
     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 = pthread_mutex_unlock(&mutex);
  checkResults("pthread_mutex_lock()\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 = pthread_mutex_destroy(&mutex);
  printf("Main completed\n");
  return 0;
}