offsetof

 

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offsetof

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Defined in header <stddef.h>
#define offsetof(type, member) /*implementation-defined*/

The macro offsetof expands to an integer constant expression of type size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified subobject, including padding if any.

Given an object o of type type with static storage duration, &(o.member) shall be an address constant expression and point to a subobject of o. Otherwise, the behavior is undefined.

If a new type is defined in type, the behavior is undefined.

(since C23)

Notes

If offsetof is applied to a bit-field member, the behavior is undefined, because the address of a bit-field cannot be taken.

member is not restricted to a direct member. It can denote a subobject of a given member, such as an element of an array member.

Even though it is specified in C23 that defining a new type in offsetof is undefined behavior, such usage is only partially supported by implementations even in earlier modes: offsetof(struct Foo { int a; }, a) is usually supported, but offsetof(struct Foo { int a, b; }, a) is not because of the comma in the definition of struct Foo.

Example

Run this code

#include <stdio.h>
#include <stddef.h>
 
struct S {
    char c;
    double d;
};
 
int main(void)
{
    printf("the first element is at offset %zu\n", offsetof(struct S, c));
    printf("the double is at offset %zu\n", offsetof(struct S, d));
}

Possible output:

the first element is at offset 0
the double is at offset 8

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

DR	Applied to	Behavior as published	Correct behavior
DR 496	C89	only structs and struct members were mentioned	unions and other subobjects are also supported

See also

size_t	unsigned integer type returned by the sizeof operator (typedef)
C++ documentation for offsetof