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c593e834e1
This splits off support for the C23 keywords alignas and alignof, from the now-deprecated stdalign module. The latter now merely provides C11 support. * MODULES.html.sh, NEWS, doc/gnulib.texi: * doc/posix-headers/stdalign.texi: Document the change. * lib/stdalign.in.h: Remove most of the definitions (which are now supplied by the alignasof module), leaving only __alignas_is_defined and __alignof_is_defined. * modules/alignasof, modules/alignasof-tests: New files. * m4/stdalign.m4 (gl_ALIGNASOF): New macro, with most of the contents of the old gl_STDALIGN_H. Do not define __alignas_is_defined or __alignof_is_defined. (gl_STDALIGN_H): Rely on gl_ALIGNASOF for most of the work. * modules/alignalloc, modules/alignof, modules/argp: * modules/crypto/md4-buffer, modules/crypto/md5-buffer: * modules/crypto/sha1-buffer, modules/crypto/sha256-buffer: * modules/crypto/sha512-buffer, modules/crypto/sm3-buffer: * modules/fts, modules/rawmemchr, modules/relocatable-prog-wrapper: * modules/stddef-tests, modules/sys_socket: Depend on alignasof, not stdalign. * modules/stdalign: Deprecate. Depend on alignasof. * modules/stdalign-tests: Move most contents to the new module alignasof-tests, and depend on that.
87 lines
3.3 KiB
Plaintext
87 lines
3.3 KiB
Plaintext
@node stdalign.h
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@section @file{stdalign.h}
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POSIX specification:@* Not in POSIX yet, but we expect it will be,
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at least temporarily until it becomes obsolete due to its phasing
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out starting in C23.
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ISO C23 (latest free draft
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@url{http://www.open-std.org/jtc1/sc22/wg14/www/docs/n3047.pdf})
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sections 6.5.3.4, 6.7.5, 7.15.
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C++11 (latest free draft
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@url{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf})
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section 18.10.
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Gnulib module: alignasof
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Portability problems fixed by Gnulib:
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@itemize
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@item
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On older C platforms @code{<stdalign.h>} must be included before using
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@code{alignas} or @code{alignof}. For example, GCC versions before 13 do not
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support these keywords, which were standardized by C23.
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On C23 and later platforms, @code{<stdalign.h>} has no effect and need
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not be included. (Gnulib-using code should not include
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@code{<stdalign.h>} without also employing Gnulib's now-deprecated
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@code{stdalign} module.)
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@item
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This header file is missing on many platforms:
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FreeBSD 6.4, NetBSD 7.1, OpenBSD 6.7, Minix 3.3.0, AIX 7.1, HP-UX 11.31, IRIX 6.5, Solaris 11.3, mingw, MSVC 14, Android 9.0.
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@item
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Clang 3.0's @code{<stdalign.h>} does not define @code{alignof}.
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@item
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The @code{alignof} macro returns too large values for
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the types @code{double} and @code{long long} in GCC 4.7.0.
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@item
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Older C platforms might not support the obsolescent
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@code{_Alignas} and @code{_Alignof} keywords or macros.
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This portability problem should not matter with code using this module,
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as such code should use @code{alignas} and @code{alignof} instead.
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@item
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In C11 and C17, @code{<stdalign.h>} defines the macros
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@code{__alignas_is_defined} and
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@code{__alignof_is_defined} to 1.
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In C23, these macros are not defined.
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This portability problem should not matter with code using Gnulib's
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@code{alignasof} module, as such code should use @code{alignas} and
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@code{alignof} without checking these two macros. (Gnulib's
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now-deprecated @code{stdalign} module defines these two macros.)
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@end itemize
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Portability problems not fixed by Gnulib:
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@itemize
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@item
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In C11 and later, the operand of @code{alignof} must be a
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parenthesized type. Recent versions of GCC support an extension in
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which the operand can also be a unary expression, as with
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@code{sizeof}. The Gnulib substitute does not support this extension.
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@item
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On most pre-C11 platforms, the operand of
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@code{alignof} cannot be a structure type containing a
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flexible array member.
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@item
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The @code{alignas} keyword or macro is not always supported.
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Supported compilers include any compiler supporting C11 or later,
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which includes GCC, IBM C, Sun C 5.9 and later,
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and MSVC 7.0 and later.
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@item
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Some compilers do not support alignment via
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@code{alignas} of @code{auto} variables (i.e.,
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variables on the stack). They diagnose and ignore the alignment: Sun
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C 5.11.
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@item
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Some linkers do not support operands of @code{alignas}
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that are greater than 8: mingw.
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@item
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Some compilers require the operand of @code{alignas}
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to be a single integer constant, not an expression: MSVC 7.0 through
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at least 10.0.
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@item
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The Sun C 5.13 (2014) compiler sometimes mishandles the alignment of multiple
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external variables that are declared close together with
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@code{alignas}. The bug is fixed in Sun C 5.15, also known as Oracle
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Developer Studio 12.6 (2017).
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@item
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You cannot assume that @code{alignas} and @code{alignof} are reserved words;
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they might be macros.
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@end itemize
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