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Re: [PATCH] [IOS] Avoid spurious EOF when writing partial bytes

From: Egeyar Bagcioglu
Subject: Re: [PATCH] [IOS] Avoid spurious EOF when writing partial bytes
Date: Sat, 27 Mar 2021 14:58:25 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 3/27/21 2:45 PM, Jose E. Marchesi wrote:
c) [your proposal] Support write-only IOS, but anything that would
     need to read fails with an exception.

     This is something of a consistency breakage; everywhere else, poke
     appears to ignore the common limitations of byte-level writing,
Agreed.  But on the other hand, it is documented and assumed that IO
devices are byte oriented (unlike the IOS abstraction built on top of
it.) so this need to read completing bytes shoulnd't be much of a

     Using an EOF exception would be quite confusing for the user as
     well; if this is implemented, please use a more accurate exception
     (maybe a new one needs to be added)
Yes, EOF is confusing.

I think we really want to introduce an E_perm exception, and raise it in
every peek* PVM instruction if the IO space doesn't support read.  Then
a dump of a non-readable IO space, for example, would raise the
exception instead of showing us an "empty" IO space.

     With this idea, if the user wishes to use a write-only IOS in a
     fully general manner, they must use another IOS as temporary
Right.  That is easy to do with .mem IOS.  If we go with this option I
will write a tutorial-like section in the manual describing how to
operate on write-only IO spaces.

Let's also keep in mind that write-only IO spaces will be very rare.
This case is handled well by the stream IOS too. Indeed we keep a
buffer there as Georgiy describes. We could reuse that code to handle
such cases in the future as well.
But that won't work for write-only files with pre existing contents.
Output streams (write-only streams) are always created anew and
"empty".  That's why the caching strategy works there.

Or I am missing something?

It would not solve the problem of writing to only a part of a byte. That has to be a readable file, we do not have any alternatives there.

The code in ios-buffer.h/c, which is currently used only by the stream IOS, can also be used as an extendable buffer space for other purposes too.


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