romke@targon.UUCP (Romke Teerenstra) (10/22/87)
This afternoon I had this weird idea to print void values, and to
cast values to void, just to see what would come out.
My thoughts were : since void is a type, I can declare such a variable.
So I can assign a value of that type to it.
So I can cast something, say an integer, to it.
So I should be ble to write a program what does just that.
To my suprise
however, this didn't even compile on the two types of machines
we have here. One compiler complained about a void type being declared
of which he didn't know the size, and the other just stopped, because
he couldn't cast a value to void.
After talking it over with two of my collegues,
We think this program is legal, but don't know for sure.
So I ask you:
1 Is this program legal?
2 If not, what is wrong with it?
3 If it is, what should be printed?
#include <sys/types.h>
main()
{
void v;
v =
(void)345;
printf("%d %c %X\n",(int)v,(char)v,(int)v);
}
As this is cross-posted, please send Follow-ups to comp.lang.c
Romkeguy%gorodish@Sun.COM (Guy Harris) (10/23/87)
> We think this program is legal, but don't know for sure. > So I ask you: > > 1 Is this program legal? No. > 2 If not, what is wrong with it? It declares a value of type "void". Guy Harris {ihnp4, decvax, seismo, decwrl, ...}!sun!guy guy@sun.com
cef@H.GP.CS.CMU.EDU (Charles Fineman) (10/23/87)
First to the declaration:
*Whenever* you try to declare something that the C compiler does not
know the size of, it complains. For example, you can say:
struct foo;
as a forward declaration and then say:
struct foo *ptr;
later in your code to get a pointer to a structure of type foo
but you can't say:
struct foo var;
because it dosn't know the size of the thing. Its the same exact
thing for the void type. Since C dosn't know what size a thing
of type void should be, you can't declare a variable of that
type.
** One iteresting side note: there was a discusion a while
** ago on some newsgroup about some folks declaring:
** typedef void *generic_pointer;
As for casting to type void, there is no problem here as long
as you realize that the *specified* semantics of this is to
throw away the value. Hence, casting anything to void makes it
*not* an rvalue (i.e. something we get a value of).
Sure, you program is sytactically correct, but then so was
that bogus fortran program that sent that poor satalite
careening off to where no man has gone before ;-)
Charles Fineman
Carnegie-Mellon University
cef@h.cs.cmu.edu (via seismo)gwyn@brl-smoke.ARPA (Doug Gwyn ) (10/27/87)
In article <359@targon.UUCP> romke@targon.UUCP (Romke Teerenstra) writes: > My thoughts were : since void is a type, I can declare such a variable. RONG >So I can assign a value of that type to it. RONG >So I can cast something, say an integer, to it. TRUE While there are "void expressions", there are no "void variables". Once you have "voided" a value, you can't do anything else with it (except void it again).