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# 1  
Old 11-01-2001
Semaphores

Dear Reader,

I'm in a multiprocess environment working with shared mem and semaphores as mutex.. The problem is -- If one of the process hooked up with the semaphore and accessing the shared mem, terminates abruptly ( or got killed ), other process which are in want of the semaphore are waiting indefnitely...

Is there any way to come out this state....

Thanks in advance..
# 2  
Old 11-01-2001
When programming such IPC's, you should consider 2 things...

1. Handle all signals so that your exiting process cleans up.
2. Use non-blocking techniques to assure no dead-lock.

The semaphore documentation states...

There are two control flags that can be used with semop():

IPC_NOWAIT
-- Can be set for any operations in the array. Makes the function return without changing any semaphore value if any operation for which IPC_NOWAIT is set cannot be performed. The function fails if it tries to decrement a semaphore more than its current value, or tests a nonzero semaphore to be equal to zero.
SEM_UNDO
-- Allows individual operations in the array to be undone when the process exits.


Any process with read permission can test whether a semaphore has a zero value. To increment or decrement a semaphore requires write permission. When an operation fails, none of the semaphores is altered.

The process blocks (unless the IPC_NOWAIT flag is set), and remains blocked until:

the semaphore operations can all finish, so the call succeeds,
the process receives a signal, or
the semaphore set is removed.

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