Re: what will be the value of #define

"raghu" <ragavakumar@xxxxxxxxx> writes:
On Mar 2, 6:28 pm, mark_blue...@xxxxxxxxx wrote:
On 2 Mar, 13:22, Eric Sosman <esos...@xxxxxxxxxxxxxxxxxxx> wrote:
raghu wrote:
Hello
I have a doubt plz clarify that
#ifndef SYSTEM_H
#define SYSTEM_H
what will be the value of SYATEM_H after this #define statement and
before that statement.
[...]
(Assuming it's just a typo): Before the #define, SYSTEM_H
is not defined and has no value at all. After it, SYSTEM_H is
defined as an object-like macro whose replacement list is empty.

To explain a little more for "raghu" - the approach is a standard
convention for ensuring that multiple inclusions of header files
(usually due to being nested in other header files) are "harmless".

If my header called "fred.h" has the form:-

#ifndef FRED_H
#define FRED_H
...
#endif /*FRED_H*/

then the first inclusion will have the macro "FRED_H" undefined, will
define the macro (as an empty macro) and do the other declarations,
definitions etc which the header defines.

Any subsequent inclusions will have "FRED_H" defined, so will do
nothing.

Yes you are correct. I am imlementing exactly like this but one thing
I didn't understand what will be the value assigned FRED_H here. when
the processor looks in to it their must be change in the value. Right.
what will be that value.

Thanks for ur reply.

Raghu, you've posted here a number of times. You've probably already
been advised not to use silly abbreviations like "plz" and "ur". They
just make what you write more difficult to read. If you want us to
take the time to read your articles, take the time to spell out the
words.

Given

#define SYSTEM_H

the identifier SYSTEM_H is a macro that expands to nothing. Any uses
of it *outside a preprocessing directive* (such as #if or #ifdef) will
simply vanish. For example, this:
SYSTEM_H int x SYSTEM_H = 42 SYSTEM_H ;
is exactly equivalent to this:
int x = 42 ;

But there's no reason to care what it expands to, because that's not
what it's used for. In the common idiom where a macro is used as a
header guard, the macro name is *only* used for the purpose of
determining whether it's defined or not, using "#ifndef"; what it
expands to is irrelevant.

--
Keith Thompson (The_Other_Keith) kst-u@xxxxxxx <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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
.

Relevant Pages

  • Re: what will be the value of #define
    ... #ifndef SYSTEM_H ... defined as an object-like macro whose replacement list is empty. ... convention for ensuring that multiple inclusions of header files ... define the macro (as an empty macro) and do the other declarations, ...
    (comp.lang.c)
  • Re: what will be the value of #define
    ... #ifndef SYSTEM_H ... defined as an object-like macro whose replacement list is empty. ... To explain a little more for "raghu" - the approach is a standard ... convention for ensuring that multiple inclusions of header files ...
    (comp.lang.c)
  • Re: [Clax86list] using common macros in NASM and C
    ... Not seeing any value in discussing the definition of a macro, ... generates a sequence of more detailed language statements." ... :with most of the Windows header files and try to create a NASM ... However, I have converted portions ...
    (comp.lang.asm.x86)
  • Re: [Clax86list] using common macros in NASM and C
    ... Not seeing any value in discussing the definition of a macro, ... generates a sequence of more detailed language statements." ... :with most of the Windows header files and try to create a NASM ... However, I have converted portions ...
    (comp.lang.asm.x86)
  • Re: what will be the value of #define
    ... convention for ensuring that multiple inclusions of header files ... #ifndef FRED_H ... then the first inclusion will have the macro "FRED_H" undefined, ...
    (comp.lang.c)