This situation is prone to deadlock of processes in an operating system and I'd like to solve it with the minimum of semaphores. Basically there are three cooperating processes that all read data from the same input device. Each process, when it gets the input device, must read two consecutive data. I want to use mutual exclusion to do this. Semaphores should be used to synchronize:

P1:                     P2:                     P3:
input(a1,a2)            input (b1,b2)           input(c1,c2)
Y=a1+c1                 W=b2+c2                 Z=a2+b1
Print (X)               X=Z-Y+W

The declaration and initialization that I think would work here are:

semaphore s=1

sa1 = 0, sa2 = 0, sb1 = 0, sb2 = 0, sc1 = 0, sc2 = 0

I'm sure that any kernel programmers that happen on this can knock this out in a minute or 2.

Diagram of cooperating Processes and one input device:

It seems like P1 and P2 would start something like:

wait(s)

input (a1/b1, a2/b2)

signal(s)