Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi>
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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 .
p 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. d -literal -offset indent (void)printf("%d\en", __alignof__(int)); .Ed
p 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. d -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.