Linux多线程3-3_线程取消

一、线程取消
    将正在运行的线程取消执行，一个线程可以取消另一个线程，线程也可以自己取消自己。当线程被取消之后，会调用清理函数    

二、取消函数
    int pthread_cancel（pthread_t tid）
    取消tid指定的线程，成功返回0。但是取消只是发送一个请求，并不意味着等待线程终止，而且发送成功也不意味着tid一定会终止

三、取消状态
    1、概念
    取消状态，就是线程对取消信号的处理方式，忽略或者响应。线程创建时默认响应取消信号
    2、函数
    int pthread_setcancelstate(int state, int *oldstate)  
    设置本线程对Cancel信号的反应，state有两种值：PTHREAD_CANCEL_ENABLE（缺省）和PTHREAD_CANCEL_DISABLE，分别表示收到信号后设为CANCLED状态和忽略CANCEL信号继续运行；old_state如果不为NULL则存入原来的Cancel状态以便恢复。

四、取消类型
    1、概念
    取消类型，是线程对取消信号的响应方式，立即取消或者延时取消。线程创建时默认延时取消
    2、函数
    int pthread_setcanceltype(int type, int *oldtype) 
    设置本线程取消动作的执行时机，type由两种取值：PTHREAD_CANCEL_DEFFERED和PTHREAD_CANCEL_ASYCHRONOUS，仅当Cancel状态为Enable时有效，分别表示收到信号后继续运行至下一个取消点再退出和立即执行取消动作（退出）；oldtype如果不为NULL则存入运来的取消动作类型值。

五、取消点
    取消一个线程，它通常需要被取消线程的配合。线程在很多时候会查看自己是否有取消请求。如果有就主动退出， 这些查看是否有取消的地方称为取消点很多地方都是包含取消点，包括：
pthread_join()、 pthread_testcancel()、pthread_cond_wait()、 pthread_cond_timedwait()、sem_wait()、sigwait()、write、read,大多数会阻塞的系统调用。你可以通过man pthreads来查看当前系统中那些操作是取消点

六、手册
PTHREAD_CANCEL(3)          Linux Programmer’s Manual         PTHREAD_CANCEL(3)
NAME
       pthread_cancel - send a cancellation request to a thread
        //向一个线程发送取消请求

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

       int pthread_cancel(pthread_t thread);

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

DESCRIPTION
       The  pthread_cancel()  function  sends  a cancellation request to the thread thread.  Whether and when the target thread reacts to the cancellation         request depends on two attributes that are under the control of that thread: its  cancelability state and type.
         //这个函数是向目标线程发送一个取消请求，目标线程会如何处理取消请求或者何时去处理都由它自己的属性来决定，线程的取消属性有取消类型
         // 和 取消状态

       A thread’s cancelability state, determined by pthread_setcancelstate(3), can be enabled (the default for new threads) or disabled.  If a thread has         disabled cancellation, then a cancellation request remains queued until  the  thread  enables cancellation.  If a thread has enabled cancellation, then         its cancelability type determines when cancellation occurs.
         //线程的取消状态是由函数 pthread_setcancelstate来设置的，可以设置为可取消（默认）或不可取消 。如果是不可取消状态，那么取消请求会一直
         // 发送直到目标线程变为可取消状态。如果线程是一个可取消的状态，那么取消类型决定了何时被取消

       A  thread’s  cancellation  type,  determined  by  pthread_setcanceltype(3),  may be either asynchronous or deferred (the default for new threads).        Asynchronous cancelability means that the thread can be canceled at any time (usually immediately,  but  the  system does not guarantee this).        Deferred cancelability means that cancellation will be delayed until the thread next calls a function that is a cancellation point.  A list of functions that        are  or  may  be  cancellation points is provided in pthreads(7).
         //一线线程的取消类型有函数 pthread_setcanceltype来修改，可以是异步的或者延时的（默认）。异步表示线程可以在任何时刻被取消（一般是立刻，
        //但操作系统不会确保永远这样）。延时取消代表取消操作会被延时直到下一个取消点。关于那些函数是取消点，你可以看手册man pthreads

       When a cancellation requested is acted on, the following steps occur for thread (in this order):     
        //一旦一个取消请求被响应了，那么以下的事情会发生

       1. Cancellation  clean-up  handlers are popped (in the reverse of the order in which they were pushed) and called.  (See
          pthread_cleanup_push(3).)
        //一个已经压栈的清除操作会被调用

       2. Thread-specific data destructors are called, in an unspecified order.  (See pthread_key_create(3).)
        //线程的特殊数据销毁程序会被调用，并且顺序不确定

       3. The thread is terminated.  (See pthread_exit(3).)
         //线程结束了

       The above steps happen asynchronously with respect to the pthread_cancel() call; the return status  of  pthread_cancel() merely informs the caller         whether the cancellation request was successfully queued.
         //以上的操作都会异步的执行， pthread_cancel() 的返回值会表示到底取消有没有成功
    
       After  a  canceled  thread has terminated, a join with that thread using pthread_join(3) obtains PTHREAD_CANCELED as the thread’s exit status.          (Joining with a thread is the only way to know that cancellation has completed.)
        //如果一个线程因为被取消而结束，那么如果有线程用pthread_join去连接它，那么 PTHREAD_CANCELED值会返回个这个join函数
RETURN VALUE
       On success, pthread_cancel() returns 0; on error, it returns a non-zero error number.
         //成功返回0，失败返回错误码

ERRORS
       ESRCH  No thread with the ID thread could be found.
        //找不到指定的线程

CONFORMING TO
       POSIX.1-2001.

NOTES
       On Linux, cancellation is implemented using signals.  Under the NPTL threading implementation, the first real-time  signal (i.e., signal 32) is used         for this purpose.  On LinuxThreads, the second real-time signal is used, if real-time signals are available, otherwise SIGUSR2 is used. PTHREAD_SETCANCELSTATE(3)  Linux Programmer’s Manual PTHREAD_SETCANCELSTATE(3)
NAME
       pthread_setcancelstate, pthread_setcanceltype - set cancelability state and type
        //设置取消状态和类型

SYNOPSIS
       #include
         //包含头文件

       int pthread_setcancelstate(int state, int *oldstate);
       int pthread_setcanceltype(int type, int *oldtype);

       Compile and link with -pthread.
        //编译连接使用线程库

DESCRIPTION
       The pthread_setcancelstate() sets the cancelability state of the calling thread to the value given in state.  The previous cancelability state of the         thread is returned in the buffer pointed to by oldstate.  The state  argument  must  have one of the following values:
        //这个函数可以设置线程的取消状态，之前的取消状态会返回到oldstate指定的缓冲区，取消状态只能是下面的值


       PTHREAD_CANCEL_ENABLE
              The  thread  is  cancelable.   This  is the default cancelability state in all new threads, including the initial thread.  The thread’s cancelability type               determines when a cancelable  thread  will  respond  to  a  cancellation request.
                //这个线程是可以取消的。这是一个线程默认的状态，包含初始化。线程的取消类型决定了线程何时响应取消请求

       PTHREAD_CANCEL_DISABLE
              The  thread  is  not  cancelable.   If  a  cancellation request is received, it is blocked until cancelability is enabled.
                 //这个线程是不可取消的。如果取消请求来了，它会阻塞，直到线程可以取消。

       The pthread_setcanceltype() sets the cancelability type of the calling thread to the value given in type.  The  previous cancelability  type  of  the         thread is returned in the buffer pointed to by oldtype.  The type argument must have one of the following values:
        // pthread_setcanceltype用来设置线程的取消类型。之前的取消类型会返回到oldtype指向的缓冲区，取消类型只能是一下的值

       PTHREAD_CANCEL_DEFERRED
              A cancellation request is deferred until the thread next calls a function  that  is  a  cancellation  point  (see pthreads(7)).  This is the default               cancelability type in all new threads, including the initial thread.
                 //取消请求被延时了，直到下一个取消点。
        
       PTHREAD_CANCEL_ASYNCHRONOUS
              The  thread  can be canceled at any time.  (Typically, it will be canceled immediately upon receiving a cancella-
              tion request, but the system doesn’t guarantee this.)
              //线程可以在任何时刻被取消（一般是立刻， 但操作系统不会确保永远这样 ）

       The set-and-get operation performed by each of these functions is atomic with respect to other threads  in  the  process calling the same function.
        //这些set和get操作都是原子的