all: add C99's "bool" define

https://mail.gnome.org/archives/networkmanager-list/2015-November/msg00036.html

include/nm-default.h

@@ -70,4 +70,51 @@
 
 /*****************************************************************************/
 
+/**
+ * The boolean type _Bool is C99 while we mostly stick to C89. However, _Bool is too
+ * convinient to miss and is effectively available in gcc and clang. So, just use it.
+ *
+ * Usually, one would include "stdbool.h" to get the "bool" define which aliases
+ * _Bool. We provide this define here, because we want to make use of it anywhere.
+ * (also, stdbool.h is again C99).
+ *
+ * Using _Bool has advantages over gboolean:
+ *
+ * - commonly _Bool is one byte large, instead of gboolean's 4 bytes (because gboolean
+ *   is a typedef for gint). Especially when having boolean fields in a struct, we can
+ *   thereby easily save some space.
+ *
+ * - _Bool type guarantees that two "true" expressions compare equal. E.g. the follwing
+ *   will not work:
+ *        gboolean v1 = 1;
+ *        gboolean v2 = 2;
+ *        g_assert_cmpint (v1, ==, v2); // will fail
+ *   For that, we often to use !! to coerce gboolean values to 0 or 1:
+ *        g_assert_cmpint (!!v2, ==, TRUE);
+ *   With _Bool type, this will be handled properly by the compiler.
+ *
+ * - For structs, we might want to safe even more space and use bitfields:
+ *       struct s1 {
+ *           gboolean v1:1;
+ *       };
+ *   But the problem here is that gboolean is signed, so that
+ *   v1 will be either 0 or -1 (not 1, TRUE). Thus, the following
+ *   fails:
+ *      struct s1 s = { .v1 = TRUE, };
+ *      g_assert_cmpint (s1.v1, ==, TRUE);
+ *   It will however work just fine with bool/_Bool while retaining the
+ *   notion of having a boolean value.
+ *
+ * Also, add the defines for "true" and "false". Those are nicely highlighted by the editor
+ * as special types, contrary to glib's "TRUE"/"FALSE".
+ */
+
+#ifndef bool
+#define bool _Bool
+#define true    1
+#define false   0
+#endif
+
+/*****************************************************************************/
+
 #endif /* __NM_DEFAULT_H__ */

libnm-core/tests/test-general.c

@@ -65,6 +65,14 @@
 #include "nm-test-utils.h"
 #include "test-general-enums.h"
 
+/* When passing a "bool" typed argument to a variadic function that
+ * expects a gboolean, the compiler will promote the integer type
+ * to have at least size (int). That way:
+ *   g_object_set (obj, PROP_BOOL, bool_val, NULL);
+ * will just work correctly. */
+G_STATIC_ASSERT (sizeof (gboolean) == sizeof (int));
+G_STATIC_ASSERT (sizeof (bool) <= sizeof (int));
+
 static void
 vpn_check_func (const char *key, const char *value, gpointer user_data)
 {