Code only works if all variables are set to 0 first. UB?

Question

This code fails randomly by correctly identifying some numeric palindromes and failing on others.

#include <stdio.h>


int main(int argc, char *argv[])
{
   int n, reverse = 0, temp;

   printf("Enter a number to check if it is a palindrome or not\n");
   scanf("%d",&n);

   temp = n;

   while( temp != 0 )
   {
      reverse = reverse * 10;
      reverse = reverse + temp%10;
      temp = temp/10;
   }

   if ( n == reverse )
      printf("%d is a palindrome number.\n", n);
   else
      printf("%d is not a palindrome number.\n", n);

   return 0;
}

For example, the above code incorrectly says "87678" isn't a numeric palindrome.

Checking the return of scanf() shows it's succeeding and printing the value of n is correct for input of 87678.

However the code correctly says "4554" is a palindrome.

However, by adding:

n = reverse = temp = 0;

before the first printf() the program appears to work correctly all the time. So what is happening in the first version? Is this some sort of undefined behavior when the variables aren't initialized before use?

EDIT: Will later provide the assembly of the compiled version that is failing to see what the compiler is doing.

An attempt to debug this was made, and the results were still ambiguous as to the actual cause of this behavior, therefore the question — Tony The Lion, Oct 29 '12 at 09:17
@DextOr They don't have an effect on the behavior of this code. — Nikos C., Oct 29 '12 at 09:18
Can you be specific on test (numbers) for which it is passing only when you add n = reverse = temp = 0; line? — sahaj, Oct 29 '12 at 09:20
try printing temp (after the scanf) on the cases it fails. As @Daniel Fischer Id expect something going wrong there. Though in any case, it is good practice to initialize variables — Thomas, Oct 29 '12 at 09:21
The code is fine but you should strictly check the input given to scanf() — Omkant, Oct 29 '12 at 09:21
What happens when you replace the line containing the `%` operation with this: `reverse = reverse + (temp - 10 * (temp / 10));` — Nikos C., Oct 29 '12 at 09:32
Can you tell us what version compiler you are using, and optionally paste bin the output of the equivalent of gcc -S (generate assembly). You might have a compiler bug?? — dave, Oct 29 '12 at 09:36
@Lundin 32 Bit Linux with GCC 4.5.2 - ubuntu 11.04. Others have noted the code works fine on a newer version of GCC... which is rather surprising to me. — Chimera, Oct 29 '12 at 09:38
Please show the result you get for 87678 (`printf()` the `reverse` var after the `while` loop finished.) — Nikos C., Oct 29 '12 at 09:39
This works just fine with Mingw GCC 4.6.2, as well as Embarcadero C++, tested on 64-bit Windows 7. — Lundin, Oct 29 '12 at 09:44
@NikosChantziaras after the loop the value of reverse was a rather large negative number. — Chimera, Oct 29 '12 at 09:53
I suppose the alternative code I gave 5 comments above doesn't change the result? — Nikos C., Oct 29 '12 at 09:55
@NikosChantziaras Didn't make a difference. Thanks for your help. I'm running memtest now. — Chimera, Oct 29 '12 at 09:56
I don't see an error in the code either. Either run it with a debugger or add some debugging statements to show the value of "reverse" each time through the loop. Maybe that will give a clue. — Jay, Jun 25 '15 at 13:55

score 4 · Accepted Answer · answered Oct 29 '12 at 09:50

4

Unless sizeof(int) is less than 4, you've either hit a compiler bug, your hardware is malfunctioning, or you have some form of data corruption going on in your system.

To answer the question: no, there's no undefined behavior anywhere in your program (assuming the scanf() really doesn't fail).

Try running memtest on your system to rule out RAM issues: http://www.memtest.org

answered Oct 29 '12 at 09:50

Nikos C.

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Running memtest now. Thanks for taking the time to help out. – Chimera Oct 29 '12 at 09:59
That pretty much leaves a compiler toolchain bug as the most likely reason. Especially if it's reproducible constantly (hardware failure usually doesn't give 100% reproducibility.) – Nikos C. Oct 29 '12 at 16:30

score 2 · Answer 2 · answered Oct 29 '12 at 09:56

2

It sounds very much like you have a compiler error since this works with later versions of gcc. I'd be very interested to see the output of gcc -S (pastebin please?) and also to know the compile command you are using. (optimization level especially).

answered Oct 29 '12 at 09:56

dave

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I will provide that assembly output when I get back into the office. Thanks. It is odd that different compilers are showing different results. – Chimera Oct 29 '12 at 09:59

score -3 · Answer 3 · edited May 23 '17 at 12:22

-3

Unlike Java, C does not have a default value for int. You can refer to this post as it discuss this similar problem.

edited May 23 '17 at 12:22

Community

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answered Oct 29 '12 at 09:20

Ian

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C does have default values for variables depending on the storage class of the variable. – Alok Save Oct 29 '12 at 09:21
6

All variables in the code are initialized, assuming the `scanf()` doesn't fail. – Nikos C. Oct 29 '12 at 09:21
@NikosChantziaras And that is what has me puzzled. – Chimera Oct 29 '12 at 09:23
1

Yeah, you're right. Assuming scanf does not fail, all these variables should have been initialized. Do you have values that fail? Does it exceed the max value of an int? – Ian Oct 29 '12 at 09:27
@IanGil Not exceeding the value of an int. – Chimera Oct 29 '12 at 10:00

Code only works if all variables are set to 0 first. UB?

3 Answers3