From: Ullrich von Bassewitz (uz_at_musoftware.de)
Date: 2003-05-06 12:08:04
Hi!
On Tue, May 06, 2003 at 11:50:06AM +0200, Spiro Trikaliotis wrote:
> Just out of curiosity: What are void data structures good for? I can't see
> any use of this, but I'm open for any hints.
>
> I'm sorry for this "C course" question.
It's not a "C course" question, because it is a non standard feature. void
data structures allows one to define arbitrary data. For example, there is no
easy way to define a structure that is equivalent to the following asm code:
table: .word 0
.asciiz "foobar"
.word 1
.asciiz "baz"
.word 2
.asciiz "lxsh"
Two ways to work around this are:
1. Define a struct like
struct tabledata {
unsigned num;
char id[8];
};
and live with the overhead generated by the strings, or
2. use a character buffer and encode the data structure byte by byte:
unsigned char tabledata[] = {
0, 0, 'f', 'o', 'b', 'a', 'r', '\0',
1, 0, 'b', 'a', 'z', '\0',
...
};
If you have an API that expects such a data structure as an argument, you
cannot use 1. The remaining possibility (2.) is ugly and gets even more ugly
if the data structure is more complex.
GEOS is an example for an application that has an API with data structures
like the one above. Forcing people to define their data structures like in 2.
would be a nice way to get them upset very quickly, so I added void data
variables. Since the data type is "void" it cannot be accessed directly. But,
when taking a pointer to this variable, it automatically yields a "void*",
which can be passed to every function that expects any pointer.
Regards
Uz
--
Ullrich von Bassewitz uz_at_musoftware.de
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