I often use mmap for very fast file I/O. Here's an example of a simplistic grep, which is not quite twice as fast as a simple perl -ne 'BEGIN { $pat = shift } print if /$pat/o' on my system. 
#!/usr/bin/perl

use warnings;
use strict;

use Sys::Mmap;

our $pat = shift;

foreach my $a (@ARGV)
{
  my $mmap;
  if (! -f $a)
  {
    warn "'$a' is not a regular file, skipping\n";
    next;
  }
  open(F, "< $a")
    or die "Couldn't open '$a': $!\n";
  mmap($mmap, 0, PROT_READ, MAP_SHARED, F)
    or die "mmap error: $!\n";
  while ($mmap =~ m/($pat)/omg)
  {
    my $pos = pos($mmap)-length($1);
    # Find the beginning and end of this line
    my $first = rindex($mmap,"\n",$pos)+1;
    my $last  = index($mmap,"\n",$pos)+1;
    print substr($mmap,$first,$last-$first);
  }
  munmap($mmap)
    or die "mmunmap error: $!\n";
  close(F)
    or die "Couldn't close '$a': $!\n";
}

[download]

That doesn't really address sharing, though. Basically, if two programs mmap in the same file, when one makes changes the other will be able to see them, which provides a simple shared memory region.

Update: Fix bug matching first line of file.

In reply to Re: anyone have some good mmap examples? by sgifford
in thread anyone have some good mmap examples? by zentara

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.