structure member changing after a condition test

[B. Wood]

I have written a simple program the has a structure with two members.

There are two strange things going on.

1. When on of the structure members is tested in a condition, the
value of the second member seems to change.

2. The order of the condition test seems to have an effect on if the
one structure member changes.

The program follows:
-------------------------------------------------------------------------------
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct LongStruct
{
unsigned long lo;
unsigned long hi;
}
LongStruct;

int
main(int argc, char * argv[])
{
LongStruct s;

s.lo = 0x00000001;
s.hi = 0x00000000;

s.lo += 0xFFFFFFFFUL;

if (s.lo < 0xFFFFFFFFUL)
{
s.hi++;
}

printf("\ns.lo tested before s.hi\n");
printf("\n#1 before s.lo test s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);

if (s.lo == 0x00000000)
{
printf("\n#2 s.lo tested == 0 s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);
}
else
{
printf("\n#2 s.lo tested != 0 s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);
}

printf("\n#1 before s.hi test s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);

if (s.hi == 0x00000001)
{
printf("\n#3 s.hi tested == 1 s.lo = %08lX s.hi = %08lX\n\n",
s.lo, s.hi);
}
else
{
printf("\n#3 s.hi tested != 1 s.lo = %08lX s.hi = %08lX\n\n",
s.lo, s.hi);
}

s.lo = 0x00000001;
s.hi = 0x00000000;

s.lo += 0xFFFFFFFFUL;

if (s.lo < 0xFFFFFFFFUL)
{
s.hi++;
}

printf("\ns.hi tested before s.lo\n");
printf("\n#1 before s.hi test s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);

if (s.hi == 0x00000001)
{
printf("\n#2 s.h1 tested == 1 s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);
}
else
{
printf("\n#2 s.hi tested != 1 s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);
}

printf("\n#1 before s.lo test s.lo = %08lX s.hi = %08lX\n",
s.lo, s.hi);

if (s.lo == 0x00000000)
{
printf("\n#3 s.lo tested == 0 s.lo = %08lX s.hi = %08lX\n\n",
s.lo, s.hi);
}
else
{
printf("\n#3 s.lo tested != 0 s.lo = %08lX s.hi = %08lX\n\n",
s.lo, s.hi);
}

return (0);

}
--------------------------------------------------------------------------------

The output of the program is as follows:

s.lo tested before s.hi

#1 before s.lo test s.lo = 00000000 s.hi = 00000001

#2 s.lo tested == 0 s.lo = 00000000 s.hi = 00000000

#1 before s.hi test s.lo = 00000000 s.hi = 00000001

#3 s.hi tested == 1 s.lo = 00000000 s.hi = 00000001


s.hi tested before s.lo

#1 before s.hi test s.lo = 00000000 s.hi = 00000001

#2 s.h1 tested == 1 s.lo = 00000000 s.hi = 00000001

#1 before s.lo test s.lo = 00000000 s.hi = 00000001

#3 s.lo tested == 0 s.lo = 00000000 s.hi = 00000001

--------------------------------------------------------------------------------

This program was compiled on a Linux machine.
The version of Linux is 2.4.7-10.
The compiler used is gcc version 2.96.

This same program builds and runs fine on a Windows machine running
Windows 2K and also a Solaris machine running SunOS 5.8.

The size of unsigned longs, unsigned ints and ints is the same on all
three platforms.

Does anyone have any ideas on what could be happening ?

 

[Greg P.]

<snip>
| s.lo tested before s.hi
|
| #1 before s.lo test s.lo = 00000000 s.hi = 00000001
|
| #2 s.lo tested == 0 s.lo = 00000000 s.hi = 00000000
<snip>

Is the line above what you are talking about? I tried executing your code on
various platforms and could not reproduce the problem. What is your Linux
running on? You may have stack frame problems (some kind of internal wild
pointer flying around). Try tagging the struct members with volatile and see
if they still change and post back.

Sorry that I can't help further =(

 

[B. Wood]

<snip>
| s.lo tested before s.hi
|
| #1 before s.lo test s.lo = 00000000 s.hi = 00000001
|
| #2 s.lo tested == 0 s.lo = 00000000 s.hi = 00000000
<snip>

Is the line above what you are talking about? I tried executing your code on
various platforms and could not reproduce the problem. What is your Linux
running on? You may have stack frame problems (some kind of internal wild
pointer flying around). Try tagging the struct members with volatile and see
if they still change and post back.

Sorry that I can't help further =(

Yes, that is the line. After the condition test between printing #1
and #2, s.hi seems to be changed to 0.

Weirder is after the printf that outputs #2 above with s.hi =
00000000, the very next line of code executed is the printf that
outputs the following line:

#1 before s.hi test s.lo = 00000000 s.hi = 00000001

Now s.hi is suddenly back to 00000001.

Here is some of the system information

model name : Pentium II
cpu MHz : 267.280

I added volatile to the member type and that worked. Thanks.
I think this might be a compiler bug. The compiler must be trying to
do some
kind of optimization that the volatile qualifier prevents.

Thanks.