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<TD><A class=left-nav-child
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<TD><A class=left-nav-child
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<H1>Posix线程编程指南(4)</H1>
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<P>级别: 初级</P>
<P><A
href="http://www-128.ibm.com/developerworks/cn/linux/thread/posix_threadapi/part4/#author">杨沙洲</A>
<BR></P>
<P>2001 年 11 月 01 日</P>
<BLOCKQUOTE>这是一个关于Posix线程编程的专栏。作者在阐明概念的基础上，将向您详细讲述Posix线程库API。本文是第四篇将向您讲述线程中止。</BLOCKQUOTE><!--START RESERVED FOR FUTURE USE INCLUDE FILES--><!-- include java script once we verify teams wants to use this and it will work on dbcs and cyrillic characters --><!--END RESERVED FOR FUTURE USE INCLUDE FILES-->
<P><A name=1><SPAN class=atitle>线程终止方式</SPAN></A></P>
<P>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。</P><BR>
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<P><A name=2><SPAN class=atitle>线程终止时的清理</SPAN></A></P>
<P>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。</P>
<P>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。</P>
<P>在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：</P>
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<TD><PRE><CODE class=section>void pthread_cleanup_push(void (*routine) (void *), void *arg)
void pthread_cleanup_pop(int execute)
</CODE></PRE></TD></TR></TBODY></TABLE><BR>
<P>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。</P>
<P>pthread_cleanup_push()/pthread_cleanup_pop()是以宏方式实现的，这是pthread.h中的宏定义：</P>
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<TD><PRE><CODE class=section>#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)); }
</CODE></PRE></TD></TR></TBODY></TABLE><BR>
<P>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。</P>
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<TD><PRE><CODE class=section>pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
</CODE></PRE></TD></TR></TBODY></TABLE><BR>
<P>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：</P>
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<TD><PRE><CODE class=section>{ int oldtype;
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
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