Ubuntu下的多线程编程

转自https://computing.llnl.gov/tutorials/pthreads/

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int     count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t)
{
  int i;
  long my_id = (long)t;

  for (i=0; i < TCOUNT; i++) {
    pthread_mutex_lock(&count_mutex);
    count++;

    /*
    Check the value of count and signal waiting thread when condition is
    reached.  Note that this occurs while mutex is locked.
    */
    if (count == COUNT_LIMIT) {
      printf("inc_count(): thread %ld, count = %d  Threshold reached. ",
             my_id, count);
      pthread_cond_signal(&count_threshold_cv);
      printf("Just sent signal.\n");
      }
    printf("inc_count(): thread %ld, count = %d, unlocking mutex\n",
	   my_id, count);
    pthread_mutex_unlock(&count_mutex);

    /* Do some work so threads can alternate on mutex lock */
    sleep(1);
    }
  pthread_exit(NULL);
}

void *watch_count(void *t)
{
  long my_id = (long)t;

  printf("Starting watch_count(): thread %ld\n", my_id);

  /*
  Lock mutex and wait for signal.  Note that the pthread_cond_wait routine
  will automatically and atomically unlock mutex while it waits.
  ！！！pthread_cond_wait进入等待时会自动解锁mutex
  Also, note that if COUNT_LIMIT is reached before this routine is run by
  the waiting thread, the loop will be skipped to prevent pthread_cond_wait
  from never returning.
  ！！!如果while循环条件跳出，会提前结束等待状态
  */
  pthread_mutex_lock(&count_mutex);
  while (count < COUNT_LIMIT) {
    printf("watch_count(): thread %ld Count= %d. Going into wait...\n", my_id,count);
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received. Count= %d\n", my_id,count);
    }
  printf("watch_count(): thread %ld Updating the value of count...\n", my_id);
  count += 125;
  printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
  printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
  pthread_mutex_unlock(&count_mutex);
  pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
  int i, rc;
  long t1=1, t2=2, t3=3;
  pthread_t threads[3];
  pthread_attr_t attr;

  /* Initialize mutex and condition variable objects */
  pthread_mutex_init(&count_mutex, NULL);
  pthread_cond_init (&count_threshold_cv, NULL);

  /* For portability, explicitly create threads in a joinable state */
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  pthread_create(&threads[0], &attr, watch_count, (void *)t1);
  pthread_create(&threads[1], &attr, inc_count, (void *)t2);
  pthread_create(&threads[2], &attr, inc_count, (void *)t3);

  /* Wait for all threads to complete */
  for (i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
  }
  printf ("Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n",
          NUM_THREADS, count);

  /* Clean up and exit */
  pthread_attr_destroy(&attr);
  pthread_mutex_destroy(&count_mutex);
  pthread_cond_destroy(&count_threshold_cv);
  pthread_exit (NULL);

}

输出

inc_count(): thread 3, count = 1, unlocking mutex
inc_count(): thread 2, count = 2, unlocking mutex
Starting watch_count(): thread 1
watch_count(): thread 1 Count= 2. Going into wait...
inc_count(): thread 3, count = 3, unlocking mutex
inc_count(): thread 2, count = 4, unlocking mutex
inc_count(): thread 3, count = 5, unlocking mutex
inc_count(): thread 2, count = 6, unlocking mutex
inc_count(): thread 3, count = 7, unlocking mutex
inc_count(): thread 2, count = 8, unlocking mutex
inc_count(): thread 3, count = 9, unlocking mutex
inc_count(): thread 2, count = 10, unlocking mutex
inc_count(): thread 3, count = 11, unlocking mutex
inc_count(): thread 2, count = 12  Threshold reached. Just sent signal.
inc_count(): thread 2, count = 12, unlocking mutex
watch_count(): thread 1 Condition signal received. Count= 12
watch_count(): thread 1 Updating the value of count...
watch_count(): thread 1 count now = 137.
watch_count(): thread 1 Unlocking mutex.
inc_count(): thread 3, count = 138, unlocking mutex
inc_count(): thread 2, count = 139, unlocking mutex
inc_count(): thread 3, count = 140, unlocking mutex
inc_count(): thread 2, count = 141, unlocking mutex
inc_count(): thread 3, count = 142, unlocking mutex
inc_count(): thread 2, count = 143, unlocking mutex
inc_count(): thread 3, count = 144, unlocking mutex
inc_count(): thread 2, count = 145, unlocking mutex
Main(): Waited and joined with 3 threads. Final value of count = 145. Done.