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From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: While learning PERL... a suggestion
Keywords: perl file learn suggest
Message-ID: <11115@jpl-devvax.JPL.NASA.GOV>
Date: 19 Jan 91 02:11:53 GMT
References: <1991Jan19.003519.23569@ux1.cso.uiuc.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 42

In article <1991Jan19.003519.23569@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
: While learning PERL I have reached the part about the various file test
: operator.  I noted that -B and -T need to read SOME of the file to make
: their appropriate determinations.  I though of another kind of test that
: could be useful, especially from pipes where the name might not be there.
: 
: Suggestion:
: 
: Add a -Z file test operator that returns TRUE if the file appears to be
: the output of the UNIX compress command.  Testing this file with -B would
: still yield TRUE since a compressed file is a subset of binary files.

I don't think Perl should have operators to check magic numbers.  It's
too easy to read the first word yourself now that there's sysread().

: I've still yet to get through the rest of PERL (probably this weekend)
: so I don't know yet if it is particularly easy to invoke uncompress -c
: or zcat to pipe input data back to a PERL script.  When reading from
: a file one can reposition back to the beginning of the file and pass
: the file descriptor on to zcat as STDIN and read from the pipe instead.
: But for something already coming in from a pipe, the suggested -Z test
: would have already taken data out of the pipe.

There's no way to do this at all under Unix, let alone Perl, without
interposing a process to supply the magic number you destructively read
out of the pipe.  You can't seek backwards on a pipe, and unless you can
convince compress to accept input without the leading magic number, you're
stuck.

As you pointed out, it wouldn't help to give Perl a -Z, since it would still
have to read the pipe.

There's one other possibility.  If you know the pipe is really a socket,
you might be able to do a recv() with the MSG_PEEK flag and read it out
non-destructively.  Likewise for streams, using I_PEEK.

: Well I hope I don't have egg all over my face with this suggestion.
: We shall see.

I'm afraid the yolk is on all of us.

Larry