Linux多线程3-5_线程清理操作

一、概念
1、线程可以安排它退出时的清理操作，这与进程的可以用atexit函数安排进程退出时需要调用的函数类似。这样的函数称为线程清理处理程序。线程
     可以建立多个清理处理程序，处理程序记录在栈中，所以这些处理程序执行的顺序与他们注册的顺序相反

pthread_cleanup_push（void （*rtn）（void*）， void *args）//注册处理程序
pthread_cleanup_pop（int excute）//清除处理程序

2、当执行以下操作时调用清理函数，清理函数的参数由args传入
    1）、调用pthread_exit
    2）、响应取消请求
    3）、用非零参数调用pthread_cleanup_pop

3、清理函数的必要性
    也许你会认为线程不需什么清理操作，我可以在退出之前把所有该办的事情办了。但是，你不能确保你的线程永远正常的退出，加入它被取消呢。
    清理操作的优越性就在于，如果线程被取消了，那么清理函数会自动调用，这一点你是办不到的

4、清理函数要注意
    pthread_cleanup_push()  和 pthread_cleanup_pop() 是用宏定义继承的，宏定义中包含{} ，因此他们两要成对出席那
    
二、手册
PTHREAD_CLEANUP_PUSH(3)    Linux Programmer’s Manual   PTHREAD_CLEANUP_PUSH(3)

NAME
       pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancellation clean-up handlers
        //注册或者销毁线程取消是的清理函数

SYNOPSIS
       #include
        //包含头文件pthread.h

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

       Compile and link with -pthread.
        //编译连接线程库

DESCRIPTION
       These functions manipulate the calling thread’s stack of thread-cancellation clean-up handlers.  A clean-up handler is a
       function that is automatically executed when a thread is canceled (or in various other circumstances  described  below);
       it might, for example, unlock a mutex so that it becomes available to other threads in the process.
        //这两个函数处理线程栈中的取消清理函数。一个清理函数会在线程被取消的时候执行，比如 它可能去解锁一个互斥量
         //让它可以在其他线程中被使用

       The  pthread_cleanup_push()  function  pushes  routine  onto the top of the stack of clean-up handlers.  When routine is
       later invoked, it will be given arg as its argument.
        // pthread_cleanup_push()函数会将它的第一个参数指向的函数压入栈的顶端，当栈顶的函数被唤醒的时候，arg就是它的参数

       The pthread_cleanup_pop() function removes the routine at the top of the stack of clean-up handlers, and optionally exe-
       cutes it if execute is non-zero.
        // The pthread_cleanup_pop()函数将栈顶的清理函数从栈中清除，如果The pthread_cleanup_pop()的参数不是0 的话，清理
        //函数在被取出的时候还会执行

       A cancellation clean-up handler is popped from the stack and executed in the following circumstances:
        //一个取消清理操作会在一下的情况弹出并执行

       1. When  a  thread is canceled, all of the stacked clean-up handlers are popped and executed in the reverse of the order
          in which they were pushed onto the stack.
        //当线程被取消的时候，所有的被压入栈的清理函数都将弹出并执行，他们执行的顺序与入栈的顺序相反

       2. When a thread terminates by calling pthread_exit(3), all clean-up handlers are executed as described in the preceding
          point.  (Clean-up handlers are not called if the thread terminates by performing a return from the thread start func-
          tion.)
        //当一个线程用pthread_exit退出，所有的清理函数回像上面描述的一样执行。如果线程用return返回，那么清理函数不会
         //执行

       3. When a thread calls pthread_cleanup_pop() with a non-zero execute argument, the top-most clean-up handler  is  popped
          and executed.
        //如果线程以非零参数调用 pthread_cleanup_pop()，栈顶的清理函数就会弹出执行

       POSIX.1  permits  pthread_cleanup_push()  and pthread_cleanup_pop() to be implemented as macros that expand to text con-
       taining '{' and '}', respectively.  For this reason, the caller must ensure that calls to  these  functions  are  paired within  the  same
       function,  and at the same lexical nesting level.  (In other words, a clean-up handler is only established during the execution of a
       specified section of code.)
        //在POSIX中，  pthread_cleanup_push()  和 pthread_cleanup_pop() 是用宏定义继承的，宏定义中包含{}。因此要确保这两个函数
         //是成对调用的