Re: union and overstamping zero values

From:

Hyman Rosen <hyrosen@mail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 22 Mar 2007 16:36:12 CST

Message-ID:

<200703221629.l2MGTcap023978@cliffclavin.cs.rpi.edu>

andrew_nuss@yahoo.com wrote:

Not portably. The NULL pointer need not to be represented
by a bit pattern that also represents the zero integer

If so, then why does this work?:
main { void* p = 0; if (p == 0) p = malloc(2000); if (p) free(p); }

A null pointer compares equal to a zero-valued integer
constant expression. That does not mean that the null
pointer is represented in memory by all-bits-zero. You
do not know what machine code is generated by the compiler
to do the comparison when your code says "if (p == 0)" or
"if (p)". It could as easily be "cmp $a0, 0xFADEDBED" as
"cmp $a0, 0".

     #include <stdio.h>
     int main() {
         static void *p;
         if (p == 0)
             printf("p is a null pointer\n");
         unsigned char *v = (unsigned char *)&p;
         for (int i = 0; i < sizeof(p); ++i)
             if (v[i] != 0)
                 printf("Null pointer is not all-bits-zero at byte
%d\n", i);
     }

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