Re: poll(): IN/OUT vs {RD,WR}NORM

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Mon, 27 May 2024 20:27:28 -0400 (EDT)
> From: Mouse <mouse%Rodents-Montreal.ORG@localhost>
> 
> In sys/sys/poll.h, I see (in 1.4T, -current according to cvsweb, and
> everything I've checked in between):
> 
> #define POLLIN          0x0001
> #define POLLPRI         0x0002
> #define POLLOUT         0x0004
> #define POLLRDNORM      0x0040
> #define POLLWRNORM      POLLOUT
> #define POLLRDBAND      0x0080
> #define POLLWRBAND      0x0100
> 
> I can understand treating POLLWRNORM as identical to POLLOUT.  But why
> the distinction between POLLRDNORM and POLLIN?  Might anyone know the
> reason and be willing to explain?

It's been this way since sysvr4, according to the archive at
<https://archive.org/details/ATTUNIXSystemVRelease4Version2>
(sysvr4.tar.bz2, svr4/uts/i386/sys/poll.h):

/*
 * Testable select events 
 */
#define POLLIN		0x0001		/* fd is readable */
#define POLLPRI		0x0002		/* high priority info at fd */
#define	POLLOUT		0x0004		/* fd is writeable (won't block) */
#define POLLRDNORM	0x0040		/* normal data is readable */
#define POLLWRNORM	POLLOUT
#define POLLRDBAND	0x0080		/* out-of-band data is readable */
#define POLLWRBAND	0x0100		/* out-of-band data is writeable */

sysvr3 didn't have the NORM/BAND ones.  Perhaps you'll find more clues
by digging through the changes from sysvr3 to sysvr4.