pthread_clean_push与pthread_cleanup_pop函数

pthread_clean_push与pthread_cleanup_pop函数：

#include <pthread.h>

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

这两个函数操作调用线程的“线程取消时的清理函数的栈”。(These functions manipulate the calling thread's stack of thread-cancellation  clean-up  handlers.)“线程取消函数”即线程被取消或者下面描述的情况发生时自动调用的函数。它一般用于释放一些资源，比如释放锁，以免其它的线程永远也不能获得锁，而造成死锁。

pthread_cleanup_push()函数执行压栈清理函数的操作，而pthread_cleanup_pop()函数执行从栈中删除清理函数的操作。

在下面三种情况下，pthread_cleanup_push()压栈的“清理函数”会被调用：

1, 线程调用pthread_exit()函数，而不是直接return.
2, 响应取消请求时，也就是有其它的线程对该线程调用pthread_cancel()函数。
3, 本线程调用pthread_cleanup_pop()函数，并且其参数非0.

所以：

1）当pthread_cleanup_pop()函数的参数为0时，仅仅在线程调用pthread_exit函数或者其它线程对本线程调用 

     pthread_cancel函数时，才在弹出“清理函数”的同时执行该“清理函数”。

2）注意pthread_exit终止线程与线程直接return终止线程的区别。

3）pthread_cleanup_push()函数与pthread_cleanup_pop()函数必须成对的出现在同一个函数中。

下面是man pthread_cleanup_push中的一个例子：

#include <pthread.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#define handle_error_en(en, msg) \
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

static int done = 0;
static int cleanup_pop_arg = 0;
static int cnt = 0;

static void cleanup_handler(void *arg)
{
        printf("Called clean-up handler\n");
        cnt = 0;
}

static void *thread_start(void *arg)
{
        time_t start, curr;

        printf("New thread started\n");
        pthread_cleanup_push(cleanup_handler, NULL);
        curr = start = time(NULL);
        while(!done){
                pthread_testcancel();           /* A cancellation point */
                if(curr < time(NULL)){
                        curr = time(NULL);
                        printf("cnt = %d\n", cnt);  /* A cancellation point */
                        cnt++;
                }
        }

        pthread_cleanup_pop(cleanup_pop_arg);
        return NULL;
}

int main(int argc, char *argv[])
{
        pthread_t thr;
        void *res;

        pthread_create(&thr, NULL, thread_start, NULL);
        sleep(2);           /* Allow new thread to run a while */

        if(argc > 1){
                if(argc > 2)
                        cleanup_pop_arg = atoi(argv[2]);
                done = 1;

        }else{
                printf("Canceling thread\n");
                pthread_cancel(thr);
        }

        pthread_join(thr, &res);
        if(res == PTHREAD_CANCELED)
                printf("Thread was canceled; cnt = %d\n", cnt);
        else
                printf("Thread terminated normally; cnt = %d\n", cnt);
        exit(EXIT_SUCCESS);
}

执行情况：

digdeep@ubuntu:~/pthread/learnthread$ ./cleanup
New thread started
cnt = 0
cnt = 1
Canceling thread
Called clean-up handler
Thread was canceled; cnt = 0
digdeep@ubuntu:~/pthread/learnthread$ ./cleanup a
New thread started
cnt = 0
cnt = 1
Thread terminated normally; cnt = 2
digdeep@ubuntu:~/pthread/learnthread$ ./cleanup a 2
New thread started
cnt = 0
cnt = 1
Called clean-up handler
Thread terminated normally; cnt = 0
digdeep@ubuntu:~/pthread/learnthread$

执行./cleanup 时，因为 argc > 1不成立，所以调用了pthread_cancel(thr);所以线程是被“取消”的，所以pthread_cleanup_push压栈的函数一点会被调用。

执行./cleanup a 时，argc > 1，但是pthread_cleanup_push()的参数cleanup_pop_arg为0，也没有调用pthread_cancel(thr);所以线程是被pthread_join(thr, &res);正常完成的，没有pthread_exit。所以不会调用pthread_cleanup_push()压栈的清理函数。

执行./cleanup a 2 时，cleanup_pop_arg ！= 0，所以即使线程不是被“取消”的，但是因为pthread_cleanup_pop的参数不是0，所以pthread_cleanup_pop函数在弹出“清理函数”的同时也会执行该“清理函数”。