The result of subtracting two pointers in C is always an integer, but the
precise data type varies from C compiler to C compiler. Likewise, the
data type of the result of sizeof also varies between compilers.
Also, it helps to have a data type with the greatest fundamental alignment.
ISO C defines standard aliases for these types, so you can refer to
them in a portable fashion. They are defined in the header file
stddef.h.
This is the signed integer type of the result of subtracting two
pointers. For example, with the declaration char *p1, *p2;, the
expression p2 - p1 is of type ptrdiff_t. This will
probably be one of the standard signed integer types (short int, int or long int), but might be a nonstandard
type that exists only for this purpose.
This is an unsigned integer type used to represent the sizes of objects.
The result of the sizeof operator is of this type, and functions
such as malloc (see Unconstrained Allocation) and
memcpy (see Copying Strings and Arrays) accept arguments of
this type to specify object sizes. On systems using the GNU C Library, this
will be unsigned int or unsigned long int.
Usage Note: size_t is the preferred way to declare any
arguments or variables that hold the size of an object.
This is an object type with the greatest fundamental alignment,
i.e., the greatest alignment needed by standard types.
Code can use alignof (max_align_t) when calculating space
needed for arbitrary collections of objects, so long as the objects’
types have a fundamental alignment and lack stricter alignment specifiers.
In the GNU C Library, the value of alignof (max_align_t) is 16 on
most architectures. However, it is 8 on 32-bit architectures that do
not require 16-byte alignment from malloc to support predefined
types.
Compatibility Note: The C11 standard introduced max_align_t;
older compilers may lack the type.