🚀 go-pugleaf

In article <1992Jul23.155158.8075@gvl.unisys.com> rickt@gvls1.gvl.unisys.com (Rick Thompson) writes:
>
>Hello,
>
>  I have a couple questions regaurding shared memory and semaphores.
>
>  1)  I've created a shared memory area using shmget and my processes
>      can communicate correctly.  However, it is possible for any one of
>      the processes to remove the partion even though the others are still
>      using it.  The processes are all running under the same user so any
>      process has permission to remove it.  Is there a way to prevent the
>      removal if the memory is still in use?

	When a process requests IPC_RMID, the actual shared memory is not
	released until no other process is attached to it.  So, you
	are not in jeopardy of deleting an in-use shared memory segment.
	However, if you do request a segment to be removed, no new
	process can attach itself to the old shared memory.
>
>  2)  I need the ability to lock out a portion of shared memory in
>      some critical regions of the code.  Is it possible using semget and
>      semctl to do this?  Again, the processes using the shared memory are
>      running under the same user.
>
	You could use a semaphore.  The critical code section
	waits for the semaphore to be released, 
	sets the semaphore, performs its operation, and then
	releases the semaphore.  Use the lock semaphore
	technique for all access to shared memory.  This will
	ensure only one process performs its function upon shared
	memory at a time.
	
	Note: watch out for race conditions.
	
	
Kenneth Cobler		ksc@cbnewsk.att.com or ken@ihlpm.att.com
AT&T Bell Laboratories	attmail!cbnewsk!ksc or attmail!ihlpm!ken
2000 N. Naperville Rd.
Naperville, IL  60566