Re: Equal yet different pointers

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From	"Pascal J. Bourguignon" <pjb@informatimago.com>
Newsgroups	comp.lang.c
Subject	Re: Equal yet different pointers
Date	Thu, 16 Nov 2017 16:14:24 +0100
Organization	Informatimago
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ram@zedat.fu-berlin.de (Stefan Ram) writes:

>   I did not invent this program, I just read it in an article
>   and then typed it in from my memory:
>
>   main.c
>
> #include <stdio.h>
> #include <string.h>
>
> int main() 
> { int i = 1; int j = 2;
>   int * p = &i + 1; int * q = &j;
>   if( !memcmp( &p, &q, sizeof( p )))
>   { *p = 3; printf( "%d %d\n", *p, *q ); }}
>
>   transcript
>
> 3 2
>
>   NB: The memcmp compares the pointers themselves,
>   not their pointees. p is only dereferenced after
>   it has been established that it is memcmp-equal
>   to q, which is a valid pointer.
>

Not here, but I have clang.

-*- mode: compilation; default-directory: "~/" -*-
Compilation started at Thu Nov 16 16:14:02

SRC="/Users/pjb/src/c/compare-pointers.c" ; EXE="compare-pointers" ; gcc -I. -L. -g3 -ggdb3 -o ${EXE} ${SRC} &&  ./${EXE} && echo status = $? ; cat $SRC ; gcc --version
different pointers
status = 0

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

int main() {
    int i = 1; int j = 2;
    int * p = &i + 1; int * q = &j;
    if(!memcmp( &p, &q, sizeof( p ))){
        *p = 3;
        printf( "%d %d\n", *p, *q );
    }else{
        printf( "different pointers\n");
    }}
    
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 9.0.0 (clang-900.0.38)
Target: x86_64-apple-darwin17.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Compilation finished at Thu Nov 16 16:14:02

-- 
__Pascal J. Bourguignon
http://www.informatimago.com

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Re: Equal yet different pointers "Pascal J. Bourguignon" <pjb@informatimago.com> - 2017-11-16 16:14 +0100

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