SPUG: C question ;)

[Scott Blachowicz]

On Sun, Mar 10, 2002 at 05:35:38PM -0800, Benjamin Franks wrote:
> ...
> int shmid=shmget((ftok("blah/blah",1), 1000*sizeof(my_struct), IPC_CREAT | 0666);
> 
> Then when I attach the shmid to a pointer, do I want to attach a pointer
> of the my_struct type, as in:
> 
> my_struct *ptr;
> ptr=shmat(shmid,NULL,0);
> 
> I guess that's where my confusion lies.  Let's say I want to work on the
> 500th element of my shared memory array of my_struct's.  How can I do it?

Well...in C, an array name is kind of like a constant ptr and array indexing
is just a form of pointer arithmetic, so once you have a pointer, you're in
business.

    /* Set all of the elements to the current time. */
    for (i = 0; i < 500; i++) ptr[i].a = ptr[i].b = time(NULL);

    /* All of these should be equivalent (unless I'm remembering wrong). */
    (ptr+499)->a = 0;
    (*(ptr+499)).a = 0;
    ptr[499].a = 0;
    499[ptr].a = 0;   /* !! */

Is that what you wanted?
-- 
Scott Blachowicz

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