Question

Consider the following code in linux/drivers/usb/hid-core.c:

static void hid_process_event (struct hid_device *hid,
                               struct hid_field *field,
                               struct hid_usage *usage,
                               __s32 value)
{
   hid_dump_input(usage, value);
   if (hid->claimed & HID_CLAIMED_INPUT)
         hidinput_hid_event(hid, field, usage, value);
#ifdef CONFIG_USB_HIDDEV
   if (hid->claimed & HID_CLAIMED_HIDDEV)
         hiddev_hid_event(hid, usage->hid, value);
#endif
}

Here the author does not want to call hiddev_hid_event() if a specific configuration option is not enabled. This is because that function will not even be present if the configuration option is not enabled.

To remove this #ifdef, the following change was made to include/linux/hiddev.h:

#ifdef CONFIG_USB_HIDDEV
   extern void hiddev_hid_event (struct hid_device *,
                                 unsigned int usage,
                                 int value);
#else
   static inline void
   hiddev_hid_event (struct hid_device
*hid,
                     unsigned int usage,
                     int value) { }
#endif

Then drivers/usb/hid-core.c was changed to:

static void hid_process_event
                           (struct hid_device *hid,
                            struct hid_field *field,
                            struct hid_usage *usage,
                            __s32 value)
{
   hid_dump_input(usage, value);
   if (hid->claimed & HID_CLAIMED_INPUT)
         hidinput_hid_event(hid, field, usage, value);
   if (hid->claimed & HID_CLAIMED_HIDDEV)
         hiddev_hid_event(hid, usage->hid, value);
}

If CONFIG_USB_HIDDEV is not enabled, the compiler will replace the call to hiddev_hid_event() with a null function call and then optimize away the if statement entirely.

What I can’t understand is how the call to hiddev_hid_event() is replaced with a null function by the compiler. The only difference I see is that return type extern void has been replaced with static inline void. Does this means that all extern functions if not defined will automatically become null function?

Ref: http://www.linuxjournal.com/article/5780?page=0,3

Answer 1

The function actually is defined, but has an empty body:

static inline void
hiddev_hid_event (struct hid_device *hid,
                 unsigned int usage,
                 int value) 
{ }

Optimizing away inline functions with empty bodies is trivial, I guess.