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Date: Sun, 10 Aug 1997 10:03:37 +0200
From: j@uriah.heep.sax.de (J Wunsch)
To: chat@FreeBSD.ORG
Subject: Re: variable sized arrays and gcc
References: <199708100722.DAA03236@skynet.ctr.columbia.edu>
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Reply-To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch)
In-Reply-To: <199708100722.DAA03236@skynet.ctr.columbia.edu>; from Bill Paul on Aug 10, 1997 03:22:28 -0400
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As Bill Paul wrote:

> Okay, maybe I haven't been paying attention, but this is the first
> time I've ever noticed that gcc would let you do this. Personally,
> I think it's damn strange, especially since _no_ other C compiler I
> can find behaves the same way. [...]
> If it's a feature, I don't think it's a 
> particularly good one since it encourages the use of non-ANSI (and 
> apparently non-portable) code. If it's a bug, it's got to be the most 
> carefully engineered bug of all time. :)

That's the problem with all local extensions a compiler provides.  I
think it's an interesting feature, and it's clearly mentioned in the
language extensions in the info file:

rrays of Variable Length
=========================

   Variable-length automatic arrays are allowed in GNU C.  These arrays
are declared like any other automatic arrays, but with a length that is
not a constant expression.  The storage is allocated at the point of
declaration and deallocated when the brace-level is exited.  For
example:

...

There are other extensions that fall into the same class, like zero-
length arrays -- could be used as placeholder in a struct where the
tail of the struct is not fixed:

struct foo {
	int len;
	char contents[0];
};

...where `len' tells you about the actual size of the array.  You
can then easily malloc() the storage for this struct by

	var = (struct foo *)malloc(sizeof(struct foo) + actual_length);
	var->len = actual_length;


Of course, if you're going to write portable code, you should not use
extensions.  But if you're going to write a quick hack for your own,
extensions can make your life easier.

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)