what is semaphore? can any body explain it in a more simple way than the manual ??

replies appreciated

Regards
raguram R

I recommend this book,
http://www.amazon.com/exec/obidos/tg...97506?v=glance

hi friend jst check out it in
OS concepts by
Galvin

Semaphore: Semaphore 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 file when the other process is accessing. This is done by assigning value to semaphore.

The value of the semaphore is initialized by the first process when the file is in access by it. When the second 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. After the first process is completed it reinitializes the semaphore value and now the second process uses it. The above example is for two processes but a semaphore can be used even when numbers of processes try to access the same file. Thus semaphores are used to coordinate access to a resource by different processes.

We have seen that semaphore can be used when numbers of processes try to access the same file. In this case we must make the semaphore available accessible to all processes so that they can read and check the value and also initialize and reinitialize the value of semaphore appropriately. For this reason only the semaphore is stored in kernel so that it can be accessed by all processes.

The value of semaphore represents thus the number of threads which are nothing but processes. In other words we found that if the value is positive then we have threads to decrement and proceed for execution without suspending. If the value of semaphore is negative then it represents that the number of threads or process is blocked and kept in suspended state. If the value of semaphore is zero then it means that there are no threads or processes in waiting state.

linuxwebmaster,
When you quote some source other source than yourself, it would be nice to give a proper attributes to the quote.

Hi Mod, I'm sorry but I didn't catch u....????