pthread_cancel函数注意事项

/**************************************************
相关函数：
     #include <pthread.h>
     int pthread_cancel(pthread_t thread)
                     成功返回0，失败返回错误码
**************************************************/

此函数是POSIX提供的用于取消同一进程中的其它线程，此函
数只发送取消请求，并不等待要取消的进程退出！线程可以
选择忽略或是选择其它动作！

需要注意的是：
当我们调用它取消一个已经获得互斥锁/匿名信号量/写锁....但
还未释放其所获得锁的线程时，如果此线程被取消，此后所有想
要获得此时执行任务的线程都将处于睡眠状态，直到此锁被释放.
为避免此问题的产生，我们可以调用一组函数：
/**************************************************
    #include <pthread.h>

    void pthread_cleanup_push(void (*routine)(void *), void *arg)
    void pthread_cleanup_pop(int execute)
        参数解释:routine为函数指针,arg为传递给routine的参数
        execute为零时，从栈中删除注册的函数，删除后将再也
        不被执行。
**************************************************/
这两个函数被称为线程清理处理程序，类似于atexit函数，我们
可以注册多个清理函数，当执行以下动作时我们所注册的函数将会
回调(执行顺序与注册顺序相反):
    1.线程从pthread_exit(void *)函数退出时。
    2.线程响应取消请求时。
    3.执行pthread_cleanup_pop函数，execute参数为非零时。

这两个线程清理处理程序必须成对出现，必须处于同一作用域中，
否则会编译出错。

实例：
    如何使用这些函数处理以上问题!
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <signal.h>

#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>

#include <pthread.h>

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;

void *count(void *arg)
{
    int i=1;
    while(1)
    {
        sleep(1);
        printf("sec: %d\n", i++);
    }
}

void handler(void *arg)
{
    printf("[%u] is cancelled.\n", (unsigned)pthread_self());
    pthread_mutex_t *pm = (pthread_mutex_t *)arg;

    pthread_mutex_unlock(pm);
}

void *routine(void *arg)
{
    #ifdef CLEANUP
    pthread_cleanup_push(handler, (void *)&m);
    #endif

    pthread_mutex_lock(&m);
    printf("[%u] lock the mutex!\n", (unsigned)pthread_self());

    /*
    ** During sleep(), if the calling thread received a cancel-
    ** request and HASN'T established any cleanup handlers to
    ** unlock the mutex, it will leave the mutex a DEAD-LOCK
    ** state.
    */
    sleep(2);
    printf("[%u]: job finished!\n", (unsigned)pthread_self());

    pthread_mutex_unlock(&m);
    printf("[%u] unlock the mutex!\n", (unsigned)pthread_self());

    /*
    ** NOTE:
    **
    ** pthread_cleanup_push() and pthread_cleanup_pop() may be
    ** implemented as macro that expand to text containing '{'
    ** and '}', respectively. For this reason, the caller must
    ** user them pairly and ensure that they are paired within
    ** a same function and at the same lexical nesting level.
    */
    #ifdef CLEANUP
    pthread_cleanup_pop(0);
    #endif

    pthread_exit(NULL);
}

int main(int argc, char **argv)
{
    pthread_t t, t1, t2;
    pthread_create(&t, NULL, count, NULL);

    pthread_create(&t1, NULL, routine, NULL);
    pthread_create(&t2, NULL, routine, NULL);
    printf("[%u] ==> t1\n", (unsigned)t1);
    printf("[%u] ==> t2\n", (unsigned)t2);
    printf("[%u] ==> main\n", (unsigned)pthread_self());

    sleep(1);
    pthread_cancel(t1);
    pthread_cancel(t2);

    sleep(2);

    pthread_mutex_lock(&m);
    printf("[%u] locked the mutex!\n",
        (unsigned)pthread_self());
    pthread_mutex_unlock(&m);

    exit(0);
}