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Date: Wed, 04 Feb 1998 08:50:05 +0100
From: Michael Schuster <Michael.Schuster@utimaco.co.at>
Organization: Utimaco Safe Concept GmbH. Linz Austria
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To: "hackers@FreeBSD.ORG" <hackers@FreeBSD.ORG>
Subject: Re: Shared memory and signals
References: <199802040717.RAA07706@cain.gsoft.com.au>
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Daniel O'Connor wrote: 
> Hmm, yes but I know of at least one program (paradise netrek :) which uses the
> fact that the shared mem segment is persistent...

Shared memory is persistent in the sense that you can do something like

        typedef struct {
                int magic;
                ....
        } my_struct;

        void  *mem;
        int   size = sizeof (my_struct);
        key_t key = MY_KEY; /* some constant key */

        fd = shmget (key, size, ...);
        mem = shmat (fd, ...);  

        if (((my_struct *) mem)->magic == MY_MAGIC) { /* re-attached */
                /* re-use memory .... */
        } else { 
                /* do what you have to do with newly attached mem */
                ((my_struct *) mem)->magic = MY_MAGIC;
        }
        
        /* ..... */

        shmdt (mem);

as often as you like, and you (and any other process) will always get
the same memory segment (although the pointers may not be the same -
they're virual addresses, of course) with the contents you left in there
the last time you used it (provided noone has changed it, of course).
Once you IPC_RM it even once though, it's gone (in fact you can only
IPC_RM it once; subsequent IPC_RMs will give you an invalid argument
error) and lost immediately for all and any process.

-- 
Michael Schuster