Message-Id: <m0yIGC2-000S2nC@inti.gov.ar>
Comments: Authenticated sender is <salvador AT natacha DOT inti DOT gov DOT ar>
From: "Salvador Eduardo Tropea (SET)" <salvador AT inti DOT gov DOT ar>
Organization: INTI
To: DJ Delorie <dj AT delorie DOT com>, eliz AT is DOT elta DOT co DOT il, blizzar AT hem1 DOT passagen DOT se,
        djgpp AT delorie DOT com, djgpp-workers AT delorie DOT com
Date: Thu, 26 Mar 1998 14:14:59 +0000
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Subject: Re: NULL redefined! :(
In-reply-to: <199803252351.SAA09923@delorie.com>
References: <m0yHoqi-000S2qC AT inti DOT gov DOT ar> (salvador AT inti DOT gov DOT ar)
Precedence: bulk

DJ Delorie <dj AT delorie DOT com> wrote:

> 
> > 2) Is equivalent to 0, that's:
> > 
> > a) char *foo=0; then (foo==0) is true.
> 
> BUT these are not equivalent:
> 
> 	__null = foo;
> 	0 == foo;
> 
> Also, looking up the value of a variable takes more CPU time than
> comparing to a constant, and the compiler can't optimize it as well.

It seems that you got it very wrong DJ:

1) You can't do __null=anything; as you can't do int=anything; 
2) __null is solved in compile time, it isn't a variable is an special kind of 
constant. That's why you can pass it to C code.

Take a look at this:

---- pp.cc: --------
#include <stdio.h>

int main(int argc, char *argv[])
{
 char *f=__null;

 printf("%d\n",f);
 
 return 0;
}
-----------------

gcc -S pp.cc

------- pp.s: --------
	.file	"pp.cc"
gcc2_compiled.:
___gnu_compiled_cplusplus:
.text
LC0:
	.ascii "%d\12\0"
	.p2align 2
.globl _main
_main:
LFB1:
	pushl %ebp
LCFI0:
	movl %esp,%ebp
LCFI1:
	subl $4,%esp
LCFI2:
	movl $0,-4(%ebp)

<<<<<<<<<<<<< Here: gcc just put a 0, no optimizations used

	movl -4(%ebp),%eax
	pushl %eax
LCFI3:
	pushl $LC0
LCFI4:
	call _printf
	addl $8,%esp
LCFI5:
	xorl %eax,%eax
	jmp L1
	.align 2,0x90
	xorl %eax,%eax
	jmp L1
	.align 2,0x90
L1:
	leave
	ret
LFE1:
[snipped]
-----------------------

As you can see that's just a trick, as I told before I didn't have the time to 
see what advantage have, but the GNU C++ library of 2.8.x uses it and not the 
0.

SET
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