mirror of
https://git.savannah.gnu.org/git/gnulib.git
synced 2025-08-16 01:22:18 +03:00
775dbeeb11
Problem reported by Joseph Myers in: https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00340.html * doc/posix-headers/stdalign.texi: Do not imply that C11 prohibits alignof(S) where S is a structure containing a flexible array member. The Gnulib substitute does not support this, but C11 does.
68 lines
2.4 KiB
Plaintext
68 lines
2.4 KiB
Plaintext
@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}. The Gnulib substitute does not support this extension.
|
|
@item
|
|
On most pre-C11 platforms, the operand of
|
|
@code{alignof}/@code{_Alignof} cannot be a structure type containing a
|
|
flexible array member.
|
|
@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 5.9 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
|