线程退出时自动释放资源

今天碰到一个问题:主线程pthread_create一个子线程A,子线程pthread_mutex_lock,然后调用其他的函数fun,最后从fun返回后再pthread_mutex_unlock.
但是如果在fun中调用了pthread_exit异常退出,那么岂不是没释放锁就退出了,这肯定会引起死锁.

解决办法一:
在fun中调用pthread_exit之前都先调用pthread_mutex_lock释放锁,
但是这就需要吧mutex作为参数传给fun,如果fun再调用了其他函数,就得一层一层的把mutex传下去,
而且要找到fun及其调用的函数中的pthread_exit然后再修改是很麻烦的,如果fun是一个第三方程序,而且退出是调用了exit而不是pthread_exit,那就更麻烦了.

解决办法二:
POSIX线程API中提供了一个pthread_cleanup_push()/pthread_cleanup_pop()函数对用于自动释放资源.
从pthread_cleanup_push()的调用点到pthread_cleanup_pop()之间的程序段中的终止动作（包括调用 pthread_exit()和取消点终止）都将执行pthread_cleanup_push()所指定的清理函数
API定义如下:

void pthread_cleanup_push(void (*routine) (void *), void *arg)
void pthread_cleanup_pop(int execute)

代码示例如下:

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

#define THREAD_NUMBER 2
pthread_mutex_t mutex;

void aa()
{
    pthread_exit(NULL);
}

void* hello1(void *arg)
{
    char *hello_str = (char *)arg;
    int oldtype;

    pthread_detach (pthread_self ());

    pthread_cleanup_push(pthread_mutex_unlock, (void *) &mutex);
    pthread_mutex_lock(&mutex);
    sleep(2);
    printf("%s\n", hello_str);
    aa();
    pthread_mutex_unlock(&mutex);
    pthread_cleanup_pop(0);
}

void* hello2(void *arg)
{
    char *hello_str = (char *)arg;

    pthread_detach (pthread_self ());
    pthread_mutex_lock(&mutex);
    sleep(1);
    printf("%s\n", hello_str);
    pthread_mutex_unlock(&mutex);
}

int main(int argc, char *argv[])
{
    int i;
    int ret_val;

    pthread_t pt[THREAD_NUMBER];
    const char *arg[THREAD_NUMBER];
    arg[0] = "hello world from thread1";
    arg[1] = "hello world from thread2";

    pthread_mutex_init(&mutex,NULL);
    printf("Begin to create threads...\n");
    ret_val = pthread_create(&pt[0], NULL, hello1, (void *)arg[0]);
    if (ret_val != 0 ) {
        printf("pthread_create error!\n");
        exit(1);
    }

    ret_val = pthread_create(&pt[1], NULL, hello2, (void *)arg[1]);
    if (ret_val != 0 ) {
        printf("pthread_create error!\n");
        exit(1);
    }

    sleep(5);
    printf("Now, the main thread returns.\n");
    return 0;
}


$ gcc -o a.out test.c -lpthread
$ ./a.out
Begin to create threads...
hello world from thread1
hello world from thread2
Now, the main thread returns.

可以看出hello1()->aa()->pthread_exit(),当线程hello1退出后锁已经释放了
其实不光是释放锁,还可以释放其他资源.

当然上述pthread_cleanup_push()/pthread_cleanup_pop()是有缺陷的,
比如线程处于PTHREAD_CANCEL_ASYNCHRONOUS状态，上述代码段就有可能出错，
因为CANCEL事件有可能在 pthread_cleanup_push()和pthread_mutex_lock()之间发生，或者在 pthread_mutex_unlock()和pthread_cleanup_pop()之间发生，从而导致清理函数unlock一个并没有加锁的 mutex变量，造成错误。
因此，在使用清理函数的时候，都应该暂时设置成PTHREAD_CANCEL_DEFERRED模式。为此，POSIX的 Linux实现中还提供了一对不保证可移植的 pthread_cleanup_push_defer_np()/pthread_cleanup_pop_defer_np()扩展函数

具体可参考下面这篇文章

reference:

Posix线程编程指南(4)