qobject: Ensure base is at offset 0

All QObject types have the base QObject as their first field. This
allows the simplification of qobject_to().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180419150145.24795-2-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message paragraph on type casts dropped, to avoid giving the
impression type casting would be okay]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
index e02270757897335ecebbd558091adc1fb29ec647..5206ff9ee1908503cdb7b7e12b319d3d13b27f14 100644 (file)
--- a/include/qapi/qmp/qobject.h
+++ b/include/qapi/qmp/qobject.h
@@ -61,9 +61,8 @@ struct QObject {
 QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
                    "The QTYPE_CAST_TO_* list needs to be extended");
 
-#define qobject_to(type, obj) ({ \
-    QObject *_tmp = qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)); \
-    _tmp ? container_of(_tmp, type, base) : (type *)NULL; })
+#define qobject_to(type, obj)                                       \
+    ((type *)qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)))
 
 /* Initialize an object to default values */
 static inline void qobject_init(QObject *obj, QType type)

diff --git a/qobject/qobject.c b/qobject/qobject.c
index 23600aa1c1f07513075cd3a9e1ae3c6288015bfa..87649c5be5864b16109cb22fe09a5d27d62b52b9 100644 (file)
--- a/qobject/qobject.c
+++ b/qobject/qobject.c
@@ -16,6 +16,15 @@
 #include "qapi/qmp/qlist.h"
 #include "qapi/qmp/qstring.h"
 
+QEMU_BUILD_BUG_MSG(
+    offsetof(QNull, base) != 0 ||
+    offsetof(QNum, base) != 0 ||
+    offsetof(QString, base) != 0 ||
+    offsetof(QDict, base) != 0 ||
+    offsetof(QList, base) != 0 ||
+    offsetof(QBool, base) != 0,
+    "base qobject must be at offset 0");
+
 static void (*qdestroy[QTYPE__MAX])(QObject *) = {
     [QTYPE_NONE] = NULL,               /* No such object exists */
     [QTYPE_QNULL] = NULL,              /* qnull_ is indestructible */