http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54188
Bug #: 54188
Summary: Inconsistent __alignof__(long long)
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: keith.s.thomp...@gmail.com
The __alignof__ operator applied to a struct type with a single long
long member yields 4. This is inconsistent; a struct's alignment
should be at least as large as the alignment of any of its members.
I'm using gcc 4.7.0 on Ubuntu 12.04 x86.
gcc -v says:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/4.7/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.0-7ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --disable-bootstrap --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-gnu-unique-object --enable-plugin --enable-objc-gc
--enable-targets=all --disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu
--target=i686-linux-gnu
Thread model: posix
gcc version 4.7.0 (Ubuntu/Linaro 4.7.0-7ubuntu3)
Demo program (alignof_bug.c):
========================================
#include <stdio.h>
#include <stddef.h>
/*
* See <http://stackoverflow.com/q/11825081/827263>,
* posted by PiotrNycz
*/
/*
* The ALIGNOF macro yields the alignment in bytes of a given type,
* determined by how the compiler aligns a member of the type in
* a struct following a single char member.
*/
#define ALIGNOF(type) ((int)(offsetof(struct {char c; type t;}, t)))
struct wrapper {
long long ll;
};
int main(void) {
printf("__alignof__(long long) = %d\n",
(int)__alignof__(long long));
printf("__alignof__(struct wrapper) = %d\n",
(int)__alignof__(struct wrapper));
printf("ALIGNOF(long long) = %d\n",
(int)ALIGNOF(long long));
printf("ALIGNOF(struct wrapper) = %d\n",
(int)ALIGNOF(struct wrapper));
if (__alignof__(long long) > __alignof__(struct wrapper)) {
puts("Inconsistent __alignof__, long long vs. struct");
}
if (__alignof__(long long) != ALIGNOF(long long)) {
puts("Inconsistent alignment for long long");
}
return 0;
}
========================================
Output:
========================================
__alignof__(long long) = 8
__alignof__(struct wrapper) = 4
ALIGNOF(long long) = 4
ALIGNOF(struct wrapper) = 4
Inconsistent __alignof__, long long vs. struct
Inconsistent alignment for long long
========================================
A response to
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10360
suggests that __alignof__ needn't necessarily yield the same result
as the offset of a member within a struct. I'm not sure I agree
with that, but the main problem I'm reporting here refers to the
alignment of the struct type itself, compared to the alignment of
one of its members.
