The Ghost in the Binary: Undefined Behavior

Undefined behavior is the boogeyman of systems programming.

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Undefined behavior is the boogeyman of systems programming. The language specification refuses to say what happens. The compiler is free to assume it cannot occur. The program runs anyway, until one day it does not, and the failure is shaped like nothing in the source.

The unsettling part is the temporal distance between cause and effect. The bug was committed two years ago. It surfaces today because a new optimisation pass decided to trust an assumption no human ever wrote down.

Treat undefined behavior as a category, not a curiosity. Lint for it. Sanitize for it. Assume that the absence of a crash today is not evidence of correctness — only of the compiler not yet having found a reason to punish you.