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

[Chet Ramey]

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:
> https://lists.freebsd.org/pipermail/freebsd-hackers/2011-November/036863.html
> The BUGS section mentioned in this FreeBSD discussion can be found on
> the FreeBSD man page:
> https://www.freebsd.org/cgi/man.cgi?query=lseek&sektion=2
>
> 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/