Address of a constant

[K. Jennings]

I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable? Naively, what I would like to
do is something like

y = f(&123456) ;

which of course results in a syntax errors.

[Ben Pfaff]

"K. Jennings" <kj*******@resu rgence.netwrite s:

I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;

Note that 123456 is outside the portable range of int.

int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable? Naively, what I would like to
do is something like

y = f(&123456) ;

which of course results in a syntax errors.

In C99, you can use a compound literal, e.g.
y = f(&(int) {123456});
or
y = f((int[]) {123456});
(I'm no C99 expert so it's possible that one or both of these is
wrong for some reason.)

I don't think there's a way to do this in C89.
--
"Programmer s have the right to be ignorant of many details of your code
and still make reasonable changes."
--Kernighan and Plauger, _Software Tools_

[CBFalconer]

"K. Jennings" wrote:

>
I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer
to f without using the intermediate x variable? Naively, what I
would like to do is something like

y = f(&123456) ;

which of course results in a syntax errors.

Why not "int f(int);", which is more direct and safer.

--
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<http://www.securityfoc us.com/columnists/423>
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cbfalconer at maineline dot net

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[K. Jennings]

On Fri, 25 May 2007 12:13:06 -0400, CBFalconer wrote:

"K. Jennings" wrote:

>>
I have a function f with the following prototype: int f(void *). From
my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable? Naively, what I would like
to do is something like

y = f(&123456) ;

which of course results in a syntax errors.

Why not "int f(int);", which is more direct and safer.

Because in the case that I am interested in the data buffer fed
to f can contain lots of different data types, that will be dealt with
appropriately in the right contexts.

[Chris Dollin]

K. Jennings wrote:

I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable?

You can't.

Naively, what I would like to
do is something like

y = f(&123456) ;

which of course results in a syntax errors.

Even if it worked, what's `f` going to do with it? It's
handed a pointer-to-object, but it doesn't know what
kind of object. It can pass it along to another
function that wants a `void*`, but the same applies
to /that/. It can store it in a `void*` variable,
but if that's to be pointful, someone somewhere has
to know what to do with it. It could `free` or `realloc`
it, but since your examples don't pass mallocated
store, that would be ... unwise.

Are you /sure/ that prototype is the one you want?

--
Untyped Hedgehog
The shortcuts are all full of people using them.

[Richard Heathfield]

Chris Dollin said:

K. Jennings wrote:

>I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable?

You can't.

Yes, he can, by changing the prototype of f so that it expects int
rather than int *. Then he can pass the integer value like this:

y = f(123456);

provided, of course, that INT_MAX >= 123456 on his system.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Christopher Benson-Manica]

Chris Dollin <eh@electriched gehog.netwrote:

K. Jennings wrote:

y = f(&123456) ;

which of course results in a syntax errors.

Even if it worked, what's `f` going to do with it?

The same thing f would do with a pointer to an automatic int,
presumably. Obviously f() is intended to have some way of knowing
what the type of its argument is (as OP explicitly stated elsethread).

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

[Christopher Benson-Manica]

K. Jennings <kj*******@resu rgence.netwrote :

int x = 123456 ;
int y ;

y = f(&x) ;

My question is, how can I pass the 123456 integer to f
without using the intermediate x variable? Naively, what I would like to
do is something like

y = f(&123456) ;

which of course results in a syntax errors.

As has been stated, you can't do this. It's worth thinking about the
question of "Why not?", which is (*) that it would require such
integer constants to have storage associated with them, much like
string literals have associated storage. (Note that you can (**)
compute the address of a string literal; the resulting pointer has
type "pointer to array of char".)

(*) - Add grain of salt here; I am not a guru.

(**) - Assuming my informal test with gcc and my reading of the
standard are correct.

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

[Army1987]

"K. Jennings" <kj*******@resu rgence.netha scritto nel messaggio
news:pa******** *************@r esurgence.net.. .

I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer to f
without using the intermediate x variable? Naively, what I would like to
do is something like

y = f(&123456) ;

which of course results in a syntax errors.

What are you trying to do, considering that the argument of f isn't even
const?
Supposing f is
int f(void *v)
{
return ++*(int *)v;
}
What do you expect to happen? All next occurrences of 123456 to behave as
if they were 123457?
More seriously, if f is actually a int f(const void*, const void*) you
use for qsort, and you want to use it to compare two int constants
without rewriting it, you may use temporary variables:

int flag;
{
tmp1 = 123456;
tmp2 = 42;
flag = f(&tmp1, &tmp2);
}

But you'd better tell *what* you are trying to do, than telling *how* you
want to do that, which could be the wrong way.

[CBFalconer]

"K. Jennings" wrote:

CBFalconer wrote:

>"K. Jennings" wrote:

>>>
I have a function f with the following prototype: int f(void *).
From my main() (or whatever) I can do things like

int x = 123456 ;
int y ;

y = f(&x) ;

which is fine. My question is, how can I pass the 123456 integer
to f without using the intermediate x variable? Naively, what I
would like to do is something like

y = f(&123456) ;

which of course results in a syntax errors.

Why not "int f(int);", which is more direct and safer.

Because in the case that I am interested in the data buffer fed to
f can contain lots of different data types, that will be dealt with
appropriately in the right contexts.

Then you better redefine f so that it knows the type involved, or
define a carrying struct which imparts that info. You will have to
use a void*, since you can't convert pointer types arbitrarily.
But a void* can carry any data pointers information, and can be
auto-converted back to the original type.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
<http://kadaitcha.cx/vista/dogsbreakfast/index.html>
cbfalconer at maineline dot net

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