what will be the value of #define

[raghu]

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.

Thanking you all

Bye

[Richard Heathfield]

raghu said:

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.

Your #define doesn't affect any identifier by the name of SYATEM_H -
but, presuming you meant SYSTEM_H, its value prior to the #ifndef
depends on whether it exists. If it didn't, then its value after the
#define (in your example) will be 0.

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

[Eric Sosman]

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.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(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.

--
Eric Sosman
es*****@acm-dot-org.invalid

[mark_bluemel]

On 2 Mar, 13:22, Eric Sosman <[e-mail] e:

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.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(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.

[Richard Heathfield]

Eric Sosman said:

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.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(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.

Ah, good point. I realise now that my parallel reply may seem a little
odd, but of course I was thinking of its value when used in
preprocessor directives such as #if.

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

[raghu]

On Mar 2, 6:28 pm, mark_blue...@po box.com wrote:

On 2 Mar, 13:22, Eric Sosman <[e-mail] e:

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.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(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.

Bye

[santosh]

raghu wrote:

On Mar 2, 6:28 pm, mark_blue...@po box.com wrote:

On 2 Mar, 13:22, Eric Sosman <[e-mail] e:

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.

(Just in case it's a trick question): Impossible to tell,
based on the information provided.

(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.

No particular value will be assigned. The symbol FRED_H is defined and
will remain defined until explicitly undefined with an #undef
directive. If you want, you can also do:

#ifndef FRED_H
#define FRED_H 1

/* ... */

#endif /* FRED_H */

[Keith Thompson]

"raghu" <ra*********@gm ail.comwrites:

On Mar 2, 6:28 pm, mark_blue...@po box.com wrote:

>On 2 Mar, 13:22, Eric Sosman <[e-mail] e:

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_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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[santosh]

Keith Thompson wrote:

"raghu" <ra*********@gm ail.comwrites:

On Mar 2, 6:28 pm, mark_blue...@po box.com wrote:

<snip>

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.

<snip>

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 ;

A minor question:

Is it simply removed or is it replaced by a space character?

<snip>

[Richard Heathfield]

Keith Thompson said:

<snip>

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.

I have already stopped bothering to read his articles, for precisely
this reason. When he starts posting intelligibly, I will consider
reading what he has to say.

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