Newsgroups: comp.lang.perl
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Subject: Re: anyone gotten mmap to work w/ perl?
Message-ID: <1991Apr9.051702.22978@jpl-devvax.jpl.nasa.gov>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
References: <535@appserv.Eng.Sun.COM>
Date: Tue, 9 Apr 1991 05:17:02 GMT

In article <535@appserv.Eng.Sun.COM> lm@sun.UUCP (Larry McVoy) writes:
: 
: The subject line says it all, I want to use mmap(2) from perl.  I know I
: can do:
: 
: sub mmap() {
: 	local($fh, $len, $prot) = @_;
: 
: 	return (syscall(SYS_mmap(0, len, $prot, MAP_SHARED, fileno($fh), 0));
: }
: 
: The problem is the return address.  I need to fake out perl into
: believing that the address is a string that is $len bytes long but
: should not be freed.  Is there an easy way of doing this short of
: wacking on perl itself?

Mmm.  Not that I can think of, though there might be some way of bootstrapping
an inplace modification of *name into a generalized poke routine.  There
might also be something you can do with the 'p' pack/unpack specifier--I
haven't thought about that much.

Apart from that, you could use the usub hooks to link mmap into uperl.o as
a subroutine.

However, there's no way to guarantee that perl won't realloc a variable
unless you're careful about the operations you apply to it.  Assigning
to a substr() of the same length is always safe.  Some substitutions are
safe and some aren't.  (The safe ones are the ones with a constant RHS
that is known to be shorter than the LHS.)  read() and sysread() are safe
as long as you don't ask for more than the length of the string.  Most
other operations are safe as long as the final length of the string is
less that what it thinks is currently allocated.

What you'd really kinda like is a way to mmap a particular variable, and
let Perl worry about it when it gets reallocated...

Larry