libfuse: don't force -D_FILE_OFFSET_BITS=64 in pkgconfig file.

FUSE_CFLAGS defines -D_FILE_OFFSET_BITS=64.  There are three problems
with this:

(1) A larger program using libfuse might have modules compiled with
and without FUSE_CFLAGS, which, if LFS is not enabled and the platform
is 32 bit, would result in a fatal mix of 32 and 64 bit off_t.  (This
would, of course, be a bug, but I think there is a better way to
detect this -- see below)

(2) Programs may need to be adjusted to support LFS.  It's the
intention of the LFS standard that the _programmer_ enables LFS once
the program has been checked/adjusted.

(3) _FILE_OFFSET_BITS does not need to be defined at all on 64 bit
Linux.  64 bit off_t is the default there.

So I think it's better not to force -D_FILE_OFFSET_BITS=64, and
because of (3) I also think you shouldn't test for it.

However off_t must still be 64 bits, so how to enforce that?  C1X will
define static assertions[1], and these can be used to check the size
of off_t.

Not all compilers support static assertions yet, although several do.
Therefore I have surrounded the static assertion with a conservative
check that the compiler is GCC >= 4.6.  In the long run, this test can
be removed and you can just use 'static_assert'.

1 files changed, 14 insertions, 5 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index 78d3ce4..af16203 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -27,11 +27,6 @@
 #define FUSE_MAKE_VERSION(maj, min)  ((maj) * 10 + (min))
 #define FUSE_VERSION FUSE_MAKE_VERSION(FUSE_MAJOR_VERSION, FUSE_MINOR_VERSION)
 
-/* This interface uses 64 bit off_t */
-#if _FILE_OFFSET_BITS != 64
-#error Please add -D_FILE_OFFSET_BITS=64 to your compile flags!
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -483,4 +478,18 @@ void fuse_remove_signal_handlers(struct fuse_session *se);
 }
 #endif
 
+
+/*
+ * This interface uses 64 bit off_t.
+ *
+ * On 32bit systems please add -D_FILE_OFFSET_BITS=64 to your compile flags!
+ */
+
+#if defined(__GNUC__) && (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 6) && !defined __cplusplus
+_Static_assert(sizeof(off_t) == 8, "fuse: off_t must be 64bit");
+#else
+struct _fuse_off_t_must_be_64bit_dummy_struct \
+	{ unsigned _fuse_off_t_must_be_64bit:((sizeof(off_t) == 8) ? 1 : -1); };
+#endif
+
 #endif /* _FUSE_COMMON_H_ */