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?

Answer 1

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.

Answer 2

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.