Date: Thu, 17 Aug 2006 09:17:00 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: tofik@oxygen.az Cc: usb@freebsd.org Subject: Re: USB_ATTACH_SETUP macros question Message-ID: <20060817.091700.-432836888.imp@bsdimp.com> In-Reply-To: <35146.85.132.32.38.1155817977.squirrel@85.132.32.38> References: <35069.85.132.32.38.1155816624.squirrel@85.132.32.38> <20060817121307.GA804@gremlin.foo.is> <35146.85.132.32.38.1155817977.squirrel@85.132.32.38>
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In message: <35146.85.132.32.38.1155817977.squirrel@85.132.32.38>
"Tofig Suleymanov" <tofik@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
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