@node stdalign.h
@section @file{stdalign.h}

POSIX specification:@* Not in POSIX yet, but we expect it will be.
ISO C11 (latest free draft
@url{http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf})
sections 6.5.3.4, 6.7.5, 7.15.
C++11 (latest free draft
@url{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf})
section 18.10.

Gnulib module: stdalign

Portability problems fixed by Gnulib:
@itemize
@item
This header file is missing on most circa-2011 platforms.
@item
Clang 3.0's @code{<stdalign.h>} does not define @code{alignof}/@code{_Alignof}.
@item
The @code{alignof} and @code{_Alignof} macros return too large values for
the types @code{double} and @code{long long} in GCC 4.7.0.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
In ISO C11, the operand of @code{alignof}/@code{_Alignof} must be a
parenthesized type.  Recent versions of GCC support an extension in
which the operand can also be a unary expression, as with
@code{sizeof}.  We don't know of any compilers that support
@code{alignof}/@code{_Alignof} without also supporting this extension;
if you find one please let us know.
@item
@code{_Alignas} and @code{alignas} are not always supported;
on platforms lacking support, the
macro @code{__alignas_is_defined} is not defined.
Supported compilers include GCC, IBM C, Sun C 5.11 and later,
and MSVC 7.0 and later.
@item
Some compilers do not support alignment via
@code{alignas}/@code{_Alignas} of @code{auto} variables (i.e.,
variables on the stack).  They diagnose and ignore the alignment: Sun
C 5.11.
@item
Some linkers do not support operands of @code{_Alignas}/@code{alignas}
that are greater than 8: mingw.
@item
Some compilers require the operand of @code{_Alignas}/@code{alignas}
to be a single integer constant, not an expression: MSVC 7.0 through
at least 10.0.
@item
The Sun C 5.11 compiler sometimes mishandles the alignment of multiple
external variables that are declared close together with
@code{_Alignas}/@code{alignas}.  This compiler bug causes the Gnulib
module @code{stdalign-tests} to fail.  The Sun Studio Developer Bug
Report Review Team assigned the internal review ID 2125432 (dated
2011-11-01) to this issue.
@item
@code{<stdalign.h>} must be #included before @code{_Alignas} and
@code{_Alignof} can be used.
@item
You cannot assume that @code{_Alignas} and @code{_Alignof} are reserved words;
they might be macros.
@end itemize