offsetof

Header: <stddef.h>

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.

# Declarations

#define offsetof(type, member) /*implementation-defined*/

# 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 specifying a new type containing an unparenthesized comma in offsetof is undefined behavior, such usage is generally not supported even in earlier modes: offsetof(struct Foo { int a, b; }, a) generally fails to compile.

typeof can be used to avoid the bad effect of commas in the definition of a new type, e.g. offsetof(typeof(struct { int i, j; }), i) is well-defined.

# Example

#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));
}

# Defect reports

# See also

• size_t
• sizeof
• C++ documentation