Messages in this thread | | | Date | Tue, 13 Jul 2004 11:53:08 -0700 | From | Elladan <> | Subject | Re: [PATCH] Use NULL instead of integer 0 in security/selinux/ |
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On Sun, Jul 11, 2004 at 11:47:58PM +0200, Olaf Titz wrote: > > For some of us who are extremely familiar with C your argument is > > confusing. You make statements that sound like they are about the > > definition of the C programming language when in fact they are > > criticism of a given C programming style. > > > > Since I am already making distinctions 0 as the integer value and > > 0 as the pointer constant when 0 is implicitly introduced. It is > > really not confusing to me in the case of manifest constants. > > So the real question is why C has no "null" token like Java or Pascal > and re-uses the "0" token (which is really no token by itself but a > numeric-constant token which happens to have a special value). > > If your argument holds that "0" in a pointer context really is a > special token like Java's "null" (which is explicitly defined by the > standard as a pointer different from any other pointer) then it would > be possible to implement a compiler which not only defines NULL to > -1L, as someone mentioned here, but actually generates an all-ones bit > pattern out of the constant 0 when used in a pointer context, yet > generates an all-zeros bit pattern when used in an integer context. > It also would have to implement the implicit null-comparison in a > boolean context appropriately.
This indeed is possible, and has been implemented in the past. Some segmented architectures use different segments for different data types, and a segment tag is part of the appropriate null pointer. Also, obviously, some pointer types are different widths on some architectures.
The C languages was defined in such a way that it works fine on such an architecture. int *p = 0; will always result in the appropriate null pointer for integers.
The one exception is if you have a variadic function, in which case you have to cast the null pointer to the right type, eg. printf("%s\n", (char*)0);
What's not allowed in conformant portable C is clearing pointers to null using memset. That just results in a zero bit pattern.
Eg.,
struct foo { int *bar; } x; memset(&x, 0, sizeof(x));
However, implicit initializers are valid, and will fill in pointer types with the appropriate null value, eg:
struct foo { int *bar; } x = {};
In addition, defining NULL as (void*)0 is completely broken. A void* null pointer is not valid as a null pointer for some other type - void* is a transitional type, it simply holds the bit values of other pointers so you can cast them back later.
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