Path: utzoo!mnetor!uunet!sdrc!scjones
From: scjones@sdrc.UUCP (Larry Jones)
Newsgroups: comp.os.vms
Subject: Re: C puzzle
Message-ID: <232@sdrc.UUCP>
Date: 24 Feb 88 02:55:10 GMT
References: <8802221044.AA21745@ucbvax.Berkeley.EDU>
Organization: Structural Dynamics Research Corp., Cincinnati
Lines: 41
Summary: bad code

In article <8802221044.AA21745@ucbvax.Berkeley.EDU>, hamm@BIOVAX.RUTGERS.EDU writes:
< 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.  

Yep, that's awful code alright.

< 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?

There is no precise definition.  For efficiency, these operations can be
defered until the end of the statement.

----
Larry Jones                         UUCP: uunet!sdrc!scjones
SDRC                                MAIL: 2000 Eastman Dr., Milford, OH  45150
                                    AT&T: (513) 576-2070
"When all else fails, read the directions."