Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!OSU-20.IRCC.OHIO-STATE.EDU!JONES-D
From: JONES-D@OSU-20.IRCC.OHIO-STATE.EDU (Dave Jones)
Newsgroups: comp.os.vms
Subject: Re: old prompt question which I never saw answered.
Message-ID: <8806281241.AA21486@ucbvax.Berkeley.EDU>
Date: 12 Jun 88 16:18:30 GMT
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Sender: daemon@ucbvax.BERKELEY.EDU
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Mr. Haining's statements regarding how VMS issues prompts are wrong.  Whether 
a prompt appears has nothing to do with whether SYS$INPUT is the same as 
SYS$OUTPUT.  The prompt is written to SYS$INPUT ONLY if SYS$INPUT is a 
terminal, prompts are never sent to SYS$OUTPUT.

The VMS terminal driver has a feature called read-with-prompt that 
treats the display of a prompt and reading of input as a single operation.
DCL and most VMS utilities use read-with-prompt when reading from a terminal
and a normal read when reading from a non-terminal (e.g. mailbox) device.

An additional caveat about assigning SYS$INPUT to a mailbox is that
DCL will 'read ahead' 1 record from the mailbox, so you can't simply use
read attention AST's to synchronize processing of output to SYS$OUTPUT
with the commands you are sending to SYS$INPUT.

(Note that this disscussion concerns the operation of DCL and VMS utilities,
 user-written and third-party software may actually write their prompts
 using the write function.)
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David Jones
Ohio State University
1981 Neil Ave. Rm. 406
Columbus OH, 43210
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