gnulib/doc/posix-headers/stdalign.texi

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

ISO C23 specification:@* @url{https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf} sections 6.5.3.4, 6.7.5, 7.15.

C++11 specification:@* @url{https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf} section 18.10.

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdalign.h.html}

Gnulib module: alignasof
@mindex alignasof

Portability problems fixed by Gnulib:
@itemize
@item
On older C platforms @code{<stdalign.h>} must be included before using
@code{alignas} or @code{alignof}.  For example, GCC versions before 13 do not
support these keywords, which were standardized by C23.
On C23 and later platforms, @code{<stdalign.h>} has no effect and need
not be included.
@mindex stdalign-h
(Gnulib-using code should not include
@code{<stdalign.h>} without also employing Gnulib's now-deprecated
@code{stdalign-h} module.)
@item
This header file is missing on many platforms:
FreeBSD 6.4, NetBSD 7.1, OpenBSD 7.5, Minix 3.3.0, AIX 7.1, HP-UX 11.31, Solaris 11.3, mingw, MSVC 14, Android 9.0.
@item
Clang 3.0's @code{<stdalign.h>} does not define @code{alignof}.
@item
The @code{alignof} macro returns too large values for
the types @code{double} and @code{long long} in GCC 4.7.0.
@item
Older C platforms might not support the obsolescent
@code{_Alignas} and @code{_Alignof} keywords or macros.
This portability problem should not matter with code using this module,
as such code should use @code{alignas} and @code{alignof} instead.
@item
In C11 and C17, @code{<stdalign.h>} defines the macros
@code{__alignas_is_defined} and
@code{__alignof_is_defined} to 1.
In C23, these macros are not defined.
This portability problem should not matter with code using Gnulib's
@code{alignasof} module, as such code should use @code{alignas} and
@code{alignof} without checking these two macros.  (Gnulib's
now-deprecated @code{stdalign-h} module defines these two macros.)
@item
@c https://github.com/llvm/llvm-project/issues/81472
Clang 17 does not support an @code{alignas} attribute in the
specifier-qualifier position of a member declaration.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
In C11 and later, the operand of @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}.  The Gnulib substitute does not support this extension.
@item
On most pre-C11 platforms, the operand of
@code{alignof} cannot be a structure type containing a
flexible array member.
@item
The @code{alignas} keyword or macro is not always supported.
Supported compilers include any compiler supporting C11 or later,
which includes GCC, IBM C, Sun C 5.9 and later,
and MSVC 7.0 and later.
@item
Some compilers do not support alignment via
@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}
that are greater than 8: mingw.
@item
Some compilers require the operand of @code{alignas}
to be a single integer constant, not an expression: MSVC 7.0 through
at least 10.0.
@item
The Sun C 5.13 (2014) compiler sometimes mishandles the alignment of multiple
external variables that are declared close together with
@code{alignas}.  The bug is fixed in Sun C 5.15, also known as Oracle
Developer Studio 12.6 (2017).
@item
You cannot assume that @code{alignas} and @code{alignof} are reserved words;
they might be macros.
@end itemize