C Programming/stddef.h
stddef.h is a header file in the standard library of the C programming language that defines the macros NULL and offsetof as well as the types ptrdiff_t, wchar_t, and size_t[1].
Inclusion
editIn C, one includes the header file "stddef.h", in this way:
#include <stddef.h>
In C++, one includes the header file "cstddef", in this way:
#include <cstddef>
Namespace
editThe header file "stddef.h" places its definitions in the global scope; the header file "cstddef" places size_t and ptrdiff_t in namespace std.
NULL
editA macro that expands to a null pointer constant. It may be defined as ((void*)0), 0 or 0L depending on the compiler and the language.
offsetof(type, member)
editA functional macro that is used to determine the byte offset of the indicated member field in the specified structure type.
Type size_t
editThe type size_t represents the appropriate type for representing the size of objects of memory areas, and for use in dereferencing the elements of an array. It has an implementation-dependent size; usually but not necessarily, it has a 32-bit representation on 32-bit systems and a 64-bit representation on 64-bit systems. It is unsigned.
This type has in some implementations a signed variant with name ssize_t, that is defined in the UNIX header file "unistd.h". For GNU C the type ssize_t is defined in "stddef.h" and thus resides in the same file as size_t.
Type wchar_t
editAn implementation-specific "wide character" type, which is predefined in the C++ programming language but requires the header "stddef.h" or "wchar.h" in the C programming language.
Type ptrdiff_t
editThe type ptrdiff_t is a type that can hold the result of subtracting two pointers which point to two items of the same object. The underlying type of ptrdiff_t varies from implementation to implementation.
A Object is maybe bigger than PTRDIFF_MAX. A subtracting of two pointers which have a bigger difference than PTRDIFF_MAX / PTRDIFF_MIN result in a undefined behavior.