Um, there are the cases new == old and new > old. And all the more specific cases new == old + n. I haven’t seen the code so this is just speculation, but there are plenty of ways how an unexpected, "can never happen" comparison result causes immediate UB because there’s no execution path to handle it, causing garbage to be returned from a function (and if that garbage was supposed to be a pointer, well…) or even execution never hitting a `ret` and just proceeding to execute whatever is next in memory.

Another super easy way to enter UB land by assuming an integer is nonnegative is array indexing.

  int foo[5] = { … }
  foo[i % 5] = bar;

Everything is fine as long as i isn’t negative. But if it is… (note that negative % positive == negative in C)

▲ account42 4 days ago | parent [-]

Dividing by a difference that is suddenly zero is another possibility.

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ogurechny 4 days ago | parent [-]

The error states that the window can't be created. It might be the problem with parameters to the window creation function (that should not depend on game state), or maybe the system is out of memory. Resources allocated in memory are never cleaned up because cleanup time overflows?

Doom4CE (this port) was based on WinDoom, which only creates the program window once at startup, then switches the graphical mode, and proceeds to draw on screen independently, processing the keyboard and mouse input messages. I'm not sure, but maybe Windows CE memory management forced the programmer to drop everything and start from scratch at the load of each level? Then why do we see the old window?

There are various 32 bit integer counters in Doom code. I find it quite strange that the author neither names the specific one, nor what it does, nor tries to debug what happens by simply initialising it with some big value.

Moreover, 2^32 divided by 60 frames per second, then by 60 seconds, 60 minutes, 24 hours, 30 days, and 12 months gives us a little less than 2.5 years. However, Doom gameplay tick (or “tic”), on which everything else is based, famously happens only 35 times a second, and is detached from frame rendering rate on both systems that are too slow (many computers at the time of release), or too fast (most systems that appeared afterwards). 2^32 divided by 35, 60 seconds, etc. gives us about 4 years until overflow.

Would be hilarious if it really is such an easy mistake.

	▲	BearOso 4 days ago \| parent [-]
		The VGA 320x200 mode, either 13h or "Mode Y", ran at 70.086 Hz, so that adding up to ~2.5 years is just coincidental. It's a shame the source code for doom isn't available, and that the author couldn't just link directly to a specific line in a gitweb repository. /s