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Compilers use int 3 for padding and ud2 for unreachable code

devblogs.microsoft.com by Raymond Chen Editor score 7/10

The explanation separates the one-byte int 3 used where no code exists from ud2 marking execution of a path that should be unreachable. Knowing the distinction helps engineers interpret compiler output and failures involving invalid control flow.

What

x86 compilers use int 3 for padding where code should not exist and ud2 for paths that should be unreachable.

How

The one-byte int 3 avoids execution from the middle of a two-byte ud2; reaching ud2 indicates an unexpected code path.

Why it matters

Distinguishing the instructions helps engineers diagnose invalid function pointers, breakpoints, and logic flaws.

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