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<H1>Posix线程编程指南(3)</H1>
<P id=subtitle><EM>线程同步</EM></P><IMG class=display-img
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<P>级别: 初级</P>
<P><A
href="http://www-128.ibm.com/developerworks/cn/linux/thread/posix_threadapi/part3/#author">杨沙洲</A>
<BR></P>
<P>2001 年 10 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。本文是第三篇将向您讲述线程同步。</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>互斥锁</SPAN></A></P>
<P>尽管在Posix
Thread中同样可以使用IPC的信号量机制来实现互斥锁mutex功能，但显然semphore的功能过于强大了，在Posix
Thread中定义了另外一套专门用于线程同步的mutex函数。</P>
<P><A name=N10048><SPAN class=smalltitle>1． 创建和销毁</SPAN></A></P>
<P>有两种方法创建互斥锁，静态方式和动态方式。POSIX定义了一个宏PTHREAD_MUTEX_INITIALIZER来静态初始化互斥锁，方法如下：
pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;
在LinuxThreads实现中，pthread_mutex_t是一个结构，而PTHREAD_MUTEX_INITIALIZER则是一个结构常量。</P>
<P>动态方式是采用pthread_mutex_init()函数来初始化互斥锁，API定义如下： int
pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutexattr_t
*mutexattr) 其中mutexattr用于指定互斥锁属性（见下），如果为NULL则使用缺省属性。</P>
<P>pthread_mutex_destroy()用于注销一个互斥锁，API定义如下： int
pthread_mutex_destroy(pthread_mutex_t *mutex)
销毁一个互斥锁即意味着释放它所占用的资源，且要求锁当前处于开放状态。由于在Linux中，互斥锁并不占用任何资源，因此LinuxThreads中的pthread_mutex_destroy()除了检查锁状态以外（锁定状态则返回EBUSY）没有其他动作。</P>
<P><A name=N10057><SPAN class=smalltitle>2． 互斥锁属性</SPAN></A></P>
<P>互斥锁的属性在创建锁的时候指定，在LinuxThreads实现中仅有一个锁类型属性，不同的锁类型在试图对一个已经被锁定的互斥锁加锁时表现不同。当前（glibc2.2.3,linuxthreads0.9）有四个值可供选择：</P>
<UL>
<LI>PTHREAD_MUTEX_TIMED_NP，这是缺省值，也就是普通锁。当一个线程加锁以后，其余请求锁的线程将形成一个等待队列，并在解锁后按优先级获得锁。这种锁策略保证了资源分配的公平性。
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