USB_ATTACH_SETUP macros question

[M. Warner Losh]

In message: <35146.85.132.32.38.1155817977.squirrel at 85.132.32.38>
            "Tofig Suleymanov" <tofik at oxygen.az> writes:
: 
: > On Thu, Aug 17, 2006 at 05:10:02PM +0500, Tofig Suleymanov wrote:
: >> Hello hackers,
: >>
: >> is there anybody to explain why do we have infinite loop inside of
: >> USB_ATTACH_SETUP macros inside of usb_port.h ? How does his loop gets
: >> escaped when we use it in some usb driver ?
: >>
: >> #define USB_ATTACH_SETUP \
: >>         do { \
: >>                 sc->sc_dev = self; \
: >>                 device_set_desc_copy(self, devinfo); \
: >>                 device_printf(self, "%s\n", devinfo); \
: >>         } while (0);
: >>
: >> Thank you  !
: >>
: >> Tofig Suleymanov
: >>
: > This isn't an infinite loop, in fact it will never be executed.
: >
: > Baldur
: >
: 
: Made a little test which shows that this code is going to be executed only
: once.How did i miss that :) Now the other question comes next:
: Why don not we use just:
: #define USB_ATTACH_SETUP \
: sc->sc_dev = self; \
: device_set_desc_copy(self, devinfo); \
: device_printf(self, "%s\n", devinfo);

Actually, it is neither an infinite loop, nor is it a never execute
loop.

the do { ... } while (0); is a way to make a macro lexically identical
to a function call so that if you do something like:

	if (blah)
		USB_ATTACH_SETUP;

it will parse as you intended.  Which is to say it will parse exactly
the same as:

	if (blah) {
		USB_ATTACH_STUP;
	}

Without the do while construct, your example would cause different
results in the latter and former cases.

Warner