In C99 there are three levels of not fully specified behaviour: 1. Undefined: anything is permitted, the standard imposes no requirement whatsoever. A typical

In C99 there are three levels of not fully specified behaviour: 1. Undefined: anything is permitted, the standard imposes no requirement whatsoever. A typical example is what happens when a null pointer is dereferenced. 2. Unspecified: anything from a constrained set is permitted. Examples include the order of evaluation of function arguments (all must be evaluated once though any order is allowed). 3. Implementation-defined: the implementation is free to choose the behaviour (possibly from a given set), but must document its choice. An example is the representation of signed integers.