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  • pthread clean up

    https://www.ibm.com/developerworks/cn/linux/thread/posix_threadapi/part4/

    http://www.cnblogs.com/xfiver/archive/2013/01/23/2873725.html

    线程终止方式

    一般来说,Posix的线程终止有两种情况:正常终止和非正常终止。线程主动调用pthread_exit()或者从线程函数中return都将使线程正常退出,这是可预见的退出方式;非正常终止是线程在其他线程的干预下,或者由于自身运行出错(比如访问非法地址)而退出,这种退出方式是不可预见的。


    线程终止时的清理

    不论是可预见的线程终止还是异常终止,都会存在资源释放的问题,在不考虑因运行出错而退出的前提下,如何保证线程终止时能顺利的释放掉自己所占用的资源,特别是锁资源,就是一个必须考虑解决的问题。

    最经常出现的情形是资源独占锁的使用:线程为了访问临界资源而为其加上锁,但在访问过程中被外界取消,如果线程处于响应取消状态,且采用异步方式响应,或者在打开独占锁以前的运行路径上存在取消点,则该临界资源将永远处于锁定状态得不到释放。外界取消操作是不可预见的,因此的确需要一个机制来简化用于资源释放的编程。

    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)

    pthread_cleanup_push()/pthread_cleanup_pop()采用先入后出的栈结构管理,void routine(void *arg)函数在调用pthread_cleanup_push()时压入清理函数栈,多次对pthread_cleanup_push()的调用将在清理函数栈中形成一个函数链,在执行该函数链时按照压栈的相反顺序弹出。

    execute参数表示执行到pthread_cleanup_pop()时是否在弹出清理函数的同时执行该函数,为0表示不执行,非0为执行;这个参数并不影响异常终止时清理函数的执行。

    补充说明:
    
    1. 线程创建
    
    pthread_create()函数返回值0,表示创建成功,线程id保存载tidp中;失败则返回非零,需自行处理,不会修改errno值
    
    2. 线程终止
    
    a. 任一线程调用exit, _Exit, _exit都将导致整个进程终止;
    
    b. 单个线程退出方式有三种:
    
      1> 线程执行函数start_rtn()中使用return返回,返回值为线程退出码;
    
      2> 被同一个进程的其他线程使用pthread_cancel()取消;
    
      3> 线程自身调用了pthread_exit();
    
    说明:pthread_join(pthread_t tid, void **rval_ptr)函数会阻塞调用线程,直到tid线程通过上述三种方式终止退出,且return/pthread_exit()方式会设置相应线程退出码rval_ptr,而pthread_cancel()取消的线程,将退出码设置为PTHREAD_CANCELED.
    
    3. 线程清理处理程序(thread cleanup handler)
    
    3.a> pthread_cleanup_push()与pthread_cleanup_pop()均为<pthread.h>中实现的宏定义,具体实现如下:
    
    ?
    pthread_cleanup_push and pthread_cleanup_pop are macros and must always 
       be used in matching pairs at the same nesting level of braces.  */ 
    #  define pthread_cleanup_push(routine, arg)  
      do {                                         
        __pthread_cleanup_class __clframe (routine, arg) 
      
    /* Remove a cleanup handler installed by the matching pthread_cleanup_push. 
       If EXECUTE is non-zero, the handler function is called. */
    #  define pthread_cleanup_pop(execute)  
        __clframe.__setdoit (execute);                         
      } while (0) 
     可见push/pop中的{/}是一一对应的,因此pthread_cleanup_push/pop()也应一一对应出现,否则编译出错。
    
    3.b> 当线程执行下列之一操作时调用清理函数,thread_cleanup_push由栈结构实现,注意清理程序调用的顺序,先入后出。
    
      1: 调用pthread_exit()时,而直接return不会出发清理函数;
    
      2: 相应取消请求pthread_cancel()时;
    
      3: 使用非零execute参数调用pthread_cleanup_pop()时;
    
    尤其需注意pthread_cleanup_pop()参数不同及此语句所处位置不同而有不同效果。
    
    看此代码实例,注意return或pthread_exit()位置不同导致pthread_cleanup_pop()不同参数的效果变化。
    include <pthread.h> 
    void testPointerSize() 
    { 
        void *tret; 
        printf("size of pointer in x86-64:%d
    ",sizeof(tret));   
        //result is 8 in x86-64. 
        //which is 4 in x86-32. 
      
        printf("size of int in x86-64:%d
    ",sizeof(int));    
        //result is 4 in x86-64. 
        //which is also 4 in x86-32. 
    } 
    void cleanup(void *arg) 
    { 
        printf("cleanup:%s
    ",(char *)arg); 
    } 
    void * thr_fn1(void *arg) 
    { 
        printf("thread 1 start
    "); 
        pthread_cleanup_push(cleanup, "thread 1 first handler"); 
        pthread_cleanup_push(cleanup, "thread 1 second handler"); 
        if(arg) 
            return ((void *)1);//arg !=0 ,return here. 
    //  return here will not triger any cleanup. 
        pthread_cleanup_pop(0); 
        pthread_cleanup_pop(1); 
        return ((void *)2);//will not run this 
    } 
    void * thr_fn2(void *arg) 
    { 
        printf("thread 2 start
    "); 
        pthread_cleanup_push(cleanup, "thread 2 first handler"); 
        pthread_cleanup_push(cleanup, "thread 2 second handler"); 
        pthread_cleanup_pop(0); 
        pthread_cleanup_pop(1); 
        return ((void *)2); 
    //  return here can triger cleanup second handler; 
    } 
      
    void * thr_fn3(void *arg) 
    { 
        printf("thread 3 start
    "); 
        pthread_cleanup_push(cleanup, "thread 3 first handler"); 
        pthread_cleanup_push(cleanup, "thread 3 second handler"); 
        if(arg) 
            pthread_exit((void *)3); 
        //pthread_exit() here will triger both cleanup first&second handler. 
        pthread_cleanup_pop(1); 
        pthread_cleanup_pop(0); 
        pthread_exit((void *)3);//wont run this 
    } 
    void * thr_fn4(void *arg) 
    { 
        printf("thread 4 start
    "); 
        pthread_cleanup_push(cleanup, "thread 4 first handler"); 
        pthread_cleanup_push(cleanup, "thread 4 second handler"); 
        pthread_cleanup_pop(1); 
        pthread_cleanup_pop(0); 
        pthread_exit((void *)4); 
        //pthread_exit() here will triger cleanup second handler. 
    } 
      
    int main(void) 
    { 
        testPointerSize(); 
        int err; 
        pthread_t tid1, tid2, tid3, tid4; 
        void *tret; 
          
        err = pthread_create(&tid1, NULL, thr_fn1, (void *)1); 
        err = pthread_join(tid1,&tret);  
        printf("thread 1 exit code %d
    ",(int)tret); 
          
        err = pthread_create(&tid2, NULL, thr_fn2, (void *)2); 
        err = pthread_join(tid2, &tret); 
        printf("thread 2 exit code %d
    ",(int)tret); 
      
        err = pthread_create(&tid3, NULL, thr_fn3, (void *)3); 
        err = pthread_join(tid3,&tret);  
        printf("thread 3 exit code %d
    ",(int)tret); 
          
        err = pthread_create(&tid4, NULL, thr_fn4, (void *)4); 
        err = pthread_join(tid4, &tret); 
        printf("thread 4 exit code %d
    ",(int)tret); 
    } 
    运行结果:
    
    ?
    [root@hello testData]# ./test  
    size of pointer in x86-648 
    size of int in x86-644 
    thread 1 start 
    thread 1 exit code 1 
    thread 2 start 
    cleanup:thread 2 first handler 
    thread 2 exit code 2 
    thread 3 start 
    cleanup:thread 3 second handler 
    cleanup:thread 3 first handler 
    thread 3 exit code 3 
    thread 4 start 
    cleanup:thread 4 second handler 
    thread 4 exit code 4 

      由上述测试程序总结如下:

    1> push与pop间的return,将导致清理程序不被触发;

    2> 位于pop之后return,由pop的参数确定是否触发清理程序,非零参数触发,零参数不触发;

    3> push/pop间的pthread_exit(),将触发所有清理函数;

    4>位于pop之后的pthread_exit()时,pop参数决定是否触发清理程序;

    其实,上述四种情况只是测试验证了前文3.b所说三个条件,加深理解。

    参考文献:

    1. Posix线程编程指南(4)

    2. <UNIX环境高级编程(第2版)> P295-296程序

    3. pthread_cleanup_push()/pthread_cleanup_pop()的详解

    4. Linux中vim的列编辑实例 (Mark记录)

    pthread_cleanup_push()/pthread_cleanup_pop()是以宏方式实现的,这是pthread.h中的宏定义:

    #define pthread_cleanup_push(routine,arg)                                     
      { struct _pthread_cleanup_buffer _buffer;                                   
        _pthread_cleanup_push (&_buffer, (routine), (arg));
    #define pthread_cleanup_pop(execute)                                          
        _pthread_cleanup_pop (&_buffer, (execute)); }

    可见,pthread_cleanup_push()带有一个"{",而pthread_cleanup_pop()带有一个"}",因此这两个函数必须成对出现,且必须位于程序的同一级别的代码段中才能通过编译。在下面的例子里,当线程在"do some work"中终止时,将主动调用pthread_mutex_unlock(mut),以完成解锁动作。

    pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
    pthread_mutex_lock(&mut);
    /* do some work */
    pthread_mutex_unlock(&mut);
    pthread_cleanup_pop(0);

    必须要注意的是,如果线程处于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,功能与以下代码段相当:

    { int oldtype;
     pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
     pthread_cleanup_push(routine, arg);
     ...
     pthread_cleanup_pop(execute);
     pthread_setcanceltype(oldtype, NULL);
     }
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  • 原文地址:https://www.cnblogs.com/diegodu/p/3881194.html
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