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From: Dave.Harris@f14.n15.z1.fidonet.org (Dave Harris)
Newsgroups: comp.lang.c
Subject: post-decrement quirk?
Message-ID: <14467.2860B4F2@stjhmc.fidonet.org>
Date: 20 Jun 91 13:57:02 GMT
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 >From: julbro@auto-trol.com (Julie Brown)

 >Can anyone explain to me why the following does not work 
 >(at least on the Sun):

 >     q = q--;

As I have been reminded several times in the last week myself, this statement 
is undefined.  Modifing an address multiple time between sequence points is 
undefined.  Both pre and post yeild 4 when I try this with what I use (TC++). 
Being undefined, your machine could yeild or do anything including crash on 
this statement (so I've been told).  q-- all by its lonesome self is 
sufficient.


 >'q' does not get decremented. No matter what the order of 
 >operations, I would expect 'q' to get decremented at some
 >point. It doesn't make a whole lot of sense to do this until 
 >you put it in a more meaningful context:

In machine language terms your compiler is possibly doing something like:

R1 = Q
Dec Q
Q = R1

where Q your variable and R1 is a register.  Just speculating of course.  
Fiddle with the optimizer switches and see if it acts differently.


 >#include <stdio.h>
 >main()
 >{
 >   int count = 5;

 >   count = (count > 0) ? count-- : 0;
 >   printf("count %d\n", count);
 >}

 >the output from this program is :
 >      count 5 

count = (count > 0) ? count-1 : 0;
count = max(0,count-1);
count -= (count>0);

no shortage of ways to do this.

 >pre-increment works just fine (q = --q;).

Dec Q
R1 = Q
Q = R1

Speculating again of course.
 


 

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