a semaphor is nothing but a term used in unix for a

variable which acts as a counter.for instance there may be

times when two processes try to access the same file

simultaneously.in this event we must control the access of

the when the other process is accessing

this is done by assigning value to a semaphore.

the value of the semaphore is initialized by the 1st

process when the file is in access by it.when the 2nd

process try to access the file it checks the value of the

semaphore and if it finds the value as initialized it does

not access the file.