Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!ucbvax!BIOVAX.RUTGERS.EDU!hamm
From: hamm@BIOVAX.RUTGERS.EDU
Newsgroups: comp.os.vms
Subject: C puzzle
Message-ID: <8802221044.AA21745@ucbvax.Berkeley.EDU>
Date: 15 Feb 88 22:57:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: <hamm@biovax.rutgers.edu>
Organization: The Internet
Lines: 95

Here's one for all you C virtuosos.

We recently had to port a C program from Unix to VMS, and one of the many 
things which exploded was a line looking like this:

    for (i=0;i<LIMIT;) c3[i] = c2[i] = c1[i++];         /*1*/

Now, I'm a complete novice at C, but this certainly looks like awful code
to me.  It's not entirely clear what the line is intended to do.  From the 
context, I assumed that what was intended was:

    for (i=0;i<LIMIT;i++) c3[i] = c2[i] = c1[i];        /*2*/

whereas what I thought would happen based on my limited knowledge of C was:

    for (i=0;i<LIMIT;i++) c3[i+1] = c2[i+1] = c1[i];    /*3*/

which would certainly explain the resulting walk in address space when the
three arrays are of size [LIMIT].

A test program shows that /*3*/ is indeed what happens under VMS, but that
/*2*/ is what happens under (Sun) Unix.

I've looked in vain (though perhaps not thoroughly enough) in K&R for a 
precise definition of when increments such as i++ or ++i should be done;  
the best I can find are statements which simply say "after evaluation" vs. 
"before evaluation" - but what's the granularity of "evaluation"?  Should
it be interpreted to mean "of a complete statement" or to mean "of a
complete expression", in this case, an array index expression?

Is the apparent (Sun) Unix interpretation consistently held in all cases
on that system?  On other Unix systems?  Is VMS actually *wrong*, or is
this one of those "left to implementation" decisions?  How does the
impending C standard define this?

Any and all informed comments on this would be welcome.  I attach the
test program below in case you want to try it.

Greg
------------------------------------------------------------------------------
Gregory H. Hamm                           || Phone:  (201)932-4864
Director, Molecular Biology Computing Lab ||  
Waksman Institute/CABM                    || BITNET: hamm@biovax
P.O. Box 759, Rutgers University          || ARPA:   hamm@biovax.rutgers.edu
Piscataway, NJ 08854 * USA                ||
------------------------------------------------------------------------------

#include stdio
#define LIMIT  4        /* to prevent walking off the end of arrays */
main ()
{
    int i; 
    int c1[LIMIT+1], c2[LIMIT+1], c3[LIMIT+1];

    for (i=0;i<LIMIT;i++) c1[i] = i;
    for (i=0;i<LIMIT;i++) c2[i] = 0;
    for (i=0;i<LIMIT;i++) c3[i] = 0;

    showc(c1,c2,c3);
    for (i=0;i<LIMIT;) c3[i] = c2[i] = c1[i++];     /* what does this do? */
    showc(c1,c2,c3);
}
showc(c1,c2,c3)
    int c1[], c2[], c3[];
{
    int i; 
    
    printf ("c1: ");
    for (i=0;i<LIMIT;i++) printf("%d ",c1[i]);    printf ("\n");
    printf ("c2: ");
    for (i=0;i<LIMIT;i++) printf("%d ",c2[i]);    printf ("\n");
    printf ("c3: ");
    for (i=0;i<LIMIT;i++) printf("%d ",c3[i]);    printf ("\n");
}

     
Results on Vax/VMS :
     
c1: 0 1 2 3
c2: 0 0 0 0
c3: 0 0 0 0
c1: 0 1 2 3
c2: 0 0 1 2
c3: 0 0 1 2
     
Results on Sun :
     
c1: 0 1 2 3
c2: 0 0 0 0
c3: 0 0 0 0
c1: 0 1 2 3
c2: 0 1 2 3
c3: 0 1 2 3
     
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