Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!ucsd!mvb.saic.com!ncr-sd!iss-rb!tortuga!ewing
From: ewing@tortuga.SanDiego.NCR.COM (David Ewing)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: ANOTHER SAS C BUG
Summary: This is not a bug !!!
Keywords: sequence points, side effects
Message-ID: <1991Apr1.224954.12574@SanDiego.NCR.COM>
Date: 1 Apr 91 22:49:54 GMT
References: <1991Mar31.035009.13183@csis.dit.csiro.au>
Sender: @SanDiego.NCR.COM
Reply-To: ewing@tortuga.SanDiego.NCR.COM (David Ewing)
Organization: NCR Corporation, Rancho Bernardo
Lines: 70

In article <1991Mar31.035009.13183@csis.dit.csiro.au> dave@csis.dit.csiro.au (David Campbell) writes:
>/*  Here is some source which demonstrates a bug in the SAS C compiler */
>
>#include <stdio.h>
>
>char *str="hello\n";
>
>main() {
>	char *ptr;
>	ptr = str;
>	*ptr = '\001' + *ptr++;
>	printf(str);
>}
>/* Should print "iello" through the incrementing of str[0]
>   Instead prints "hillo"

This is not a bug in SAS C, as a careful reading of the ANSI standard
will confirm.  The behavior of your code is clearly undefined.

From the standard :

" 2.1.2.3 Program Execution 
...
Evaluation of an expression may produce side effects.  At certain specified
points in the execution sequence called sequence points, all side effects
of previous evaluations shall be complete and no side effects of subsequent
evaluations shall have taken place...

3.3 Expressions
...
Between the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression.  Furthermore,
the prior value shall be accessed only to determine the value to be stored.
(footnote 34) ... the order of evaluation of subexpressions and the order in 
which side effects take place are both unspecified....

footnote 34. This paragraph renders undefined statement expressions such as 
   i = ++i + 1; ..."

Also see appendix B on sequence points.

This is nothing new to ANSI C.  Kernighan and Ritchie warn :

   "When side effects (assignment to actual variables) takes place is
    left to the discretion of the compiler, since the best order
    strongly depends on machine architecture."

Furthermore, as another astute reader has already commented,
modifying char *str="hello\n"; is also non-portable and undefined.
This should be char str[] = "hello\n";   -->

"3.5.7 Initialization
...
   char s[] = "abc" ...
The contents of the array are modifiable. On the other hand, the 
declaration
   char *p = "abc";
defines p with type 'pointer to char' that is initialized to point
to an object with type 'array of char' with length 4 whose elements 
are initialized with a character string literal.  If an attempt 
is made to use p to modify the contents of the array, the behavior
is undefined."

Please take the time to THOROUGHLY understand C before you disparage
a compiler.

*********************************************
David A. Ewing
ewing@tortuga.sandiego.ncr.com
*********************************************