Provided by: libsys-mmap-perl_0.16-1build2_amd64 
NAME
Sys::Mmap - uses mmap to map in a file as a Perl variable
SYNOPSIS
use Sys::Mmap;
new Mmap $str, 8192, 'structtest2.pl' or die $!;
new Mmap $var, 8192 or die $!;
mmap($foo, 0, PROT_READ, MAP_SHARED, FILEHANDLE) or die "mmap: $!";
@tags = $foo =~ /<(.*?)>/g;
munmap($foo) or die "munmap: $!";
mmap($bar, 8192, PROT_READ|PROT_WRITE, MAP_SHARED, FILEHANDLE);
substr($bar, 1024, 11) = "Hello world";
mmap($baz, 8192, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, STDOUT);
$addr = mmap($baz, 8192, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANON, STDOUT);
Sys::Mmap::hardwire($qux, $addr, 8192);
DESCRIPTION
The Mmap module uses the POSIX mmap call to map in a file as a Perl variable. Memory access by mmap may
be shared between threads or forked processes, and may be a disc file that has been mapped into memory.
Sys::Mmap depends on your operating system supporting UNIX or POSIX.1b mmap, of course.
Note that PerlIO now defines a ":mmap" tag and presents mmap'd files as regular files, if that is your
cup of joe.
Several processes may share one copy of the file or string, saving memory, and concurrently making
changes to portions of the file or string. When not used with a file, it is an alternative to SysV shared
memory. Unlike SysV shared memory, there are no arbitrary size limits on the shared memory area, and
sparce memory usage is handled optimally on most modern UNIX implementations.
Using the "new()" method provides a "tie()"'d interface to "mmap()" that allows you to use the variable
as a normal variable. If a filename is provided, the file is opened and mapped in. If the file is smaller
than the length provided, the file is grown to that length. If no filename is provided, anonymous shared
inheritable memory is used. Assigning to the variable will replace a section in the file corresponding to
the length of the variable, leaving the remainder of the file intact and unmodified. Using "substr()"
allows you to access the file at an offset, and does not place any requirements on the length argument to
substr() or the length of the variable being inserted, provided it does not exceed the length of the
memory region. This protects you from the pathological cases involved in using "mmap()" directly,
documented below.
When calling "mmap()" or "hardwire()" directly, you need to be careful how you use the variable. Some
programming constructs may create copies of a string which, while unimportant for smallish strings, are
far less welcome if you're mapping in a file which is a few gigabytes big. If you use PROT_WRITE and
attempt to write to the file via the variable you need to be even more careful. One of the few ways in
which you can safely write to the string in-place is by using "substr()" as an lvalue and ensuring that
the part of the string that you replace is exactly the same length. Other functions will allocate other
storage for the variable, and it will no longer overlay the mapped in file.
new Mmap VARIABLE, LENGTH, OPTIONALFILENAME
Maps LENGTH bytes of (the contents of) OPTIONALFILENAME if OPTINALFILENAME is provided, otherwise
uses anonymous, shared inheritable memory. This memory region is inherited by any "fork()"ed
children. VARIABLE will now refer to the contents of that file. Any change to VARIABLE will make an
identical change to the file. If LENGTH is zero and a file is specified, the current length of the
file will be used. If LENGTH is larger then the file, and OPTIONALFILENAME is provided, the file is
grown to that length before being mapped. This is the preferred interface, as it requires much less
caution in handling the variable. VARIABLE will be tied into the "Mmap" package, and "mmap()" will be
called for you.
Assigning to VARIABLE will overwrite the beginning of the file for a length of the value being
assigned in. The rest of the file or memory region after that point will be left intact. You may use
substr() to assign at a given position:
substr(VARIABLE, POSITION, LENGTH) = NEWVALUE
mmap(VARIABLE, LENGTH, PROTECTION, FLAGS, FILEHANDLE, OFFSET)
Maps LENGTH bytes of (the underlying contents of) FILEHANDLE into your address space, starting at
offset OFFSET and makes VARIABLE refer to that memory. The OFFSET argument can be omitted in which
case it defaults to zero. The LENGTH argument can be zero in which case a stat is done on FILEHANDLE
and the size of the underlying file is used instead.
The PROTECTION argument should be some ORed combination of the constants PROT_READ, PROT_WRITE and
PROT_EXEC or else PROT_NONE. The constants PROT_EXEC and PROT_NONE are unlikely to be useful here but
are included for completeness.
The FLAGS argument must include either MAP_SHARED or MAP_PRIVATE (the latter is unlikely to be useful
here). If your platform supports it, you may also use MAP_ANON or MAP_ANONYMOUS. If your platform
supplies MAP_FILE as a non-zero constant (necessarily non-POSIX) then you should also include that in
FLAGS. POSIX.1b does not specify MAP_FILE as a FLAG argument and most if not all versions of Unix
have MAP_FILE as zero.
mmap returns undef on failure, and the address in memory where the variable was mapped to on success.
munmap(VARIABLE)
Unmaps the part of your address space which was previously mapped in with a call to "mmap(VARIABLE,
...)" and makes VARIABLE become undefined.
munmap returns 1 on success and undef on failure.
hardwire(VARIABLE, ADDRESS, LENGTH)
Specifies the address in memory of a variable, possibly within a region you've "mmap()"ed another
variable to. You must use the same percaustions to keep the variable from being reallocated, and use
"substr()" with an exact length. If you "munmap()" a region that a "hardwire()"ed variable lives in,
the "hardwire()"ed variable will not automatically be "undef"ed. You must do this manually.
Constants
The Mmap module exports the following constants into your namespace
MAP_SHARED MAP_PRIVATE MAP_ANON MAP_ANONYMOUS MAP_FILE
PROT_EXEC PROT_NONE PROT_READ PROT_WRITE
Of the constants beginning MAP_, only MAP_SHARED and MAP_PRIVATE are defined in POSIX.1b and only
MAP_SHARED is likely to be useful.
BUGS
Scott Walters doesn't know XS, and is just winging it. There must be a better way to tell Perl not to
reallocate a variable in memory...
The tie() interface makes writing to a substring of the variable much less efficient. One user cited his
application running 10-20 times slower when "new Mmap" is used than when mmap() is called directly.
Malcolm Beattie has not reviewed Scott's work and is not responsible for any bugs, errors, omissions,
stylistic failings, importabilities, or design flaws in this version of the code.
There should be a tied interface to hardwire() as well.
Scott Walter's spelling is awful.
hardwire() will segfault Perl if the mmap() area it was refering to is munmap()'d out from under it.
munmap() will segfault Perl if the variable was not successfully mmap()'d previously, or if it has since
been reallocated by Perl.
AUTHOR
Malcolm Beattie, 21 June 1996.
Updated for Perl 5.6.x, additions, Scott Walters, Feb 2002.
Aaron Kaplan kindly contributed patches to make the C ANSI compliant and contributed documentation as
well.