As an aside, we also note another use for semaphores. They can be used with PARBEGIN .. PAREND to enable any precedence graph to be realized.
| Our earlier precedence graph | Solution | |
|---|---|---|
S1
| \
S2 S3
| / \
S4 S5
\ /
S6
|
S1; PARBEGIN BEGIN S2; V(a); END; BEGIN S3; V(b); V(c); END; BEGIN P(a); P(b); S4; END; BEGIN P(c); S5; END; PAREND; S6; |
-- all semaphores initially 0 -- semaphore a guarantees S2 precedes S4 -- semaphores b, c guarantee that S3 -- precedes both S4 and S5 |