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From: guineau@wjg.enet.dec.com
Newsgroups: comp.sys.amiga.tech
Subject: Re: A bug in ARP 1.3?
Message-ID: <6865@shlump.nac.dec.com>
Date: 13 Dec 89 19:17:09 GMT
References: <148@bucsb.UUCP> <14752@boulder.Colorado.EDU> <3598@jhunix.HCF.JHU.EDU>
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> What does the following line mean?
>
>     Wait(1 << Window->UserPort->mp_SigBit)
>
> I know it causes my program to wait for a response in the IDCMP port,
> but how exactly does this work?
>
> ->  means structure member notation for pointers
>
> <<  means shift left, a bitwise operator, which can also be enterpreted
>    as multiplication by 2
>
> So, why should shifting the mp_SigBit member of the window's UserPort
>  One bit position to the left magically cause my application to wake up 
> from a sleeping condition and recognize the presence of an incoming message?


It doesn't. The   << (and >>)  C operators mean 

	a << b

Shift a left by b bit positions.  (shift right for >> )

So  Wait(1 << Window->UserPort->mp_SigBit)

means shift a 1 left to the position corresponding to the signal number.
So for example
an  mp_SigBit of 3 coresponds to a bit mask of  binary  1000 (i.e. bit
position 3).

Wait needs a bit mask, while mp_SigBit is the signal number.

>                                 -Paul Castonguay


John Guineau