Using Dispatch Semaphores to Regulate the Use of Finite Resources

The semantics for using a dispatch semaphore are as follows:

When you create the semaphore (using the
dispatch_semaphore_create
function), you can specifya positive integer indicating the number of resources available.

In each task, call
dispatch_semaphore_wait
to wait on the semaphore.

When the wait call returns, acquire the resource and do your work.

When you are done with the resource, release it and signal the semaphore by calling the dispatch_semaphore_signal
function.

// Create the semaphore, specifying the initial pool size
dispatch_semaphore_t fd_sema = dispatch_semaphore_create(getdtablesize() / 2);
// Wait for a free file descriptor
dispatch_semaphore_wait(fd_sema, DISPATCH_TIME_FOREVER);
fd = open("/etc/services", O_RDONLY);
// Release the file descriptor when done
close(fd);
dispatch_semaphore_signal(fd_sema);

When you create the semaphore, you specify the number of available resources. This value becomes the initialcount variable for the semaphore. Each time you wait on the semaphore, the
dispatch_semaphore_waitfunction decrements that count variable by 1. If the resulting value
is negative, the function tells the kernel toblock your thread. On the other end, the
dispatch_semaphore_signal
function increments the countvariable by 1 to indicate that a resource has been freed up. If there are tasks blocked and waiting for a resource,one of them is subsequently unblocked and allowed to do
its work.