sys: util: improve IS_EMPTY() implementation

The current implementation relies on preprocessor concatenation to
work. This makes it incompatible with any content which expansion
is not a valid preprocessor token such as strings, pointers, etc.
and therefore limits its usefulness. Replace it with an implementation
that can cope with all cases.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

Author: npitre

include/sys/util_internal.h

@@ -67,9 +67,23 @@
 #define __DEBRACKET(...) __VA_ARGS__
 
 /* Used by IS_EMPTY() */
-#define Z_IS_EMPTY_(...) Z_IS_EMPTY__(__VA_ARGS__)
-#define Z_IS_EMPTY__(a, ...) Z_IS_EMPTY___(_ZZ##a##ZZ0, __VA_ARGS__)
-#define Z_IS_EMPTY___(...) GET_ARG_N(3, __VA_ARGS__)
+/* reference: https://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/ */
+#define Z_HAS_COMMA(...) \
+	NUM_VA_ARGS_LESS_1_IMPL(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, \
+	 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
+	 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, \
+	 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0)
+#define Z_TRIGGER_PARENTHESIS_(...) ,
+#define Z_IS_EMPTY_(...) \
+	Z_IS_EMPTY__( \
+		Z_HAS_COMMA(__VA_ARGS__), \
+		Z_HAS_COMMA(Z_TRIGGER_PARENTHESIS_ __VA_ARGS__), \
+		Z_HAS_COMMA(__VA_ARGS__ (/*empty*/)), \
+		Z_HAS_COMMA(Z_TRIGGER_PARENTHESIS_ __VA_ARGS__ (/*empty*/)))
+#define Z_CAT5(_0, _1, _2, _3, _4) _0 ## _1 ## _2 ## _3 ## _4
+#define Z_IS_EMPTY__(_0, _1, _2, _3) \
+	Z_HAS_COMMA(Z_CAT5(Z_IS_EMPTY_CASE_, _0, _1, _2, _3))
+#define Z_IS_EMPTY_CASE_0001 ,
 
 /* Used by LIST_DROP_EMPTY() */
 /* Adding ',' after each element would add empty element at the end of

include/sys/util_macro.h

@@ -210,15 +210,6 @@ extern "C" {
  * This macro may be used with COND_CODE_1() and COND_CODE_0() while
  * processing <tt>__VA_ARGS__</tt> to avoid processing empty arguments.
  *
- * Note that this macro is intended to check macro names that evaluate
- * to replacement lists being empty or containing numbers or macro name
- * like tokens.
- *
- * @note Not all arguments are accepted by this macro and compilation will fail
- *	 if argument cannot be concatenated with literal constant. That will
- *	 happen if argument does not start with letter or number. Example
- *	 arguments that will fail during compilation: .arg, (arg), "arg", {arg}.
- *
  * Example:
  *
  *	#define EMPTY
@@ -234,9 +225,9 @@ extern "C" {
  * In above examples, the invocations of IS_EMPTY(...) return @p true,
  * @p false, and @p true; @p some_conditional_code is included.
  *
- * @param a macro to check for emptiness
+ * @param ... macro to check for emptiness (may be <tt>__VA_ARGS__</tt>)
  */
-#define IS_EMPTY(a) Z_IS_EMPTY_(a, 1, 0,)
+#define IS_EMPTY(...) Z_IS_EMPTY_(__VA_ARGS__)
 
 /**
  * @brief Remove empty arguments from list.

tests/unit/util/test.inc

@@ -354,6 +354,10 @@ static void test_IS_EMPTY(void)
 		     "Expected to be empty");
 	zassert_false(IS_EMPTY(test_IS_EMPTY_NOT_EMPTY),
 		      "Expected to be non-empty");
+	zassert_false(IS_EMPTY("string"),
+		      "Expected to be non-empty");
+	zassert_false(IS_EMPTY(&test_IS_EMPTY),
+		      "Expected to be non-empty");
 }
 
 static void test_LIST_DROP_EMPTY(void)