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create:
Select a filename.
Create XSI semaphore array {S = 0, W = 0, X = 1, Q = 0 and N = 0}
with random key. Store the semaphore array's key in decimal form
on the first line in the selected file.
Create XSI shared memory, with an allocation of 2048 bytes, with a
random key. Store the shared memory's key in decimal form on the
second line in the selected file.
broadcast:
with P(X):
Z(W)
Write NUL-terminate message to shared memory
with V(N): -- (1)
Q := 0
Z(S)
-- (1) may be omitted if semaphores are known that
P(·), Z(·), V(·) cannot create a race condition
with a processes running Z(·).
listen:
with V(S):
forever:
V(Q)
Z(Q)
Read NUL-terminated message from shared memory
if breaking:
break
with V(W):
with P(S):
Z(S)
Z(N)
`V(a)` means that semaphore a is released.
`P(a)` means that semaphore a is acquired.
`Z(a)` means that the process waits for semaphore a to become 0.
`with P(a)` that `P(a)` is done before the entering the scope,
and `V(a)` is done when exiting the scope. It also means that
these actions [P(a) and V(a)] are undone when the process exits,
or if the call fails.
`with V(a)` that `V(a)` is like `with P(a)` that `P(a)` and
`V(a)` are exchanged.
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