ARRAY_SIZE: check for type

We can use a gcc extension to ensure that ARRAY_SIZE() is handed an array,
not a pointer.  This is especially important when code is changed from a
fixed array to a pointer.  I assume the Intel compiler doesn't support
__builtin_types_compatible_p.

[jdike@addtoit.com: uml: update UML definition of ARRAY_SIZE]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/arch/um/include/user_util.h b/arch/um/include/user_util.h
index 73ffea1829f..b52bc408483 100644
--- a/arch/um/include/user_util.h
+++ b/arch/um/include/user_util.h
@@ -8,8 +8,19 @@
 
 #include "sysdep/ptrace.h"
 
-/* Copied from kernel.h */
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+/* Copied from kernel.h and compiler-gcc.h */
+
+/* Force a compilation error if condition is true, but also produce a
+   result (of value 0 and type size_t), so the expression can be used
+   e.g. in a structure initializer (or where-ever else comma expressions
+   aren't permitted). */
+#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
+
+/* &a[0] degrades to a pointer: a different type from an array */
+#define __must_be_array(a) \
+  BUILD_BUG_ON_ZERO(__builtin_types_compatible_p(typeof(a), typeof(&a[0])))
+
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
 
 #define CATCH_EINTR(expr) while ((errno = 0, ((expr) < 0)) && (errno == EINTR))