about preprocessor directives...

Hi,

OFten i see programs starting with preprocessor directives such as #ifdef and ifndef and #define and etc and etc........

BUt i don't understand one thing. #ifdef is followed by a constants or words, or watever u call it ( e.g. NDEBUG ). And many of these hav underscores or 2 underscores in between the word(s), in front, or after it. WHY ?
Where are these 'words' defined? Where can i find out which 'words' are available? What do the underscores mean?

pls explain, i think this topic is very important.

thnx in advance

These are mangled names that make a header file unique. If the name is already defined then the header file has been opened already and it won't be opened again. It's both an inclusion guard as well as a unique identifier for a header file. The underscores you see are simply a common practice for name mangling, consider a header file myHeader.h. To mangle the name most people would use
MYHEADER_H_

If you see leading underscores then it's wrong, any identifier starting with an underscore is reserved by the implementation for further updates.

-Prelude

>Where are these 'words' defined?

You define them by yourself.

#define MYDEF 1001

Now everywhere in my code where MYDEF appears it will be replaced by 1001 by the preprocessor.

#ifdef MYDEBUG
printf ("Debug Info: .....\n");
#endif

If you defined MYDEBUG at the beginning of your file, or in a header-file you included, then the compiler will compile the debug info statement. If MYDEBUG is not included, the debug code will not be compiled.

This is called a compiler switch. You can define your own compiler switches to make it possible to add functionality or leave functionality away.

#ifndef MYDEF

This is the opponent of #ifdef.

In a macro in a program in my C Unleashed book, i see '__FILE__' and '__LINE__', ar they compiler specific?

Where can i find compiler specific 'mangled names' ?

Also, i didn't define NDEBUG anywhere, so i guess some mangled names are not programmer-defined.

You're missing the point of preprocessor directives. Speficicly the '#ifdef' / '#ifndef' / #if directives.

#ifdef UNIX
#include "myunixheaders.h"
#else
#include "mydosheaders.h"
#endif

Now then, here, I need to either define UNIX or my program will compile as a DOS applicatioin. If I want a unix compile, before I compile, I edit the file, and before the #ifdef line, I add something like:

#define UNIX

See? Another example would be if I wanted to compile my application so the debug lines were displayed.

#ifdef DEBUGON
#define DEBUG(x) fprintf( stderr, x );
#endif

... more code ...

#ifdef DEBUGON
DEBUG( "Allocating memory for %d nodes.\n", node_count );
#endif

Quzah.

consider this,



#ifndef YOU
#define YOU 10
#endif

this code of fragment, determines if YOU were define..
if you were not define, then he would define YOU as 10

#define is a macro substitution...
meaning, before your program is compiled,
it changes YOU into 10..
that is the reason why it is called a preprocessor...


and this stuffs are use in header files...

try this out..

#include<stdio.h>
#include<stdio.h>

int main(){
balh blah balh
return 0;
}


you can include stdio.h many times but,
the compiler will treat it only once
because it has been define..

you can ask some of the guys from this board...

they will surely teach you more that i said....



thx

I get your point, but how do u explain the magled names '__FILE__' and '__LINE__' ? They compiler-specific or what?

yea,


the ANSI standard specifies five built-in predefined macro names. they are

__LINE__
__FILE__
__DATE__
__TIME__
__STDC__

e.g.

chack this out


#include<stdio.h>

#line 50
int main(){
printf("%d",__LINE__);
return 0;
}


this will printf 52 on the screen...

line is primarily used for debuggin porpoes and special application..



did i explain well...

tnx...

Thnx u explained very well....

Is there any documentation on what u just said, like ANSI-standard predefined mangled names?

I am using Dev C++, which uses the MingGw compiler i suppose.

im am very sorry...



i dont know about such documentation


by the way, what is your purpose that you want to know about this advance stuff?

Doesn't matter i found it thnx anyway

Why do i learn? Coz i am reading C Unleashed now and i'm trying to learn advanced topics of C.. It's interesting u know...

:D wish i could have that book....

tnx again

anytime man

You can order that book. I couldn't find it in a bookshop, so i ordered it. Came in after a week, and the discounted price for me is $25 AUD, while the original price is like a hundred.

do you have msn installed on your pc?
i want to know...

tnhx

i use msn. email: [email protected]

I would be happy to add u/.