11
\$\begingroup\$

Which values of x and y will cause a crash with some C compilers?

int f(int x, int y) {
    return (y==0) ? 0 : (x/y);
}
\$\endgroup\$
8
  • \$\begingroup\$ Since C's ternary operator shortcuts, I would say none would. This question doesn't seem to fit the format for this site, which is focused on program puzzles and code golf. See the faq for details codegolf.stackexchange.com/faq. \$\endgroup\$ Jan 12, 2012 at 14:48
  • \$\begingroup\$ This isn't code golf, but is a puzzle. There is an answer, and it's just a couple of numbers. \$\endgroup\$
    – ugoren
    Jan 12, 2012 at 14:51
  • \$\begingroup\$ I stand corrected. \$\endgroup\$ Jan 12, 2012 at 14:59
  • 2
    \$\begingroup\$ Actually, judging by the K&R book, this function really must never crash. But by the ANSI C standard, the behavior in the particular crashing case is undefined, and with x86 compilers it crashes. \$\endgroup\$
    – ugoren
    Jan 12, 2012 at 15:05
  • 1
    \$\begingroup\$ @dmckee, If you give the right answer, you're the winner. What cretirion could be more clear and objective? There's only one answer (or do you have another example?) \$\endgroup\$
    – ugoren
    Jan 12, 2012 at 18:10

2 Answers 2

7
\$\begingroup\$

-2147483648 (INT_MIN) and -1

#include <stdio.h>
#include <limits.h>
int f(int x, int y) {
    return (y==0) ? 0 : (x/y);
}
int main() {
    int r = f(INT_MIN, -1);
    printf("%d\n", r);
    return 0;
}

$ gcc -Wall division.c && ./a.out # => zsh: floating point exception ./a.out

\$\endgroup\$
4
  • \$\begingroup\$ Indeed. Though this should give a warning, because 2147483648 isn't a valid integer. \$\endgroup\$
    – ugoren
    Jan 12, 2012 at 14:57
  • 1
    \$\begingroup\$ Yes, that's why I used INT_MIN after, to use a valid int. I guess the reason is 2147483648 is not a valid int, since INT_MAX is 2^31-1 with 32-bit int. \$\endgroup\$
    – eregon
    Jan 12, 2012 at 14:59
  • \$\begingroup\$ Ah. Two's complement. I missed that. \$\endgroup\$ Jan 12, 2012 at 15:00
  • \$\begingroup\$ Yes, it should compile cleanly with INT_MIN (which is -2147483648). \$\endgroup\$
    – ugoren
    Jan 12, 2012 at 15:02
3
\$\begingroup\$

The right answer is already given, but I immediately thought about Microsoft Pex.

Pex automatically generates test suites with high code coverage. Right from the Visual Studio code editor, Pex finds interesting input-output values of your methods, which you can save as a small test suite with high code coverage. Microsoft Pex is a Visual Studio add-in for testing .NET Framework applications

After adding your puzzle in the sandbox site, it finds the answer in a few seconds, the same as eregons answer. (click ask pex)

Note: it does it in C#, but the language is not really relevant.

  • x: int.MinValue
  • y: -1
  • Exception: OverflowException
  • Message: Arithmetic operation resulted in an overflow.
\$\endgroup\$
2
  • 1
    \$\begingroup\$ Nice. It surely doesn't brute-force it, because it wouldn't end in a few seconds. I guess someone in MS realized that numbers around 0 and MAX_INT are always interesting. \$\endgroup\$
    – ugoren
    Jan 17, 2012 at 15:19
  • \$\begingroup\$ Hopefully it's a little more clever than that. It might look at (x/y) and know that INT_MIN, -1, 0 etc. are all problem cases for that expression, and try to reverse engineer a way to produce those values at the time of evaluation. \$\endgroup\$
    – Clueless
    Jan 18, 2012 at 21:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.