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I think this is a bug in glibc


From: Noel David Torres Tao
Subject: I think this is a bug in glibc
Date: Sat, 09 Dec 2000 /-:40:45 -1300

I don't know if this is a bug in glibc, a bug in gcc or
only in my C knowledge, but I think the first because I
think a variable can't have two different values at the
same time.

Using an array in a struct, I found that the same array
element has different values in a printf() call in two
situations.
I expected something like this:

address@hidden ~]$ ./struct
Memory directions and values
0xbffff9ac  80483bb
0xbffff9b0  804960c
0xbffff9b4  8049620

Using listing and listing.array[0] in separate printf()
calls
listing=80483bb listing.array[0]=80483bb
Using listing and listing.array[0] in the same printf()
call
listing=80483bb listing.array[0]=80483bb
address@hidden ~]$ 

and I found exactly this

address@hidden pruebatablas]$ ./struct
Memory directions and values
0xbffff9ac  80483bb
0xbffff9b0  804960c
0xbffff9b4  8049620

Using listing and listing.array[0] in separate printf()
calls
listing=80483bb listing.array[0]=80483bb
Using listing and listing.array[0] in the same printf()
call
listing=80483bb listing.array[0]=804960c
address@hidden pruebatablas]$ 


The source code to reproduce the effect is the follow:

/*Program "struct" to reproduce a bug*/
/*Compile with gcc -o struct struct.c*/
#include <stdio.h>

main()
{

struct list
{
 int array[2];
 int value;
}
listing;

printf("Memory directions and values\n");
printf("%p
%8x\n",&listing.array[0],*(&listing.array[0]));
printf("%p
%8x\n",&listing.array[0]+1,*(&listing.array[0]+1));
printf("%p
%8x\n",&listing.array[0]+2,*(&listing.array[0]+2));
printf("\n");
printf("Using listing and listing.array[0] in separate
printf() calls\n");
printf("listing=%x ",listing);
printf("listing.array[0]=%x\n",listing.array[0]);
printf("Using listing and listing.array[0] in the same
printf() call\n");
printf("listing=%x
listing.array[0]=%x\n",listing,listing.array[0]);
}

Thanks for your attention and time. Im using a
GNU/Linux system (pentium MMX) with Red Hat Linux 6.2,
in which the packets are

address@hidden ~]$ rpm -qa|grep libc
glibc-2.1.3-15
glibc-devel-2.1.3-15
libc-5.3.12-31
address@hidden ~]$ 

and the compiler is

address@hidden ~]$ gcc --version
egcs-2.91.66
address@hidden ~]$ 

Again, many thanks for your time and attention.

Sincerely:
Noel David Torres Taño
Spain

address@hidden

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