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Re: Why does "mapfile -d delim" (delim != '\n') use unbuffered read?

From: Chet Ramey
Subject: Re: Why does "mapfile -d delim" (delim != '\n') use unbuffered read?
Date: Tue, 4 May 2021 09:28:04 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.0

On 5/4/21 2:31 AM, Koichi Murase wrote:
I'm still thinking about whether it is possible to use buffered read
in more cases of mapfile.

(1) Suggestion: use buffered read for a regular file for delim != '\n'?

It won't work on any system that doesn't return -1/ESPIPE when you try
to lseek on a terminal device.

I found a related interesting discussion in a FreeBSD mailing list:
The BUGS section mentioned in this FreeBSD discussion can be found on
the FreeBSD man page:

lseek(2) may succeed on non-seekable streams including some terminal
device (mentioned in Chet's reply) and a tape device (mentioned in the
above FreeBSD discussion).

Let's take a step back. I think we can make the code simpler and still
handle all the cases we need to. I don't think tape devices are worth
worrying about.

The issue with the BSDs and terminal devices is that the current code
only checks for -1/ESPIPE, but they return a different value for errno
(EINVAL? I forget.) I believe AIX returns something else. Rather than
check for specific errno values, we can just decide not to buffer input
if lseek fails for any reason.

``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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