Hi, the definition of NULL is not the problem, the problem is why the NULL macro is expanded to 0 and not to another value. I would prefer 42 (see the hitchhikers guide through the galaxy) and i think chinese would prefer 666.
Rolf F. mspgcc-users@lists.sourceforge.net schrieb am 29.03.04 09:49:56: > > Hello, > > Here some info > > From The book "The C programming language" by Brain W. Kernighan and Dennis > M. Ritchie" ISBN 0-13-110370-9 ISBN 0-13-110362-8 > Page 102 Chapter 5.4 > > "Pointers and integers are not interchangeable. Zero is the sole exception: > the constant zero may be assigned to a pointer, and a pointer may be > compared with the constant zero. The symbolic constant NULL is often used in > place of the Zero, as a mnemonic to indicate more clearly that this is a > special value for a pointer. NULL is defined in <stdio.h>. " > > Robert > > > -----Original Message----- > From: mspgcc-users-ad...@lists.sourceforge.net > [mailto:mspgcc-users-ad...@lists.sourceforge.net] On Behalf Of > nobo...@web.de > Sent: 2004 March 28 00:34 > To: mspgcc-users@lists.sourceforge.net > Subject: Re: [Mspgcc-users] Why #define NULL 0? > > Hi, > > > as far as i know the compiler does if(!foo) => if(0==foo) =>if(NULL==foo) > at compile time. > > i forgot to say that chapter 6.5.3 (C99) says that the unary operator ! can > be applied to a pointer type; that's the reason why it works although on > first look it seems to work only if the NULL macro is expanded to 0. > > > > But generally it's not possible to compare a pointer with an integer, > because they the have diffent type and often do not have the same size. On > sparc64 the user-space pointer have 32 bit while the kernel-space pointers > have 64 bit, so on that platform comparing integers with pointers is a bad > idea. > > Therefore gcc generally produces a warning if he finds such ugly code. > > > > > > >> After all, ain't unary logical operations with pointers illegal? > > > > A pointer is an object of size sizeof(pointer); there is no reason why not > to apply logical operations with that object. > > I forgot to say that maybe for some operators (look into the standard) you > have to use a union to apply them (to the other member of the union): > > union foo > { > type *p_p; > char a_c[sizeof(type *)]; > }; > I > If sizeof(int) == sizeof(type *) than you can use int as the second member. > I > Rolf F. > > > > > ------------------------------------------------------- > This SF.Net email is sponsored by: IBM Linux Tutorials > Free Linux tutorial presented by Daniel Robbins, President and CEO of > GenToo technologies. Learn everything from fundamentals to system > administration.http://ads.osdn.com/?ad_id=1470&alloc_id=3638&op=click > _______________________________________________ > Mspgcc-users mailing list > Mspgcc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/mspgcc-users > > > > ------------------------------------------------------- > This SF.Net email is sponsored by: IBM Linux Tutorials > Free Linux tutorial presented by Daniel Robbins, President and CEO of > GenToo technologies. Learn everything from fundamentals to system > administration.http://ads.osdn.com/?ad_id=1470&alloc_id=3638&op=click > _______________________________________________ > Mspgcc-users mailing list > Mspgcc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/mspgcc-users
