Implementation Defined Behaviour

Implementation defined behaviour describes an aspect of the C language that may vary from implementation to implementation but whose behaviour is predictable and is documented. For example:

When a signed integer is shifted right, the high order bit may be filled with zeroes (logical shift) or by propagating the sign bit (arithmetic shift).
The "plain" char data type may be signed or unsigned.
A "plain" int bit-field may be signed or unsigned.
The remainder on signed integer division may be a positive or negative quantity