Described restrictions for common HAL object methods.
Inheritance of HAL object is performed by composing a child structure of a
single parent structure located at offset 0 followed by new data members
and function pointers in the child structure.
For example,
struct child {
struct parent common;
int a_data_member;
void (*a_method)(struct child *c, int v);
};
HAL code assumes this layout when accessing child structures given a pointer
to a parent structure such that users write code like the following...
void child_method(struct *parent, int v) {
struct child * c = (struct child*)parent;
// do stuff with c
}
Code above will break if a member is added before "common" in "struct child".
This change adds comments that describe the restriction on the location of
parent HAL objects within a derived HAL object. HAL objects that already
have comments that describe the required location of parent objects are not
modified.
Change-Id: Ibe4300275286ef275b2097534c84f1029d761d87
diff --git a/include/hardware/hwcomposer.h b/include/hardware/hwcomposer.h
index afb4e99..049edea 100644
--- a/include/hardware/hwcomposer.h
+++ b/include/hardware/hwcomposer.h
@@ -471,10 +471,22 @@
/*****************************************************************************/
typedef struct hwc_module {
+ /**
+ * Common methods of the hardware composer module. This *must* be the first member of
+ * hwc_module as users of this structure will cast a hw_module_t to
+ * hwc_module pointer in contexts where it's known the hw_module_t references a
+ * hwc_module.
+ */
struct hw_module_t common;
} hwc_module_t;
typedef struct hwc_composer_device_1 {
+ /**
+ * Common methods of the hardware composer device. This *must* be the first member of
+ * hwc_composer_device_1 as users of this structure will cast a hw_device_t to
+ * hwc_composer_device_1 pointer in contexts where it's known the hw_device_t references a
+ * hwc_composer_device_1.
+ */
struct hw_device_t common;
/*