man3/offsetof.3

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  24 .\"   /usr/lib/gcc/i486-linux-gnu/4.1.1/include/stddef.h
  25 .\"   glibc-doc
  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 \fBoffsetof\fP() returns the offset of the field
  35 \fImember\fP from the start of the structure \fItype\fP.
  36
  37 This macro is useful because the sizes of the fields that compose
  38 a structure can vary across implementations,
  39 and compilers may insert different numbers of padding
  40 bytes between fields.
  41 Consequently, an element's offset is not necessarily
  42 given by the sum of the sizes of the previous elements.
  43
  44 A compiler error will result if
  45 \fImember\fP is not aligned to a byte boundary
  46 (i.e., it is a bit field).
  47 .SH "RETURN VALUE"
  48 \fBoffsetof\fP() returns the offset of the given element within the
  49 given type, in units of bytes.
  50 .SH EXAMPLE
  51 On a Linux/x86 system, when compiled using the default \fBgcc\fP(1)
  52 options, the program below produces the following output:
  53 .fi
  54
  55     $ ./a.out
  56     offsets: i=0; c=4; d=8 a=16
  57     sizeof(struct s)=16
  58
  59 .nf
  60 .nf
  61 #include <stddef.h>
  62 #include <stdio.h>
  63 #include <stdlib.h>
  64
  65 int
  66 main(void)
  67 {
  68     struct s {
  69         int i;
  70         char c;
  71         double d;
  72         char a[];
  73     };
  74
  75     /* Output is compiler dependent */
  76
  77     printf("offsets: i=%ld; c=%ld; d=%ld a=%ld\\n",
  78             (long) offsetof(struct s, i),
  79             (long) offsetof(struct s, c),
  80             (long) offsetof(struct s, d),
  81             (long) offsetof(struct s, a));
  82     printf("sizeof(struct s)=%ld\\n", (long) sizeof(struct s));
  83
  84     exit(EXIT_SUCCESS);
  85 }
  86 .fi
  87 .SH "CONFORMING TO"
  88 C89, C99, POSIX.1-2001.
  89 .\" .SH SEE ALSO
  90 .\" FIXME . When one day readdir_r(3) is documented, it should have
  91 .\" a SEE ALSO that refers to this page.