offsetof

Section: C Library Functions (3)
Updated: 2022-12-29
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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 boundary (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); }


 

Index

NAME
LIBRARY
SYNOPSIS
DESCRIPTION
RETURN VALUE
STANDARDS
EXAMPLES
Program source

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Time: 12:36:47 GMT, March 29, 2024