.\" $NetBSD: __alignof__.3,v 1.5 2011/04/14 06:56:28 wiz Exp $
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.Dd December 20, 2010
.Dt __ALIGNOF__ 3
.Os
.Sh NAME
.Nm __alignof__
.Nd GNU extension for alignment of an object
.Sh SYNOPSIS
.Ft int
.Fn __alignof__ "void x"
.Sh DESCRIPTION
The
.Fn __alignof__
operator returns the alignment of its operand.
The operand can be a type or an expression.
If the operand is a
.Sq lvalue ,
the return value represents the required alignment of the underlying type,
not the actual alignment of the specified
.Sq lvalue .
.Pp
The returned value is specific to the architecture and the
.Tn ABI .
If the architecture does not impose strict alignment requirements,
.Fn __alignof__
returns the minimum required alignment.
If
.Xr __aligned 3
is used to increase the alignment,
.Fn __alignof__
returns the specified alignment.
.Sh EXAMPLES
The syntax is comparable to the
.Fn sizeof
operator.
If the architecture aligns integers along 32-bit address boundaries,
the following should print the value 4.
.Bd -literal -offset indent
(void)printf("%d\en", __alignof__(int));
.Ed
.Pp
On the other hand, the following example should print the value 1,
even though this is unlikely to be the actual alignment of the
structure member.
.Bd -literal -offset indent
struct align {
	int  x;
	char y;
} a;

(void)printf("%d\en", __alignof__(a.y));
.Ed
.Sh SEE ALSO
.Xr gcc 1 ,
.Xr attribute 3 ,
.Xr offsetof 3 ,
.Xr typeof 3
.Sh CAVEATS
This is a non-standard, compiler-specific extension.