What does this result from printf indicate?

Question

I periodically get extraneous results when I run a program I have constructed. Instead of printing a float, I get this: -1.#IO. What does this result indicate? A divide by zero?

In my experience, if the result was an overflow then it should still print correctly, just a an incorrect value (perhaps some large negative).

Here is what I am getting most of the time:

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Here is what happens about every 6-7 runs.

enter image description here

If I run the program using a constant seed (assuming that seed produces correct results), then everything works fine indefinitely. However, if I run using a random seed, like time(0), I tend to see this program explode at run-time.

In short, has anyone experienced this result before? Just looking for a starting point to dive into the code.

Actually, I didn't think about that. I am using a C89 compiler included with Visual Studio 2012. I migrated to writing C programs in the IDE and was told that it does VS does not support a C99 compiler. Perhaps this is the issue. The only libraries I am using are the ANSI C ones (like stdio, stlid, time, math, etc). — sherrellbc
– sherrellbc, Commented Feb 25, 2014 at 5:29
Worked fine using C99 compiler. I dare not even ask what could have caused this using a C89 compiler. It could have been anything. — sherrellbc
– sherrellbc, Commented Feb 25, 2014 at 5:35
What is the format string you're using? Looks like some fixed decimal format that may not fit inside the fixed character restrictions. — Joachim Isaksson
– Joachim Isaksson, Commented Feb 25, 2014 at 5:40
You got this special output maybe because your compiler/libc wants to inform you that some error happened, and this specific output may depend on the specific compiler/libc you are using. — Lee Duhem
– Lee Duhem, Commented Feb 25, 2014 at 5:44

Stack Exchange Broke The Law · Accepted Answer · 2014-02-25 05:56:44Z

On Windows, infinity is printed as 1.#INF. A quirk in the standard library means that rounding it to three decimal places gives 1.#IO ('F' >= '5', so it increments the 'N').

For the same reason, -1.#IO is negative infinity.

And yes, you could get this result by dividing by zero.

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