AlphonseTai's Blog

由于最近在看开源代码，手里的开源代码为了能够比较好的可移植性，在代码中加了很多宏，并在linux用了pthread库。

其中对于pthread的条件变量理解很是疑惑，看pthread手册后还是有不解，感觉条件变量用起来太奇怪，在pthread_cond_wait时，互斥到底有没有释放等问题上没搞明白，上代码调试后遍便豁然开朗。先贴一个修改过的官方代码例子：

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

#include <iostream>

#include <unistd.h>

using namespace std;

#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.
  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.
  */
  pthread_mutex_lock(&count_mutex);
  while (1) {
    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);
    //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);
}

void cond_test()
{
	cout << "---------- cond_test ----------" << endl;

	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);
	usleep(1000*20);
	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);
}

首先说一下例子，主线程创建一个inc线程和watch线程，每次inc线程激活条件变量，这时watch线程被激活。
为了测试在pthread_cond_wait函数进入是，互斥有没有被释放掉。因此注释掉高亮部分的代码，发现inc线程永远不会被激活。说明在pthread_cond_wait时会将互斥进行释放，接下来等有空一定要抽空看看pthread的实现代码。以及比较一下vs是如何将pthread库进行伪封装后在windows上提供使用的。