sem_init(…): What is the value parameter for?

Question

In a class, we've had to use semaphores to accomplish work with threads. The prototype (and header file) of sem_init is the following:

int sem_init(sem_t *sem, int pshared, unsigned int value);

but I don't understand what the value variable is used for. According to opengroup.org:

value is an initial value to set the semaphore to

"value is a value..." How does that help, what is it used for?

Solution 1

sem_init() initializes a pointed to semaphore (first parameter), with value (last parameter), and finally I believe this is actually what you were asking, int pshared you can think of like a flag. If pshared == 1 then semaphore can be forked.

EDIT: semaphore has int value because you would use a function such as sem_wait(sem_t* sem) to decrement pointed to semaphore. If it's negative, then block.

Solution 2

Semaphore value represents the number of common resources available to be shared among the threads. If the value is greater than 0, then the thread calling sem_wait need not wait. It just decrements the value by 1 and proceeds to access common resource. sem_post will add a resource back to the pool. So it increments the value by 1. If the value is 0, then we will wait till somebody has done sem_post.

Solution 3

In order to understand the "value" in sem_init(sem_t *sem, int pshared, unsigned int value)

I think we need synchronize it with how we use the semaphore in codes: when we want to wait for the semaphore, we call sem_wait (&mutex), and if we want to notify to make other threads' sem_wait (&mutex) run, we need call sem_post (&mutex). But the problem is if no any other threads call sem_post (&mutex) at first(in the system initialization), what should the sem_wait (&mutex) do? It should pass or wait for other threads call sem_post (&mutex)? The answer is in the "value" in the sem_init(). If it's zero, it means we must wait for some other threads to call sem_post() one time, then it could pass. If it's one, then the first time calling for sem_wait() will pass at once, but the second time sem_wait() will wait(if no sem_post called during them). So the value in sem_init() means how many times we could execute sem_wait() without really waiting without any sem_post(). And we could consider the sem_wait() as: /deadlock waiting until value is changed to bigger than 0 by another thread/

while (value<=0) {}

value--;

And consider the sem_post() as:

value++;

Comments

[SomethingSomething]

What does it mean "with a value" ? Does it mean that it is locked/unlocked ? Does it specify the number of possible aquirings of the semaphore?

[JohnyTex]

I believe it is this intial value: "sem_wait() decrements (locks) the semaphore pointed to by sem. If the semaphore's value is greater than zero, then the decrement proceeds, and the function returns, immediately. If the semaphore currently has the value zero, then the call blocks until either it becomes possible to perform the decrement (i.e., the semaphore value rises above zero), or a signal handler interrupts the call."