offsetof — offset of a structure member
#include <stddef.h>
size_t offsetof( |
type, |
member); |
The macro offsetof() returns
the offset of the field member from the start of the
structure type.
This macro is useful because the sizes of the fields that compose a structure can vary across implementations, and compilers may insert different numbers of padding bytes between fields. Consequently, an element's offset is not necessarily given by the sum of the sizes of the previous elements.
A compiler error will result if member is not aligned to a byte
boundary (i.e., it is a bit field).
offsetof() returns the
offset of the given member within the given
type, in units of
bytes.
On a Linux/i386 system, when compiled using the default gcc(1) options, the program below produces the following output:
$ ./a.out offsets: i=0; c=4; d=8 a=16 sizeof(struct s)=16
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
struct s {
int i;
char c;
double d;
char a[];
};
/* Output is compiler dependent */
printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\n",
(long) offsetof(struct s, i),
(long) offsetof(struct s, c),
(long) offsetof(struct s, d),
(long) offsetof(struct s, a));
printf("sizeof(struct s)=%ld\n", (long) sizeof(struct s));
exit(EXIT_SUCCESS);
}