想象一下，每个人都想上厕所而浴室的钥匙是有限的。如果剩下的钥匙不够多，那个人就需要等待。因此，可以将信号量看作是表示不同进程(上厕所的人)可以请求访问的卫生间(系统资源)可用的一组键。

现在想象一下两个过程同时去洗手间。这不是一个好的情况，信号量被用来防止这种情况。不幸的是，信号量是一种自愿机制，进程(我们的上厕所的人)可以忽略它(即，即使有钥匙，有人仍然可以把门踢开)。

二进制/互斥量和计数信号量之间也有区别。

请登录http://www.cs.columbia.edu/~jae/4118/lect/L05-ipc.html查看课堂讲稿。

信号量是一种锁定资源的方法，以保证在执行一段代码时，只有这段代码可以访问该资源。这可以防止两个线程并发访问一个资源，从而导致问题。

@Craig:

信号量是一种锁定 资源，这样才有保证 当执行一段代码时， 只有这段代码可以访问 该资源。这保留了两个线程 从并发访问资源， 这可能会导致问题。

这不仅仅局限于一个线程。信号量可以配置为允许固定数量的线程访问资源。

硬件或软件标志。在多任务系统中，信号量是一个变量，其值表示公共资源的状态。需要资源的进程检查信号量以确定资源状态，然后决定如何继续。

Michael Barr的文章《互斥量和信号量揭秘》很好地介绍了互斥量和信号量的不同之处，以及什么时候应该和不应该使用它们。我在这里摘录了几个关键段落。

关键在于应该使用互斥来保护共享资源，而应该使用信号量来发送信号。通常不应该使用信号量来保护共享资源，也不应该使用互斥量来发送信号。例如，在使用信号量来保护共享资源方面，使用bouncer类比是有问题的——您可以这样使用它们，但这可能会导致难以诊断错误。

While mutexes and semaphores have some similarities in their implementation, they should always be used differently. The most common (but nonetheless incorrect) answer to the question posed at the top is that mutexes and semaphores are very similar, with the only significant difference being that semaphores can count higher than one. Nearly all engineers seem to properly understand that a mutex is a binary flag used to protect a shared resource by ensuring mutual exclusion inside critical sections of code. But when asked to expand on how to use a "counting semaphore," most engineers—varying only in their degree of confidence—express some flavor of the textbook opinion that these are used to protect several equivalent resources.

...

在这一点上，一个有趣的类比是使用浴室钥匙的想法来保护共享资源-浴室。如果一家商店只有一间浴室，那么一把钥匙就足以保护这一资源，防止多人同时使用。

如果有多个浴室，人们可能会试图用相同的键来设置多个键——这类似于误用信号量。一旦你有了一个键，你实际上不知道哪个浴室是可用的，如果你沿着这条路走下去，你可能最终会使用互斥锁来提供该信息，并确保你没有使用已经被占用的浴室。

A semaphore is the wrong tool to protect several of the essentially same resource, but this is how many people think of it and use it. The bouncer analogy is distinctly different - there aren't several of the same type of resource, instead there is one resource which can accept multiple simultaneous users. I suppose a semaphore can be used in such situations, but rarely are there real-world situations where the analogy actually holds - it's more often that there are several of the same type, but still individual resources, like the bathrooms, which cannot be used this way.

...

The correct use of a semaphore is for signaling from one task to another. A mutex is meant to be taken and released, always in that order, by each task that uses the shared resource it protects. By contrast, tasks that use semaphores either signal or wait—not both. For example, Task 1 may contain code to post (i.e., signal or increment) a particular semaphore when the "power" button is pressed and Task 2, which wakes the display, pends on that same semaphore. In this scenario, one task is the producer of the event signal; the other the consumer.

...

Here an important point is made that mutexes interfere with real time operating systems in a bad way, causing priority inversion where a less important task may be executed before a more important task because of resource sharing. In short, this happens when a lower priority task uses a mutex to grab a resource, A, then tries to grab B, but is paused because B is unavailable. While it's waiting, a higher priority task comes along and needs A, but it's already tied up, and by a process that isn't even running because it's waiting for B. There are many ways to resolve this, but it most often is fixed by altering the mutex and task manager. The mutex is much more complex in these cases than a binary semaphore, and using a semaphore in such an instance will cause priority inversions because the task manager is unaware of the priority inversion and cannot act to correct it.

...

The cause of the widespread modern confusion between mutexes and semaphores is historical, as it dates all the way back to the 1974 invention of the Semaphore (capital "S", in this article) by Djikstra. Prior to that date, none of the interrupt-safe task synchronization and signaling mechanisms known to computer scientists was efficiently scalable for use by more than two tasks. Dijkstra's revolutionary, safe-and-scalable Semaphore was applied in both critical section protection and signaling. And thus the confusion began. However, it later became obvious to operating system developers, after the appearance of the priority-based preemptive RTOS (e.g., VRTX, ca. 1980), publication of academic papers establishing RMA and the problems caused by priority inversion, and a paper on priority inheritance protocols in 1990, 3 it became apparent that mutexes must be more than just semaphores with a binary counter.

互斥:资源共享

信号:信号

在没有仔细考虑副作用的情况下，不要用一种药物来替代另一种药物。

互斥:对资源的独占成员访问

信号量:对资源的n个成员访问

也就是说，互斥可以用来同步对计数器、文件、数据库等的访问。

信号量可以做同样的事情，但支持固定数量的同时调用者。例如，我可以将我的数据库调用包装在一个信号量(3)中，这样我的多线程应用程序将最多3个同时连接到数据库。所有的尝试都将被阻塞，直到三个插槽中的一个打开。它们让简单的节流变得非常简单。