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 Message-ID: <19970810100337.ZV59622@uriah.heep.sax.de> In-Reply-To: <199708100722.DAA03236@skynet.ctr.columbia.edu>; from Bill Paul on Aug 10, 1997 03:22:28 -0400 References: <199708100722.DAA03236@skynet.ctr.columbia.edu>
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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. ;-)
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