man3/offsetof.3

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  26 .TH OFFSETOF 3 2006-05-23 GNU "Linux Programmer's Manual"
  27 .SH NAME
  28 offsetof \- offset of a structure member
  29 .SH SYNOPSIS
  30 \fB#include <stddef.h>
  31
  32 \fBsize_t offsetof(\fItype\fP, \fPmember\fP);
  33 .SH DESCRIPTION
  34 The macro
  35 .BR offsetof ()
  36 returns the offset of the field
  37 \fImember\fP from the start of the structure \fItype\fP.
  38
  39 This macro is useful because the sizes of the fields that compose
  40 a structure can vary across implementations,
  41 and compilers may insert different numbers of padding
  42 bytes between fields.
  43 Consequently, an element's offset is not necessarily
  44 given by the sum of the sizes of the previous elements.
  45
  46 A compiler error will result if
  47 \fImember\fP is not aligned to a byte boundary
  48 (i.e., it is a bit field).
  49 .SH "RETURN VALUE"
  50 .BR offsetof ()
  51 returns the offset of the given element within the
  52 given type, in units of bytes.
  53 .SH EXAMPLE
  54 On a Linux/x86 system, when compiled using the default
  55 .BR gcc (1)
  56 options, the program below produces the following output:
  57 .fi
  58
  59     $ ./a.out
  60     offsets: i=0; c=4; d=8 a=16
  61     sizeof(struct s)=16
  62
  63 .nf
  64 .nf
  65 #include <stddef.h>
  66 #include <stdio.h>
  67 #include <stdlib.h>
  68
  69 int
  70 main(void)
  71 {
  72     struct s {
  73         int i;
  74         char c;
  75         double d;
  76         char a[];
  77     };
  78
  79     /* Output is compiler dependent */
  80
  81     printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\\n",
  82             (long) offsetof(struct s, i),
  83             (long) offsetof(struct s, c),
  84             (long) offsetof(struct s, d),
  85             (long) offsetof(struct s, a));
  86     printf("sizeof(struct s)=%ld\\n", (long) sizeof(struct s));
  87
  88     exit(EXIT_SUCCESS);
  89 }
  90 .fi
  91 .SH "CONFORMING TO"
  92 C89, C99, POSIX.1-2001.
  93 .\" .SH SEE ALSO
  94 .\" FIXME . When one day readdir_r(3) is documented, it should have
  95 .\" a SEE ALSO that refers to this page.