offsetof(3) Library Functions Manual offsetof(3)
NAME
offsetof - offset of a structure member
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stddef.h>
size_t offsetof(type, member);
DESCRIPTION
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 bound-
ary (i.e., it is a bit field).
RETURN VALUE
offsetof() returns the offset of the given member within the given
type, in units of bytes.
STANDARDS
POSIX.1-2001, POSIX.1-2008, C99.
EXAMPLES
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
Program source
#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=%zu; c=%zu; d=%zu a=%zu\n",
offsetof(struct s, i), offsetof(struct s, c),
offsetof(struct s, d), offsetof(struct s, a));
printf("sizeof(struct s)=%zu\n", sizeof(struct s));
exit(EXIT_SUCCESS);
}
Linux man-pages 6.03 2022-12-29 offsetof(3)
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