NAME
offsetof —
offset of a structure
member
SYNOPSIS
#include <stddef.h>
size_t
offsetof(
type,
member);
DESCRIPTION
The
offsetof() macro expands to an integer constant expression
of type
size_t and yields the offset, in bytes, of the
field
member from the start of the structure
type.
A compiler error will result if
member is not aligned to a
byte boundary (i.e. it is a bit-field).
EXAMPLES
Regardless of the architecture and the ABI, the following example prints the
value zero for the variable
x.
struct example {
double x;
int y;
char z;
};
size_t x, y, z;
x = offsetof(struct example, x);
y = offsetof(struct example, y);
z = offsetof(struct example, z);
(void)printf("%zu %zu %zu\n", x, y, z);
SEE ALSO
__alignof__(3),
stddef(3),
typeof(3)
STANDARDS
The
offsetof() macro conforms to
ANSI
X3.159-1989 (“ANSI C89”).