stddef.h

The stddef.h header provides fundamental type aliases and macros. Many of these definitions also appear in other headers.

Excluding type aliases max_align_t, ptrdiff_t and wchar_t and function-like macro offsetof.

Type aliases

size_t

Alias for:

The sizeof operator returns a size_t value. It is widely used in the standard library to represent sizes and counts.

NULL

Expands to (void*)0
Macro for a null pointer. It has three primary use cases:

To initialise a pointer variable before assigning it a valid memory address.
To return from a function when no valid memory address can be returned, e.g. for error handling.
To pass as a function argument when it is not desired to pass a valid memory address.