# offsetof

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< c‎ | types

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)

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