NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/287

Internet Message Format | 1996-08-06 | 3.6 KB

Path: newshost.lanl.gov!tanmoy
From: [e-mail] (Tanmoy Bhattacharya)
Newsgroups: comp.lang.c,comp.std.c
Subject: Re: Integral conversion e.t.c. (was: Re: Hungarian notation)
Date: 02 Feb 1996 18:04:43 GMT
Organization: Los Alamos National Laboratory
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In-reply-to: [e-mail]'s message of Thu, 1 Feb 1996 16:56:57 GMT

In article <[e-mail]> [e-mail] (Michael Furman)
writes:
<snip>
     Not exactly. My question was not about diagnostics, but about usage
   of constructions that are not defined in standard.
   There are two kinds of undefined behavior:
   1. When operation does not have any meaning. Example: assigning a value
     to dereferenced incorrect pointer. In such cases sometimes it is very
     hard or impossible (at least on some platforms) to check it even at
     execution time and it is really possible that hard disk will be
     reformatted as a result of such operations.
   2. When operation has meaning but can not be portable defined for all
     platforms. Example: all number conversions that depends on internal
     representation or some CPU specific (like rounding). But I have more
     clear example: "asm" keyword. I found it is defined in C++ draft and
     I believe it is defined in C standard.
     Was it inserted just to give programmers some way to implement undefined
     behavior (with some chance to have disk reformatted)? I do not think so.

First, asm is not a C keyword.

Next, undefined behaviour doesn't have types. Undefined constructs
have no meaning. i = i++ is just as meaningless as #include <memory.h>
: I have no idea what either of them means. (Note that asm { mov ax,
bx } on the other hand is a syntax error.) An implementation or
another standard is allowed to extend the language by providing
meaning for undefined constructs.

   In C++ draft standard there are two words used in such cases:
   "undefined behavior" and "implementation defined behavior" - and I think
   they are used accordingly for 1. and 2. As I understand now in C standard
   there is no such clear division. But it does not mean that the problem
   does not exist and I believe it is very wrong to claim that in every
   case where behavier is not defined by C standard we may loose all
   data from our hard disk!

C does have implementation defined behaviour too. For example, whether
char is signed or unsigned is implementation defined. The
implementation documentation has to specify the choices made for the
implementation defined behaviours.  The standard very clearly
distinguishes between implementation defined and undefined behaviour,
and only very rarely, as in (int)(char*)0, allows an implementation to
define something as undefined (This seems to be the majority view
here: claims have been made that implementation always has the right).

So, for undefined behaviour, you may indeed loose all your data on the
hard disk.

Cheers
Tanmoy
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