8.44 stdalign.h

POSIX specification:
Not in POSIX yet, but we expect it will be. ISO C11 (latest free draft 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 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf) section 18.10.

Gnulib module: stdalign

Portability problems fixed by Gnulib:

• This header file is missing on most circa-2011 platforms.
• Clang 3.0’s <stdalign.h> does not define alignof/_Alignof.
• The alignof and _Alignof macros return too large values for the types double and long long in GCC 4.7.0.

Portability problems not fixed by Gnulib:

• In ISO C11, the operand of alignof/_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 sizeof. The Gnulib substitute does not support this extension.
• In ISO C11, the operand of alignof/_Alignof must be a complete type. Recent versions of GCC support an extension in which the operand can also be structure type containing a flexible array member. The Gnulib substitute does not support this extension.
• _Alignas and alignas are not always supported; on platforms lacking support, the macro __alignas_is_defined is not defined. Supported compilers include GCC, IBM C, Sun C 5.9 and later, and MSVC 7.0 and later.
• Some compilers do not support alignment via alignas/_Alignas of auto variables (i.e., variables on the stack). They diagnose and ignore the alignment: Sun C 5.11.
• Some linkers do not support operands of _Alignas/alignas that are greater than 8: mingw.
• Some compilers require the operand of _Alignas/alignas to be a single integer constant, not an expression: MSVC 7.0 through at least 10.0.
• The Sun C 5.11 compiler sometimes mishandles the alignment of multiple external variables that are declared close together with _Alignas/alignas. This compiler bug causes the Gnulib module 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.
• <stdalign.h> must be #included before _Alignas and _Alignof can be used.
• You cannot assume that _Alignas and _Alignof are reserved words; they might be macros.