详解Linux进程间通信——使用信号量
<p><span><strong>一、什么是信号量</strong></span></p>
<p>
为了防止出现因多个程序同时访问一个共享资源而引发的一系列问题，我们需要一种方法，它可以通过生成并使用令牌来授权，在任一时刻只能有一个执行线程访问代码的临界区域。临界区域是指执行数据更新的代码需要独占式地执行。而信号量就可以提供这样的一种访问机制，让一个临界区同一时间只有一个线程在访问它，也就是说信号量是用来调协进程对共享资源的访问的。</p>
<p>
信号量是一个特殊的变量，程序对其访问都是原子操作，且只允许对它进行等待（即P(信号变量))和发送（即V(信号变量))信息操作。最简单的信号量是只能取0和1的变量，这也是信号量最常见的一种形式，叫做二进制信号量。而可以取多个正整数的信号量被称为通用信号量。这里主要讨论二进制信号量。</p>
<p>
<span><strong>二、信号量的工作原理</strong></span></p>
<p>
由于信号量只能进行两种操作等待和发送信号，即P(sv)和V(sv),他们的行为是这样的：</p>
<p>
P(sv)：如果sv的值大于零，就给它减1；如果它的值为零，就挂起该进程的执行</p>
<p>
V(sv)：如果有其他进程因等待sv而被挂起，就让它恢复运行，如果没有进程因等待sv而挂起，就给它加1.</p>
<p>
举个例子，就是两个进程共享信号量sv，一旦其中一个进程执行了P(sv)操作，它将得到信号量，并可以进入临界区，使sv减1。而第二个进程将被阻止进入临界区，因为当它试图执行P(sv)时，sv为0，它会被挂起以等待第一个进程离开临界区域并执行V(sv)释放信号量，这时第二个进程就可以恢复执行。</p>
<p>
<span><strong>三、Linux的信号量机制</strong></span></p>
<p>
Linux提供了一组精心设计的信号量接口来对信号进行操作，它们不只是针对二进制信号量，下面将会对这些函数进行介绍，但请注意，这些函数都是用来对成组的信号量值进行操作的。它们声明在头文件sys/sem.h中。</p>
<p>
<strong>1、semget函数</strong></p>
<p>
它的作用是创建一个新信号量或取得一个已有信号量，原型为：</p>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">semget(key_t key, </code><code class="cpp color1 bold">int</code> <code class="cpp plain">num_sems, </code><code class="cpp color1 bold">int</code> <code class="cpp plain">sem_flags);</code>
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第一个参数key是整数值（唯一非零），不相关的进程可以通过它访问一个信号量，它代表程序可能要使用的某个资源，程序对所有信号量的访问都是间接的，程序先通过调用semget函数并提供一个键，再由系统生成一个相应的信号标识符（semget函数的返回值），只有semget函数才直接使用信号量键，所有其他的信号量函数使用由semget函数返回的信号量标识符。如果多个程序使用相同的key值，key将负责协调工作。</p>
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第二个参数num_sems指定需要的信号量数目，它的值几乎总是1。</p>
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第三个参数sem_flags是一组标志，当想要当信号量不存在时创建一个新的信号量，可以和值IPC_CREAT做按位或操作。设置了IPC_CREAT标志后，即使给出的键是一个已有信号量的键，也不会产生错误。而IPC_CREAT | IPC_EXCL则可以创建一个新的，唯一的信号量，如果信号量已存在，返回一个错误。</p>
<p>
semget函数成功返回一个相应信号标识符（非零），失败返回-1.</p>
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<strong>2、semop函数</strong></p>
<p>
它的作用是改变信号量的值，原型为：</p>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">semop(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">sem_id, </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sembuf *sem_opa, </code><code class="cpp color1 bold">size_t</code> <code class="cpp plain">num_sem_ops);</code>
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sem_id是由semget返回的信号量标识符，sembuf结构的定义如下：</p>
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<code class="cpp keyword bold">struct</code> <code class="cpp plain">sembuf{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">short</code> <code class="cpp plain">sem_num;</code><code class="cpp comments">//除非使用一组信号量，否则它为0 </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">short</code> <code class="cpp plain">sem_op;</code><code class="cpp comments">//信号量在一次操作中需要改变的数据，通常是两个数，一个是-1，即P（等待）操作， </code>
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<code class="cpp spaces"> </code><code class="cpp comments">//一个是+1，即V（发送信号）操作。 </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">short</code> <code class="cpp plain">sem_flg;</code><code class="cpp comments">//通常为SEM_UNDO,使操作系统跟踪信号， </code>
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<code class="cpp spaces"> </code><code class="cpp comments">//并在进程没有释放该信号量而终止时，操作系统释放信号量 </code>
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<code class="cpp plain">}; </code>
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<strong>3、semctl函数</strong></p>
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该函数用来直接控制信号量信息，它的原型为：</p>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">semctl(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">sem_id, </code><code class="cpp color1 bold">int</code> <code class="cpp plain">sem_num, </code><code class="cpp color1 bold">int</code> <code class="cpp plain">command, ...);</code>
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如果有第四个参数，它通常是一个union semum结构，定义如下：</p>
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<code class="cpp keyword bold">union</code> <code class="cpp plain">semun{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">val; </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">semid_ds *buf; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">unsigned </code><code class="cpp color1 bold">short</code> <code class="cpp plain">*arry; </code>
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<code class="cpp plain">};</code>
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前两个参数与前面一个函数中的一样，command通常是下面两个值中的其中一个</p>
<ul>
<li>
SETVAL：用来把信号量初始化为一个已知的值。p 这个值通过union semun中的val成员设置，其作用是在信号量第一次使用前对它进行设置。</li>
<li>
IPC_RMID：用于删除一个已经无需继续使用的信号量标识符。</li>
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<span><strong>四、进程使用信号量通信</strong></span></p>
<p>
下面使用一个例子来说明进程间如何使用信号量来进行通信，这个例子是两个相同的程序同时向屏幕输出数据，我们可以看到如何使用信号量来使两个进程协调工作，使同一时间只有一个进程可以向屏幕输出数据。注意，如果程序是第一次被调用（为了区分，第一次调用程序时带一个要输出到屏幕中的字符作为一个参数），则需要调用set_semvalue函数初始化信号并将message字符设置为传递给程序的参数的第一个字符，同时第一个启动的进程还负责信号量的删除工作。如果不删除信号量，它将继续在系统中存在，即使程序已经退出，它可能在你下次运行此程序时引发问题，而且信号量是一种有限的资源。</p>
<p>
在main函数中调用semget来创建一个信号量，该函数将返回一个信号量标识符，保存于全局变量sem_id中，然后以后的函数就使用这个标识符来访问信号量。</p>
<p>
源文件为seml.c，代码如下：</p>
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<code class="cpp preprocessor">#include <unistd.h> </code>
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<code class="cpp preprocessor">#include <sys/types.h> </code>
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<code class="cpp preprocessor">#include <sys/stat.h> </code>
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<code class="cpp preprocessor">#include <fcntl.h> </code>
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<code class="cpp preprocessor">#include <stdio.h> </code>
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<code class="cpp preprocessor">#include <string.h> </code>
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<code class="cpp preprocessor">#include <sys/sem.h> </code>
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<code class="cpp keyword bold">union</code> <code class="cpp plain">semun </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">val; </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">semid_ds *buf; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">unsigned </code><code class="cpp color1 bold">short</code> <code class="cpp plain">*arry; </code>
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<code class="cpp plain">}; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">sem_id = 0; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">set_semvalue(); </code>
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<code class="cpp keyword bold">static</code> <code class="cpp keyword bold">void</code> <code class="cpp plain">del_semvalue(); </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">semaphore_p(); </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">semaphore_v(); </code>
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<code class="cpp spaces"> </code>
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<div class="line number24 index23 alt1">
<code class="cpp color1 bold">int</code> <code class="cpp plain">main(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">argc, </code><code class="cpp color1 bold">char</code> <code class="cpp plain">*argv[]) </code>
</div>
<div class="line number25 index24 alt2">
<code class="cpp plain">{ </code>
</div>
<div class="line number26 index25 alt1">
<code class="cpp spaces"> </code><code class="cpp color1 bold">char</code> <code class="cpp plain">message = </code><code class="cpp string">'X'</code><code class="cpp plain">; </code>
</div>
<div class="line number27 index26 alt2">
<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">i = 0; </code>
</div>
<div class="line number28 index27 alt1">
<code class="cpp spaces"> </code>
</div>
<div class="line number29 index28 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//创建信号量 </code>
</div>
<div class="line number30 index29 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT); </code>
</div>
<div class="line number31 index30 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number32 index31 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(argc > 1) </code>
</div>
<div class="line number33 index32 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number34 index33 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//程序第一次被调用，初始化信号量 </code>
</div>
<div class="line number35 index34 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(!set_semvalue()) </code>
</div>
<div class="line number36 index35 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number37 index36 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"Failed to initialize semaphore\n"</code><code class="cpp plain">); </code>
</div>
<div class="line number38 index37 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(EXIT_FAILURE); </code>
</div>
<div class="line number39 index38 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number40 index39 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//设置要输出到屏幕中的信息，即其参数的第一个字符 </code>
</div>
<div class="line number41 index40 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">message = argv; </code>
</div>
<div class="line number42 index41 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(2); </code>
</div>
<div class="line number43 index42 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number44 index43 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">for</code><code class="cpp plain">(i = 0; i < 10; ++i) </code>
</div>
<div class="line number45 index44 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number46 index45 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//进入临界区 </code>
</div>
<div class="line number47 index46 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(!semaphore_p()) </code>
</div>
<div class="line number48 index47 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(EXIT_FAILURE); </code>
</div>
<div class="line number49 index48 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//向屏幕中输出数据 </code>
</div>
<div class="line number50 index49 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"%c"</code><code class="cpp plain">, message); </code>
</div>
<div class="line number51 index50 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//清理缓冲区，然后休眠随机时间 </code>
</div>
<div class="line number52 index51 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">fflush</code><code class="cpp plain">(stdout); </code>
</div>
<div class="line number53 index52 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(</code><code class="cpp functions bold">rand</code><code class="cpp plain">() % 3); </code>
</div>
<div class="line number54 index53 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//离开临界区前再一次向屏幕输出数据 </code>
</div>
<div class="line number55 index54 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"%c"</code><code class="cpp plain">, message); </code>
</div>
<div class="line number56 index55 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">fflush</code><code class="cpp plain">(stdout); </code>
</div>
<div class="line number57 index56 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//离开临界区，休眠随机时间后继续循环 </code>
</div>
<div class="line number58 index57 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(!semaphore_v()) </code>
</div>
<div class="line number59 index58 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(EXIT_FAILURE); </code>
</div>
<div class="line number60 index59 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(</code><code class="cpp functions bold">rand</code><code class="cpp plain">() % 2); </code>
</div>
<div class="line number61 index60 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number62 index61 alt1">
<code class="cpp spaces"> </code>
</div>
<div class="line number63 index62 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(10); </code>
</div>
<div class="line number64 index63 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"\n%d - finished\n"</code><code class="cpp plain">, getpid()); </code>
</div>
<div class="line number65 index64 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number66 index65 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(argc > 1) </code>
</div>
<div class="line number67 index66 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number68 index67 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//如果程序是第一次被调用，则在退出前删除信号量 </code>
</div>
<div class="line number69 index68 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(3); </code>
</div>
<div class="line number70 index69 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">del_semvalue(); </code>
</div>
<div class="line number71 index70 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number72 index71 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(EXIT_SUCCESS); </code>
</div>
<div class="line number73 index72 alt2">
<code class="cpp plain">} </code>
</div>
<div class="line number74 index73 alt1">
<code class="cpp spaces"> </code>
</div>
<div class="line number75 index74 alt2">
<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">set_semvalue() </code>
</div>
<div class="line number76 index75 alt1">
<code class="cpp plain">{ </code>
</div>
<div class="line number77 index76 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//用于初始化信号量，在使用信号量前必须这样做 </code>
</div>
<div class="line number78 index77 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">union</code> <code class="cpp plain">semun sem_union; </code>
</div>
<div class="line number79 index78 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number80 index79 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sem_union.val = 1; </code>
</div>
<div class="line number81 index80 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(semctl(sem_id, 0, SETVAL, sem_union) == -1) </code>
</div>
<div class="line number82 index81 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
</div>
<div class="line number83 index82 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">1; </code>
</div>
<div class="line number84 index83 alt1">
<code class="cpp plain">} </code>
</div>
<div class="line number85 index84 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number86 index85 alt1">
<code class="cpp keyword bold">static</code> <code class="cpp keyword bold">void</code> <code class="cpp plain">del_semvalue() </code>
</div>
<div class="line number87 index86 alt2">
<code class="cpp plain">{ </code>
</div>
<div class="line number88 index87 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//删除信号量 </code>
</div>
<div class="line number89 index88 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">union</code> <code class="cpp plain">semun sem_union; </code>
</div>
<div class="line number90 index89 alt1">
<code class="cpp spaces"> </code>
</div>
<div class="line number91 index90 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(semctl(sem_id, 0, IPC_RMID, sem_union) == -1) </code>
</div>
<div class="line number92 index91 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"Failed to delete semaphore\n"</code><code class="cpp plain">); </code>
</div>
<div class="line number93 index92 alt2">
<code class="cpp plain">} </code>
</div>
<div class="line number94 index93 alt1">
<code class="cpp spaces"> </code>
</div>
<div class="line number95 index94 alt2">
<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">semaphore_p() </code>
</div>
<div class="line number96 index95 alt1">
<code class="cpp plain">{ </code>
</div>
<div class="line number97 index96 alt2">
<code class="cpp spaces"> </code><code class="cpp comments">//对信号量做减1操作，即等待P（sv） </code>
</div>
<div class="line number98 index97 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sembuf sem_b; </code>
</div>
<div class="line number99 index98 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_num = 0; </code>
</div>
<div class="line number100 index99 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_op = -1;</code><code class="cpp comments">//P() </code>
</div>
<div class="line number101 index100 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_flg = SEM_UNDO; </code>
</div>
<div class="line number102 index101 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(semop(sem_id, &sem_b, 1) == -1) </code>
</div>
<div class="line number103 index102 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number104 index103 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"semaphore_p failed\n"</code><code class="cpp plain">); </code>
</div>
<div class="line number105 index104 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
</div>
<div class="line number106 index105 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number107 index106 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">1; </code>
</div>
<div class="line number108 index107 alt1">
<code class="cpp plain">} </code>
</div>
<div class="line number109 index108 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number110 index109 alt1">
<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">semaphore_v() </code>
</div>
<div class="line number111 index110 alt2">
<code class="cpp plain">{ </code>
</div>
<div class="line number112 index111 alt1">
<code class="cpp spaces"> </code><code class="cpp comments">//这是一个释放操作，它使信号量变为可用，即发送信号V（sv） </code>
</div>
<div class="line number113 index112 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sembuf sem_b; </code>
</div>
<div class="line number114 index113 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_num = 0; </code>
</div>
<div class="line number115 index114 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_op = 1;</code><code class="cpp comments">//V() </code>
</div>
<div class="line number116 index115 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sem_b.sem_flg = SEM_UNDO; </code>
</div>
<div class="line number117 index116 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(semop(sem_id, &sem_b, 1) == -1) </code>
</div>
<div class="line number118 index117 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number119 index118 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"semaphore_v failed\n"</code><code class="cpp plain">); </code>
</div>
<div class="line number120 index119 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
</div>
<div class="line number121 index120 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number122 index121 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">1; </code>
</div>
<div class="line number123 index122 alt2">
<code class="cpp plain">} </code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
<p>
运行结果如下：</p>
<p>
<img title="详解Linux进程间通信——使用信号量" alt="详解Linux进程间通信——使用信号量" src="https://zhuji.jb51.net/uploads/img/202305/40891af711ee6ac0e29e35c9ce56deb4.jpg"></p>
<p>
注：这个程序的临界区为main函数for循环不的semaphore_p和semaphore_v函数中间的代码。</p>
<p>
<strong>例子分析 ：</strong>同时运行一个程序的两个实例，注意第一次运行时，要加上一个字符作为参数，例如本例中的字符‘O'，它用于区分是否为第一次调用，同时这个字符输出到屏幕中。因为每个程序都在其进入临界区后和离开临界区前打印一个字符，所以每个字符都应该成对出现，正如你看到的上图的输出那样。在main函数中循环中我们可以看到，每次进程要访问stdout（标准输出），即要输出字符时，每次都要检查信号量是否可用（即stdout有没有正在被其他进程使用）。</p>
<p>
所以，当一个进程A在调用函数semaphore_p进入了临界区，输出字符后，调用sleep时，另一个进程B可能想访问stdout，但是信号量的P请求操作失败，只能挂起自己的执行，当进程A调用函数semaphore_v离开了临界区，进程B马上被恢复执行。然后进程A和进程B就这样一直循环了10次。</p>
<p>
<span><strong>五、对比例子——进程间的资源竞争</strong></span></p>
<p>
看了上面的例子，你可能还不是很明白，不过没关系，下面我就以另一个例子来说明一下，它实现的功能与前面的例子一样，运行方式也一样，都是两个相同的进程，同时向stdout中输出字符，只是没有使用信号量，两个进程在互相竞争stdout。它的代码非常简单，文件名为normalprint.c，代码如下：</p>
<div class="jb51code">
<div>
<div class="syntaxhighlightercpp" id="highlighter_770053">
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<table border="0" cellpadding="0" cellspacing="0"><tbody><tr>
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<td class="code">
<div class="container">
<div class="line number1 index0 alt2">
<code class="cpp preprocessor">#include <stdio.h> </code>
</div>
<div class="line number2 index1 alt1">
<code class="cpp preprocessor">#include <stdlib.h> </code>
</div>
<div class="line number3 index2 alt2">
<code class="cpp spaces"> </code>
</div>
<div class="line number4 index3 alt1">
<code class="cpp color1 bold">int</code> <code class="cpp plain">main(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">argc, </code><code class="cpp color1 bold">char</code> <code class="cpp plain">*argv[]) </code>
</div>
<div class="line number5 index4 alt2">
<code class="cpp plain">{ </code>
</div>
<div class="line number6 index5 alt1">
<code class="cpp spaces"> </code><code class="cpp color1 bold">char</code> <code class="cpp plain">message = </code><code class="cpp string">'X'</code><code class="cpp plain">; </code>
</div>
<div class="line number7 index6 alt2">
<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">i = 0; </code>
</div>
<div class="line number8 index7 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(argc > 1) </code>
</div>
<div class="line number9 index8 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">message = argv; </code>
</div>
<div class="line number10 index9 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">for</code><code class="cpp plain">(i = 0; i < 10; ++i) </code>
</div>
<div class="line number11 index10 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
</div>
<div class="line number12 index11 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"%c"</code><code class="cpp plain">, message); </code>
</div>
<div class="line number13 index12 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">fflush</code><code class="cpp plain">(stdout); </code>
</div>
<div class="line number14 index13 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(</code><code class="cpp functions bold">rand</code><code class="cpp plain">() % 3); </code>
</div>
<div class="line number15 index14 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"%c"</code><code class="cpp plain">, message); </code>
</div>
<div class="line number16 index15 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">fflush</code><code class="cpp plain">(stdout); </code>
</div>
<div class="line number17 index16 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(</code><code class="cpp functions bold">rand</code><code class="cpp plain">() % 2); </code>
</div>
<div class="line number18 index17 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
</div>
<div class="line number19 index18 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">sleep(10); </code>
</div>
<div class="line number20 index19 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"\n%d - finished\n"</code><code class="cpp plain">, getpid()); </code>
</div>
<div class="line number21 index20 alt2">
<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(EXIT_SUCCESS); </code>
</div>
<div class="line number22 index21 alt1">
<code class="cpp plain">} </code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
</div>
</div>
<p>
运行结果如下：</p>
<p>
<img title="详解Linux进程间通信——使用信号量" alt="详解Linux进程间通信——使用信号量" src="https://zhuji.jb51.net/uploads/img/202305/e30cabc964afbfc3188f4bcf194e1dbf.jpg"></p>
<p>
<strong>例子分析：</strong></p>
<p>
从上面的输出结果，我们可以看到字符‘X'和‘O'并不像前面的例子那样，总是成对出现，因为当第一个进程A输出了字符后，调用sleep休眠时，另一个进程B立即输出并休眠，而进程A醒来时，再继续执行输出，同样的进程B也是如此。所以输出的字符就是不成对的出现。这两个进程在竞争stdout这一共同的资源。通过两个例子的对比，我想信号量的意义和使用应该比较清楚了。</p>
<p>
<span><strong>六、信号量的总结</strong></span></p>
<p>
信号量是一个特殊的变量，程序对其访问都是原子操作，且只允许对它进行等待（即P(信号变量))和发送（即V(信号变量))信息操作。我们通常通过信号来解决多个进程对同一资源的访问竞争的问题，使在任一时刻只能有一个执行线程访问代码的临界区域，也可以说它是协调进程间的对同一资源的访问权，也就是用于同步进程的。</p>
<p>
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。</p>
<p>
原文链接：http://blog.csdn.net/ljianhui/article/details/10243617</p>
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