In my textbook, Operating Systems: Internals and Design Principles (9th Edition) by William Stallings in chapter 5, it explains how semaphores work:

The fundamental principle is this: Two or more processes can cooperate by means of simple signals, such that a process can be forced to stop at a specified place until it has received a specific signal. Any complex coordination requirement can be satisfied by the appropriate structure of signals. For signaling, special variables called semaphores are used. To transmit a signal via semaphore s , a process executes the primitive semSignal (s) . To receive a signal via semaphore s, a process executes the primitive semWait (s) ; if the corresponding signal has not yet been transmitted, the process is suspended until the transmission takes place.

I can't find a description in chapter 5 of when a semaphore would issue a semSignal vs a semWait (which are also sometimes called signal and wait). Can anyone describe the conditions under which semSignal would be issued vs a semWait?


1 Answer 1


The semaphore doesn't issue those. A semaphore is an object that can be used by processes to coordinate between themselves. Thus, a process will call semSignal or semWait on a shared semaphore. We can't describe the conditions under which it will do that in general, because it's up to each process how it wants to use semaphores. Semaphores are a primitive that can be used in many ways. You should be able to find many examples online or in your textbook of using semaphores for coordination; that will give you some examples of specific processes and when those particular processes will invoke those operations on a semaphore.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.