pthread中的互斥锁和条件锁

pthread_mutex_t和pthread_cond_t.

为什么互斥锁和条件锁通常要一起使用？

目的是给资源或者临界区加锁，同时不消耗CPU资源。

如果没有条件锁，互斥锁会一直轮询，导致大量CPU时间被占用。如下代码中，如果将条件锁注掉，那么大约占18.5%的CPU时间，如果有条件锁，基本是0%。

/**                                                                                                                                                                     
 * @file: test_thread_mutex.c                                                                                                                                           
 * @brief : test whether pthread_mutex_t is enough in a multi-threaded application                                                                                       
 */

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

#define TOTAL_THREAD_NUM 10

pthread_mutex_t work_mutex;
pthread_cond_t work_cond;
int running[TOTAL_THREAD_NUM] = {0};

void *thread_function(void *arg);

static int check_collision() {
        int i = 0;
        int sum = 0;
        for (i=0; i<TOTAL_THREAD_NUM; i++) {
                sum += running[i];
        }
        if (sum > 1) {
                return 1;
        } else {
                return 0;
        }
}


int main(int argc, char *argv[]) {
        pthread_t thread[TOTAL_THREAD_NUM];
        int ret;
        int i;

        ret = pthread_mutex_init(&work_mutex, NULL);
        if (ret != 0) {
                perror("Mutex Initialization Failed");
                exit(EXIT_FAILURE);
        }
        ret = pthread_cond_init(&work_cond, NULL);
        if (ret != 0) {
                perror("Cond Initialization Failed");
                exit(EXIT_FAILURE);
        }

        for (i=0; i<TOTAL_THREAD_NUM; i++) {
                printf("Main: creating thread %d \n", i);
                ret = pthread_create(&thread[i], NULL, thread_function, (void*)i);
                if (ret != 0) {
                        perror("Create Threads Failed");
                        exit(EXIT_FAILURE);
                }
        }
        for (i=0; i<TOTAL_THREAD_NUM; i++) {
                ret = pthread_join(thread[i], NULL);
                if (ret != 0) {
                        perror("thread join failed");
                        exit(EXIT_FAILURE);
                }
        }
        pthread_mutex_destroy(&work_mutex);
        return 0;
}

void* thread_function(void *arg) {
        int i = (int)arg;
        for (;;) {
                pthread_mutex_lock(&work_mutex);
/*      printf("thread %d is running now\n", i); */
/*      fflush(stdout); */
                pthread_cond_wait(&work_cond, &work_mutex);
                running[i] = 1;
                if (check_collision() != 0) {
                        printf("Collision Occured!\n");
                }
                running[i] = 0;
                pthread_cond_signal(&work_cond);
                pthread_mutex_unlock(&work_mutex);
        }
        return NULL;
}