declaration shadows a parameter

[main()]

Hi all,

When i compile following piece of code,

# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

int main()
{
int num = 1;
fun(num);
return 0;
}

Compiler gives a warning (why not an error?),
In function `fun':
warning: declaration of `val' shadows a parameter

But what i expected is an error something like this,
error: redeclaration of `val'

Can someone explain me the warning ?
Thanks for your time.
Yugi

Don't say to GOD how big your problem is
Say to your problem how big your GOD is

[spibou]

main() wrote:

Hi all,

When i compile following piece of code,

# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

int main()
{
int num = 1;
fun(num);
return 0;
}

Compiler gives a warning (why not an error?),
In function `fun':
warning: declaration of `val' shadows a parameter

But what i expected is an error something like this,
error: redeclaration of `val'

Can someone explain me the warning ?

For ease of reference I will call val-A the parameter passed
to the function fun() and val-B the variable declared inside
fun().

What the warning means is that while val-B is in scope ,
every time you use val you will get the value stored inside
val-B not the value passed in val-A. Here's an example:

#include <stdio.h>
int main(void) {
int i=1 ;

printf("%d\n",i ) ;
{
int i=2 ;
printf("%d\n",i ) ;
}
printf("%d\n",i ) ;
return 0 ;
}

The above code will produce output
1
2
1

Shadowing a variable by another variable with the same
name can be useful in certain circumstances although
some would consider it poor style. Consider the following
example.

#define macrofoo(a) { \
int i ; \
/* Code which uses among other things i \
* as an index variable */ \
}

int foo(void) {
int i,a ;
/* More code
......
*/
for (i=1;i<10;i++)
macrofoo(a)
}

In the above example the variable i defined
inside macrofoo takes values independently
from the variable i defined inside foo(). If you
are really passionate about using the name i
for index variables and you want to use macrofoo
in more than one places then you might write
such code. For that reason , when shadowing
occurs you'd want to have a warning rather than
an error which would make the code uncompilable.

Finally , I note that the concept of shadowing appears
in other programming languages.

Spiros Bousbouras

[Chris Torek]

In article <11************ **********@b28g 2000cwb.googleg roups.com>
main() <dn****@gmail.c omwrote:

void fun(int val)
{
int val; /*problem is here*/

[snippage]

Compiler gives a warning (why not an error?) ...

The C standards require only a "diagnostic ". A warning, an error,
a beep, flashing the screen, or squirting water out of the floppy
drive (if you still have a floppy drive) can all be a "diagnostic ".
(The documentation that comes with the compiler should say something
about its diagnostics.)

See also <http://web.torek.net/torek/c/compiler.html>.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Bill Pursell]

main() wrote:

When i compile following piece of code,
# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

Compiler gives a warning (why not an error?),

gcc errors on this:

b.c:8: error: 'val' redeclared as different kind of symbol

--
Bill Pursell

[Keith Thompson]

Chris Torek <no****@torek.n etwrites:

In article <11************ **********@b28g 2000cwb.googleg roups.com>
main() <dn****@gmail.c omwrote:

>void fun(int val)
{
int val; /*problem is here*/

[snippage]

>Compiler gives a warning (why not an error?) ...

The C standards require only a "diagnostic ". A warning, an error,
a beep, flashing the screen, or squirting water out of the floppy
drive (if you still have a floppy drive) can all be a "diagnostic ".
(The documentation that comes with the compiler should say something
about its diagnostics.)

Is even a diagnostic required in this case?

C99 6.2.1p4 says:

If the declarator or type specifier that declares the identifier
appears inside a block or within the list of parameter
declarations in a function definition, the identifier has block
scope, which terminates at the end of the associated block.

I think this means that the scope of the parameter is the outer block
of the function, even though it's declared before the opening '{'.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Herbert Rosenau]

On Tue, 22 Aug 2006 04:31:27 UTC, "main()" <dn****@gmail.c omwrote:

Hi all,

When i compile following piece of code,

# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

int main()
{
int num = 1;
fun(num);
return 0;
}

Compiler gives a warning (why not an error?),
In function `fun':
warning: declaration of `val' shadows a parameter

But what i expected is an error something like this,
error: redeclaration of `val'

Can someone explain me the warning ?

Because it is NOT an error to override the name of a parameter. It may
be a mistake to do so, but the only who knows that it is a mistake is
the one who wrote the code.

As it is legal to override the name the compiler may or may not warn
you here.

Anyway you should initialise each auto variable with 0 or an value
that flags the variable as faulty. That will help you to dedect the
mistake that you have not set it right later on.

--
Tschau/Bye
Herbert

Visit http://www.ecomstation.de the home of german eComStation
eComStation 1.2 Deutsch ist da!

[Keith Thompson]

"Herbert Rosenau" <os****@pc-rosenau.dewrite s:

On Tue, 22 Aug 2006 04:31:27 UTC, "main()" <dn****@gmail.c omwrote:

> When i compile following piece of code,

# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

int main()
{
int num = 1;
fun(num);
return 0;
}

Compiler gives a warning (why not an error?),
In function `fun':
warning: declaration of `val' shadows a parameter

But what i expected is an error something like this,
error: redeclaration of `val'

Can someone explain me the warning ?

Because it is NOT an error to override the name of a parameter. It may
be a mistake to do so, but the only who knows that it is a mistake is
the one who wrote the code.

As it is legal to override the name the compiler may or may not warn
you here.

I don't believe that's correct. Both the parameter and the local
object are at the same block scope, even though the parameter is
declared before the opening '{' of the scope. See C99 6.2.1.

But as far as programmers are concerned, it doesn't matter much
whether it's legal or not; just don't do it, and be glad that the
compiler at least warned you about it.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Richard Tobin]

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.orgw rote:

>Because it is NOT an error to override the name of a parameter. It may
be a mistake to do so, but the only who knows that it is a mistake is
the one who wrote the code.

As it is legal to override the name the compiler may or may not warn
you here.

>I don't believe that's correct. Both the parameter and the local
object are at the same block scope, even though the parameter is
declared before the opening '{' of the scope. See C99 6.2.1.

That seems very surprising. It would have broken existing code to
prohibit it in the standard, and as far as I can remember I have never
used a compiler that treated it as an error (rather than a warning).

-- Richard

[Ancient_Hacker]

main() wrote:

Hi all,

When i compile following piece of code,

# include <stdio.h>

void fun(int val)
{
int val; /*problem is here*/
printf("%d\n",v al);
}

int main()
{
int num = 1;
fun(num);
return 0;
}

C has always been a bit fuzzy about parameter declarations. In the
original K&R C, the suggestion was to just mention the parameter names
in the parentheses, then associate the names with types after the
header but before the function block (somewhat like FORTRAN).

The new standard is to mention both the type and name in the header.
Which leaves open the question whether the header has its own lexical
nesting level, or is at the same level as the function body.

If the header is its own level, then it's okay to declare another
variable (or const, or type) inside the function body.

If the header is at the same level as the function body, then it should
be an error to redeclare the same name.

It appears your compiler thinks the header has its own nesting level,
but also warns you if you write this somewhat problematical
declaration.

>
Compiler gives a warning (why not an error?),
In function `fun':
warning: declaration of `val' shadows a parameter

But what i expected is an error something like this,
error: redeclaration of `val'

Can someone explain me the warning ?
Thanks for your time.
Yugi

Don't say to GOD how big your problem is
Say to your problem how big your GOD is

[Al Balmer]

On 22 Aug 2006 19:07:44 GMT, ri*****@cogsci. ed.ac.uk (Richard Tobin)
wrote:

>In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.orgw rote:

>>Because it is NOT an error to override the name of a parameter. It may
be a mistake to do so, but the only who knows that it is a mistake is
the one who wrote the code.

As it is legal to override the name the compiler may or may not warn
you here.

>>I don't believe that's correct. Both the parameter and the local
object are at the same block scope, even though the parameter is
declared before the opening '{' of the scope. See C99 6.2.1.

That seems very surprising. It would have broken existing code to
prohibit it in the standard,

I find that surprising. I think I've seen that done only twice in
several million lines of code, and both times were errors.

>and as far as I can remember I have never
used a compiler that treated it as an error (rather than a warning).

Bill Pursell posted one yesterday.

--
Al Balmer
Sun City, AZ