On FreeBSD 12.2/sparc64, in a testdir of gnulib, I see a compilation error
in most or all C++ compilation units. Such as this one:
In file included from /usr/include/stddef.h:41,
from ../gllib/stddef.h:80,
from ../config.h:8257,
from ../../gltests/test-limits-h-c++.cc:20:
/usr/include/sys/_types.h:107: error: expected `)' before '(' token
/usr/include/sys/_types.h:107: error: requested alignment is not a constant
/usr/include/sys/_types.h:109: error: expected `)' before '(' token
/usr/include/sys/_types.h:109: error: requested alignment is not a constant
The cause is that, after preprocessing, part of /usr/include/sys/_types.h
produces:
typedef struct {
long long __max_align1 __attribute__((__aligned__(offsetof
(__alignof_helper<long long>, __b))));
long double __max_align2 __attribute__((__aligned__(offsetof
(__alignof_helper<long double>, __b))));
} __max_align_t;
and this is invalid syntax, because 'offsetof' is not a defined macro at this
point.
We had a similar fix in C mode earlier:
2023-01-24 Bruno Haible <[email protected]>
alignasof, stdalign: Fix a compilation error on FreeBSD 12.0.
* m4/stdalign.m4 (gl_ALIGNASOF): In C mode, prefer __builtin_offsetof
over offsetof when possible, since __builtin_offsetof works also when
<stddef.h> has not been fully included yet.
The same thing needs to be done in C++ mode.
2023-08-21 Bruno Haible <[email protected]>
alignasof, stdalign: Fix a compilation error in C++ mode on FreeBSD 12.
* m4/stdalign.m4 (gl_ALIGNASOF): In C++ mode, prefer __builtin_offsetof
over offsetof when possible, since __builtin_offsetof works also when
<stddef.h> has not been fully included yet.
diff --git a/m4/stdalign.m4 b/m4/stdalign.m4
index 6a39ffe756..5880efb2eb 100644
--- a/m4/stdalign.m4
+++ b/m4/stdalign.m4
@@ -112,7 +112,11 @@ AC_DEFUN([gl_ALIGNASOF]
# define _Alignof(type) alignof (type)
# else
template <class __t> struct __alignof_helper { char __a; __t __b; };
-# define _Alignof(type) offsetof (__alignof_helper<type>, __b)
+# if (defined __GNUC__ && 4 <= __GNUC__) || defined __clang__
+# define _Alignof(type) __builtin_offsetof (__alignof_helper<type>, __b)
+# else
+# define _Alignof(type) offsetof (__alignof_helper<type>, __b)
+# endif
# define _GL_STDALIGN_NEEDS_STDDEF 1
# endif
# else
