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/audio.h b/include/hardware/audio.h
index b53cbff..6c28def 100644
--- a/include/hardware/audio.h
+++ b/include/hardware/audio.h
@@ -260,6 +260,11 @@
*/
struct audio_stream_out {
+ /**
+ * Common methods of the audio stream out. This *must* be the first member of audio_stream_out
+ * as users of this structure will cast a audio_stream to audio_stream_out pointer in contexts
+ * where it's known the audio_stream references an audio_stream_out.
+ */
struct audio_stream common;
/**
@@ -383,6 +388,11 @@
typedef struct audio_stream_out audio_stream_out_t;
struct audio_stream_in {
+ /**
+ * Common methods of the audio stream in. This *must* be the first member of audio_stream_in
+ * as users of this structure will cast a audio_stream to audio_stream_in pointer in contexts
+ * where it's known the audio_stream references an audio_stream_in.
+ */
struct audio_stream common;
/** set the input gain for the audio driver. This method is for
@@ -439,6 +449,11 @@
};
struct audio_hw_device {
+ /**
+ * Common methods of the audio device. This *must* be the first member of audio_hw_device
+ * as users of this structure will cast a hw_device_t to audio_hw_device pointer in contexts
+ * where it's known the hw_device_t references an audio_hw_device.
+ */
struct hw_device_t common;
/**