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From: gamma@rtp47.UUCP (Michael Meissner)
Newsgroups: net.lang.c
Subject: Re: Volatile type in ANSI C
Message-ID: <31@rtp47.UUCP>
Date: Fri, 26-Apr-85 10:22:43 EDT
Article-I.D.: rtp47.31
Posted: Fri Apr 26 10:22:43 1985
Date-Received: Mon, 29-Apr-85 07:08:50 EDT
References: <543@lll-crg.ARPA>
Reply-To: gamma@rtp47.UUCP (Michael Meissner)
Distribution: net
Organization: Data General, RTP, NC
Lines: 18
Keywords: ANSI volatile


Both flavors of volatile have their uses:

    1)	volatile int *p;	/* p is a non-volatile ptr to volatile mem */
		The compiler can optimize `p' into a register, or some such,
		but every access to the memory p points to cannot be optimized.

    2)	int * volatile q;	/* q is a volatile ptr to non-volatile mem */
		This is useful for things like multi-processor implementations,
		where q can be modified at any time (though obviously you might
		get strange results if q is not aligned correctly, and the other
		processor takes more than 1 memory cycle to update q).  Because
		q is volitle, it means that anything it points cannot be
		optimized, and probably it should read:
			volatile int * volatile q;
	
	Michael Meissner
	Data General Corporation
	...ihnp4!mcnc!rti-sel!rtp47