undefine for functions

[Pietro Cerutti]

Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

For example, let's say that I need a few declarations from stdio.h but
want to define my own printf:

#include <stdio.h>
#undefine-or-similar printf
#include "printf.h"

int main(void)
{
printf(123);
return (0);
}

Thank you.
--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Chris Dollin]

Pietro Cerutti wrote:

is there a mean to undefine a function, in a similar way as you can
undefine macros?

No.

--
Chris "in denial" Dollin

Hewlett-Packard Limited registered no:
registered office: Cain Road, Bracknell, Berks RG12 1HN 690597 England

[Ian Collins]

Pietro Cerutti wrote:

Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

For example, let's say that I need a few declarations from stdio.h but
want to define my own printf:

#include <stdio.h>
#undefine-or-similar printf
#include "printf.h"

int main(void)
{
printf(123);
return (0);
}

No, once a symbol is declared in a compilation unit, it's definition can
not be changed.

The best you can do is something gross like:

#define puts _puts
#include <stdio.h>
#undef puts

void puts( int n ) {}

int main(void) {
puts( 42 );
}

--
Ian Collins.

[Pietro Cerutti]

Ian Collins wrote:

Pietro Cerutti wrote:

>Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

[snip]

No, once a symbol is declared in a compilation unit, it's definition can
not be changed.

The best you can do is something gross like:

#define puts _puts
#include <stdio.h>
#undef puts

void puts( int n ) {}

int main(void) {
puts( 42 );
}

That achieves what I was looking for. Basically, it replaces the
occurrences of "puts" in stdio.h with "_puts", which isn't a function
name. After that, I'm able to declare and define my own version of puts.

Anyway, why I don't get warnings about puts being defined elsewhere
(libc.so)?

Great, thank you!

--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Ian Collins]

Pietro Cerutti wrote:

Ian Collins wrote:

>Pietro Cerutti wrote:

>>Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

[snip]

>No, once a symbol is declared in a compilation unit, it's definition can
not be changed.

The best you can do is something gross like:

#define puts _puts
#include <stdio.h>
#undef puts

void puts( int n ) {}

int main(void) {
puts( 42 );
}

That achieves what I was looking for. Basically, it replaces the
occurrences of "puts" in stdio.h with "_puts", which isn't a function
name. After that, I'm able to declare and define my own version of puts.

Anyway, why I don't get warnings about puts being defined elsewhere
(libc.so)?

Well I said it was gross and I should have added dangerous and not
portable as well!

Your system uses some form of week symbol for standard library functions
and your function replaced it.

--
Ian Collins.

[Pietro Cerutti]

Ian Collins wrote:

Pietro Cerutti wrote:

>Ian Collins wrote:

>>Pietro Cerutti wrote:
Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

[snip]

>>No, once a symbol is declared in a compilation unit, it's definition can
not be changed.

The best you can do is something gross like:

#define puts _puts
#include <stdio.h>
#undef puts

void puts( int n ) {}

int main(void) {
puts( 42 );
}

That achieves what I was looking for. Basically, it replaces the
occurrences of "puts" in stdio.h with "_puts", which isn't a function
name. After that, I'm able to declare and define my own version of puts.

Anyway, why I don't get warnings about puts being defined elsewhere
(libc.so)?

Well I said it was gross and I should have added dangerous and not
portable as well!

Your system uses some form of week symbol for standard library functions
and your function replaced it.

Not understood, sorry.
Could you elaborate it or give further directions, please?

Thank you!
--
Pietro Cerutti

PGP Public Key:
http://gahr.ch/pgp

[Kenneth Brody]

Ian Collins wrote:

>
Pietro Cerutti wrote:

Hi Group,
is there a mean to undefine a function, in a similar way as you can
undefine macros?

For example, let's say that I need a few declarations from stdio.h but
want to define my own printf:

[...]

No, once a symbol is declared in a compilation unit, it's definition can
not be changed.

The best you can do is something gross like:

#define puts _puts
#include <stdio.h>
#undef puts

void puts( int n ) {}

int main(void) {
puts( 42 );
}

Or:

#include <stdio.h>
#undef puts
#define puts MyPuts
...

Besides, doesn't the standard forbid you from replacing standard
library functions? For example, I have seen compilers which can
inline some standard functions, like strcpy(), so even if you
had a function called strcpy(), it wouldn't be called.

BTW, is it legal to #undef something that hasn't been #define'd?
Does the above need a #ifdef around the #undef? (My compiler
doesn't complain about it.)

Hold on... I knew I downloaded n1124.pdf for a reason...

Don't mind me. Just go about your things while I have this
conversation with myself.

6.10.3.5p2

(#undef)

It is ignored if the specified identifier is not currently
defined as a macro name.

So, to answer my own question... Yes, it is perfectly legal to
#undef something which isn't #define'd already.

--
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