Re: sed not working on /proc files

Richard B. Johnson (root@chaos.analogic.com)
Mon, 26 Apr 1999 19:29:04 -0400 (EDT)

On Mon, 26 Apr 1999, Stefan Monnier wrote:
[SNIPPED]
> >
> > It happens because files in /proc have zero length. These are not
> > real files. To use `sed` and utilities that do lseek you can `cat`
> > through a pipe as you have done. Sed's first 'lseek()' returns ESPIPE
> > which tells it to not do that again -- and everything is fine.
>
> I didn't know sed did `lseek' since it works on pipes as well.
> And actually, looking at an `strace' output, it seems it doesn't
> do any lseek:
>
[SNIPPED strace]
>
> But maybe the fstat is the culprit. Maybe sed tries to be smart and
> doesn't bother opening files of size zero ?
>

The sed on my system, which is quite old, not from a standards
distribution, does, indeed do lseek to find the file-size. Your
version does 'fstat' which will accomplish the same thing. Typically
a program that does 'lseek(0, SEEK_CUR)` expects to have the correct
return value or errno be set to ESPIPE which lets it know that it's
a pipe, socket, or fifo. This lets it work with such.

In any event, the files in /proc can't be read with `less` either
for the same reason, also `more` always shows 0%. It's the expected
nature,

Cheers,
Dick Johnson
***** FILE SYSTEM WAS MODIFIED *****
Penguin : Linux version 2.2.6 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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