stddef.h File Reference

Standard definitions for C and C++. More...

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Macros
#define	NULL   ((void *) 0)
	Null pointer constant.
#define	offsetof(type, field)   ((size_t) (&((type *) 0)->field))
	Get the offset of a member within a structure.
#define	container_of(ptr, type, member)
	Get a pointer to the container structure from a pointer to a member.
#define	likely(expr)   (!!(expr))
	Likely expression hint for branch prediction.
#define	unlikely(expr)   (!!(expr))
#define	min(a, b)   (((a) < (b)) ? (a) : (b))
	Minimum of two values.
#define	max(a, b)   (((a) > (b)) ? (a) : (b))
	Maximum of two values.
#define	clamp(v, a, b)   min(max(a, v), b)
	Clamp a value between a minimum and maximum.
#define	ifloor(x)   ((x) > 0 ? (int) (x) : (int) ((x) -0.9999999999))
	Integer floor of a floating-point value.
#define	iround(x)   ((x) > 0 ? (int) ((x) + 0.5) : (int) ((x) -0.5))
	Integer rounding of a floating-point value.
#define	iceil(x)   ((x) > 0 ? (int) ((x) + 0.9999999999) : (int) (x))
	Integer ceiling of a floating-point value.
#define	idiv255(x)   ((((int) (x) + 1) * 257) >> 16)
	Divide an integer by 255 efficiently.

Detailed Description

Standard definitions for C and C++.

This file contains several macros and definitions used in both C and C++ code. It defines types, utility macros, and compiler-specific optimizations for portability across different compilers.

Macro Definition Documentation

clamp

#define clamp	(		v,
			a,
			b
	)		min(max(a, v), b)

Clamp a value between a minimum and maximum.

This macro ensures that the value v is between a and b. If v is smaller than a, it returns a, and if v is greater than b, it returns b.

Parameters

v	Value to be clamped.
a	Minimum bound.
b	Maximum bound.

Returns

The clamped value.

container_of

#define container_of	(		ptr,
			type,
			member
	)

Value:

    ({                                                       \
        const typeof(((type *) 0)->member) *__mptr = (ptr);  \
        (type *) ((char *) __mptr - offsetof(type, member)); \
    })

Get a pointer to the container structure from a pointer to a member.

This macro computes the address of the container structure from a pointer to a member within the structure. This is useful for traversing linked data structures like lists or trees.

Parameters

ptr	Pointer to the member.
type	Type of the structure containing the member.
member	Name of the member in the structure.

Returns

Pointer to the container structure.

iceil

#define iceil

(

x

)

((x) > 0 ? (int) ((x) + 0.9999999999) : (int) (x))

Integer ceiling of a floating-point value.

Computes the ceiling of a floating-point value x, rounding up to the nearest integer.

Parameters

x	The floating-point value.

Returns

The smallest integer greater than or equal to x.

idiv255

#define idiv255

(

x

)

((((int) (x) + 1) * 257) >> 16)

Divide an integer by 255 efficiently.

Performs division of an integer by 255, with an efficient algorithm using bit shifts. This is useful for applications where division by 255 is common, such as image processing.

Parameters

x	The integer value.

Returns

The result of dividing x by 255.

ifloor

#define ifloor

(

x

)

((x) > 0 ? (int) (x) : (int) ((x) -0.9999999999))

Integer floor of a floating-point value.

Computes the integer floor of a floating-point value x. If x is positive, it truncates towards zero; otherwise, it rounds down to the nearest integer.

Parameters

x	The floating-point value.

Returns

The integer floor of x.

iround

#define iround

(

x

)

((x) > 0 ? (int) ((x) + 0.5) : (int) ((x) -0.5))

Integer rounding of a floating-point value.

Rounds a floating-point value x to the nearest integer. Rounds up if x is positive and 0.5 or more, and rounds down otherwise.

Parameters

x	The floating-point value.

Returns

The nearest integer to x.

likely

#define likely

(

expr

)

(!!(expr))

Likely expression hint for branch prediction.

Provides a hint to the compiler that the given expression is likely to be true. This helps the compiler optimize the code, improving performance in certain cases.

Parameters

expr	Expression that is likely true.

Returns

The expression itself.

max

#define max	(		a,
			b
	)		(((a) > (b)) ? (a) : (b))

Maximum of two values.

Returns the larger of two values, a and b.

Parameters

a	First value.
b	Second value.

Returns

The larger value.

min

#define min	(		a,
			b
	)		(((a) < (b)) ? (a) : (b))

Minimum of two values.

Returns the smaller of two values, a and b.

Parameters

a	First value.
b	Second value.

Returns

The smaller value.

NULL

#define NULL   ((void *) 0)

Null pointer constant.

Defines the value of NULL. In C++, NULL is defined as 0, while in C, it is defined as a void pointer of value 0. The NULL macro can be used to represent null pointers in both languages.

offsetof

#define offsetof	(		type,
			field
	)		((size_t) (&((type *) 0)->field))

Get the offset of a member within a structure.

Returns the byte offset of a given member within a structure. This is useful when working with low-level memory manipulation or in custom memory allocation schemes.

Parameters

type	The type of the structure.
member	The member of the structure whose offset is to be calculated.

Returns

The offset of the member in bytes.

unlikely

#define unlikely

(

expr

)

(!!(expr))