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From: gamma@EDN-UNIX.arpa (W. J. Showalter)
Newsgroups: comp.lang.c
Subject: Possible C Anomaly
Message-ID: <9236@brl-adm.ARPA>
Date: Fri, 11-Sep-87 10:17:39 EDT
Article-I.D.: brl-adm.9236
Posted: Fri Sep 11 10:17:39 1987
Date-Received: Sun, 13-Sep-87 09:27:02 EDT
Sender: news@brl-adm.ARPA
Lines: 62

I would like to get some comments on the following problem involving
what I consider an inconsistency in the C programming language.

Consider the statment

	z = (a > b) ? a : b;

which is taken directly from Kernighan and Ritchie on page 47.  According
to them the statement is equivalent to

if (a > b)
   z = a;
else
   z = b;


The other day I wrote the following code:
			    k = 10;
				.
				.
				.
				.
/* Example #1 */            k = (k > 1) ? k-- : 1;

My intent was to decrement k with each pass until it reached 1 and then to
keep it at one.  There are a variety of ways to accomplish this but I just
happened to try this one first.

According to K&R, the above expression is equivalent to

/* Example #2 */

if (k > 1)
   k = k--;
else
   k = 1;

Note:  In this case the else is not needed but included for
completeness.

The problem is is that the two examples behave differently.  In the first
example k does not change values.  It remains equal to 10 indefinitely.

In the second example k does change like one would expect.

I generated assembler listings for the two examples and discovered that
example 1 makes used of an interim register to house the value of k
BEFORE decrementing.  It eventually moves this value back to k, replacing
the decremented value with the original.

I would like to get some other opinions on this if anyone has the time
or interest.

Also, although it seems to me that there is a slight inconsistency within
the language I tried 3 different compilers on 3 different machine
architectures and they all handle it the same way.  I assume that there
must be some well known and accepted BNF which is used to arrive at this
consistency between the different compilers. 

Jim

gamma@edn-unix.arpa