Concepts of Programming Languages

588 Chapter 13 Concurrency

The following are concise pseudocode descriptions of wait and release:

wait(aSemaphore)

if aSemaphore’s counter > 0 then

decrement aSemaphore’s counter

else

put the caller in aSemaphore’s queue

attempt to transfer control to some ready task

(if the task ready queue is empty, deadlock occurs)

end if

release(aSemaphore)

if aSemaphore’s queue is empty (no task is waiting) then

increment aSemaphore’s counter

else

put the calling task in the task-ready queue

transfer control to a task from aSemaphore’s queue

end

We can now present an example program that implements cooperation syn-

chronization for a shared buffer. In this case, the shared buffer stores integer

values and is a logically circular structure. It is designed for use by possibly

multiple producer and consumer tasks.

The following pseudocode shows the definition of the producer and con-

sumer tasks. Two semaphores are used to ensure against buffer underflow or

overflow, thus providing cooperation synchronization. Assume that the buffer

has length BUFLEN, and the routines that actually manipulate it already exist

as FETCH and DEPOSIT. Accesses to the counter of a semaphore are specified

by dot notation. For example, if fullspots is a semaphore, its counter is

referenced by fullspots.count.

semaphore fullspots, emptyspots;

fullspots.count = 0;

emptyspots.count = BUFLEN;

task producer;

loop

-- produce VALUE --

wait(emptyspots); { wait for a space }

DEPOSIT(VALUE);

release(fullspots); { increase filled spaces }

end loop;

end producer;

task consumer;

loop

wait(fullspots); { make sure it is not empty }