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

1 files changed, 6 insertions, 0 deletions

diff --git a/include/hardware/audio_policy.h b/include/hardware/audio_policy.h
index cbaa31d..0e88361 100644
--- a/include/hardware/audio_policy.h
+++ b/include/hardware/audio_policy.h
@@ -423,6 +423,12 @@ typedef struct audio_policy_module {
 } audio_policy_module_t;
 
 struct audio_policy_device {
+    /**
+     * Common methods of the audio policy device.  This *must* be the first member of
+     * audio_policy_device as users of this structure will cast a hw_device_t to
+     * audio_policy_device pointer in contexts where it's known the hw_device_t references an
+     * audio_policy_device.
+     */
     struct hw_device_t common;
 
     int (*create_audio_policy)(const struct audio_policy_device *device,