Undefined Behaviour

If the behaviour of a C statement is described as undefined, it is impossible to make any assumptions about the result; the compiler is at liberty to do anything ! The ISO C Standard defines 97 instances of undefined behaviour. Most are obvious examples of inconsistency or constructs which clearly have no meaning; but it is important to realise that such constructs can be written without any infringement of the grammatical rules or constraints of the C language and a conforming compiler is not obliged to identify them.

Some examples:

• The result of integer overflow
• An object is modified more than once, or is modified and accessed other than to determine the new value, between two sequence points.
  (e.g. x = 2 + x++;)
• The value of a function is used, but no value was returned.
• The result of the preprocessing concatenation operator ## is not a valid preprocessing token.

See also:

Implementation Defined Behaviour
Unspecified Behaviour
Index