5 ms·

I would love to see the code that produces this bug.

• mseebach 15y ago
  Probably something like loading the 128 bytes into a byte array, then doing something to it that requires it to be \0 terminated and then the last byte is overwritten. EDIT: The suggested workaround is to run it in binary mode. As binary doesn't have a concept of \0-terminated strings, this seems to back up my theory.
  • w-ll 15y ago
    My assumption as well. It's funny how we can even produce pseudo bugs in our head when we look at how some other software misbehaves.
  • pbhjpbhj 15y ago
    If it's something that simple then how on earth could MS ship it? What tests would they have run that could miss this - I'd think that a test on a file comparison program would include cycling through a single bit alteration and checking that it was captured up to some arbitrary number of bytes (and I'd think you'd let it run > 128 bytes too). I'm not a coder, does this sort of assumption seem reasonable. If you can't get the basics right ...?
    • Zev 15y ago
      This is an easy mistake to make, and miss when looking over your code. With hindsight - and having it explained to you - it is also a very easy mistake to understand. That doesn't mean it was an easy mistake to find. I bet it went without being detected for a long time.
    • jerf 15y ago
      C is a fantastic language for failing to the get the basics right. It takes only a moment's inattention from even a C master and you've got an error like this. In almost every more modern language I can think of, the language itself would afford code that would make this particular error much harder to write, unless you write C-in-(Python/Ruby/Haskell/etc) in the first place. And of course those other languages all have their own problems, but they tend to be fewer, which is why we can write in them faster.
    • Locke1689 15y ago
      Your test suite seems broken (which is why test suites are hard to write in the first place). What if there is a bug when you have two bits changed -- one in the 128th byte and one in the 256th byte? Here's a possible test suite that could catch those bugs. Take two files. Generate all possible combinations of byte-differences between the two files up to a length of 256 bytes and flag invalid comparisons. Now, what is the running time of this test case? I'll give you a hint: this test case is unlikely to finish before the heat-death of the universe. In general it's very hard to right comprehensive test suites and the only thing this proves is that Microsoft developers are humans, not gods. In fact, this is exactly the type of weird corner-case bug that I would expect to find in code written by good developers.
      • pzxc 15y ago
        He didn't suggest that the test suite should account for all possible combinations of byte differences. He suggested that the test suite should account for all possible combinations of SINGLE byte differences. I.E., the test suite should have checked two files with only the first byte different, with only the second byte different, etc. Such a test suite would be linearly complex, not exponentially, and could easily be run before the heat death of anything. (However, to play my own devil's advocate, I'd have to say it's easy in retrospect to say "yes! there is an easy test for this that should have been written!" when in fact often the number of possible tests is astronomically large and it can be hard to pick the right ones. What if the bug was that FC.EXE didn't correctly register a difference when both the 127th and 128th bytes were the only differences? The proposed test suite would not have caught it.)
        • Locke1689 15y ago
          Your devil's advocate argument seems to be just my post. I was trying to show that, while the parent's test suite was linear running time, the number of tests in a comprehensive test suite is exponential, therefore the runtime of any completely comprehensive test suite is exponential. Choosing the correct tests to use resources on is a very difficult problem.
      • pbhjpbhj 15y ago
    • baddspellar 15y ago
      I don't know the root cause, but something this esoteric would most likely be discoverable only in a code review, or possibly a unit test. It's hard to imagine someone thinking of testing the case of 2 files that differ in every 128 bytes without reading the code.
• gst 15y ago
  I guess it's a simply off-by-one error. Most likely they are reading the file data into buffers of size 128, but then only compare the first 127 characters of each buffer (e.g., because they use < instead of <= in their loop).
  • rudiger 15y ago
    There are two hard problems in computer science: cache invalidation, naming things, and off-by-one errors.
    • ajross 15y ago
      You forgot sign bugs. It's very, very important that you have an even number of them. Many developers forget.