What are the common undefined/unspecified behavior for C that you run into? [closed]

Answer 1

I've seen a lot of relatively inexperienced programmers bitten by multi-character constants.

This:

"x"

is a string literal (which is of type char[2] and decays to char* in most contexts).

This:

'x'

is an ordinary character constant (which, for historical reasons, is of type int).

This:

'xy'

is also a perfectly legal character constant, but its value (which is still of type int) is implementation-defined. It's a nearly useless language feature that serves mostly to cause confusion.