# <stddef.h>

<head><title>&lt;stddef.h&gt;</title></head>

# `<stddef.h>`

* * *

Include the standard header **`<stddef.h>`**
to define several types and macros that are
of general use throughout the program. The
standard header `<stddef.h>` is available even in a
[freestanding implementation](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_lib_over.html#freestanding%20implementation).

        /* MACROS */
    #define NULL <either 0, 0L, or (void *)0> [0 in C++]
    #define offsetof(s-type, mbr) <size_t constant expression>
    
        /* TYPES */
    typedef si-type ptrdiff_t;
    typedef ui-type size_t;
    typedef i-type wchar_t; [keyword in C++]Copy to clipboard

## `NULL`

    #define NULL <either 0, 0L, or (void *)0> [0 in C++]Copy to clipboard

The macro yields a null pointer constant that is usable as an
[address constant expression](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_express.html#address%20constant%20expression).

## `offsetof`

    #define offsetof(s-type, mbr) <size_t constant expression>Copy to clipboard

The macro yields the offset in bytes, of type
[`size_t`](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_stddef.html#size_t),
of member `mbr` from
the beginning of structure type `s-type`,
where for `X` of type `s-type`,
`&X.mbr` is an
[address constant expression](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_express.html#address%20constant%20expression).

## `ptrdiff_t`

    typedef si-type ptrdiff_t;Copy to clipboard

The type is the signed integer type `si-type`
of an object that you declare to store the result of
subtracting two pointers.

## `size_t`

    typedef ui-type size_t;Copy to clipboard

The type is the unsigned integer type `ui-type`
of an object that you declare to store the result of the
[*sizeof*](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_express.html#sizeof%20operator) operator.

## `wchar_t`

    typedef i-type wchar_t; [keyword in C++]Copy to clipboard

The type is the integer type `i-type` of a
[wide-character constant](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_charset.html#wide-character%20constant), such as `L'X'`.
You declare an object of type `wchar_t` to hold a
[wide character](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_charset.html#Wide-Character%20Encoding).

* * *

See also the
**[Table of Contents](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_index.html#Table%20of%20Contents)** and the
**[Index](https://docs.qualcomm.com/doc/80-N3040-23/topic/_c_index.html)**.

*[Copyright](https://docs.qualcomm.com/doc/80-N3040-23/topic/c_crit_pjp.html) © 1992-2006by P.J. Plauger and Jim Brodie. All rights reserved.*

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Last Published: Oct 06, 2026

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