Ubuntu Manpage: File::Map - Memory mapping made simple and safe.

Provided by: libfile-map-perl_0.67-2_amd64

NAME

       File::Map - Memory mapping made simple and safe.

VERSION

       version 0.67

SYNOPSIS

        use File::Map 'map_file';

        map_file my $map, $filename, '+<';
        $map =~ s/bar/quz/g;
        substr $map, 1024, 11, "Hello world";

DESCRIPTION

File::Map maps files or anonymous memory into perl variables.

Advantages of memory mapping
• Unlike normal perl variables, mapped memory is (usually) shared between threads or
forked processes.

• It is an efficient way to slurp an entire file. Unlike for example File::Slurp, this
module returns almost immediately, loading the pages lazily on access. This means you
only 'pay' for the parts of the file you actually use.

• Perl usually doesn't return memory to the system while running, mapped memory can be
returned.

Advantages of this module over other similar modules
• Safety and Speed

This module is safe yet fast. Alternatives are either fast but can cause segfaults or
lose the mapping when not used correctly, or are safe but rather slow. File::Map is as
fast as a normal string yet safe.

• Simplicity

It offers a simple interface targeted at common usage patterns

• Files are mapped into a variable that can be read just like any other variable,
and it can be written to using standard Perl techniques such as regexps and
"substr".

• Files can be mapped using a set of simple functions. There is no need to know
weird constants or the order of 6 arguments.

• It will automatically unmap the file when the scalar gets destroyed. This works
correctly even in multi-threaded programs.

• Portability

File::Map supports Unix and Windows.

• Thread synchronization

It has built-in support for thread synchronization.

FUNCTIONS

Mapping
The following functions for mapping a variable are available for exportation. Note that
all of these functions throw exceptions on errors, unless noted otherwise.

map_handle $lvalue, $filehandle, $mode = '<', $offset = 0, $length = -s(*handle) - $offset

Use a filehandle to map into an lvalue. $filehandle should be a scalar filehandle. $mode
uses the same format as "open" does (it currently accepts "<", "+<", ">" and "+>").
$offset and $length are byte positions in the file, and default to mapping the whole file.

* map_file $lvalue, $filename, $mode = '<', $offset = 0, $length = -s($filename) - $offset

Open a file and map it into an lvalue. Other than $filename, all arguments work as in
map_handle.

* map_anonymous $lvalue, $length, $type

Map an anonymous piece of memory. $type can be either 'shared', in which case it will be
shared with child processes, or 'private', which won't be shared.

* sys_map $lvalue, $length, $protection, $flags, $filehandle, $offset = 0

Low level map operation. It accepts the same constants as mmap does (except its first
argument obviously). If you don't know how mmap works you probably shouldn't be using
this.

* unmap $lvalue

Unmap a variable. Note that normally this is not necessary as variables are unmapped
automatically at destruction, but it is included for completeness.

* remap $lvalue, $new_size

Try to remap $lvalue to a new size. This call is linux specific and not supported on other
systems. For a file backed mapping a file must be long enough to hold the new size,
otherwise you can expect bus faults. For an anonymous map it must be private, shared maps
can not be remapped. Use with caution.

Auxiliary
* sync $lvalue, $synchronous = 1

Flush changes made to the memory map back to disk. Mappings are always flushed when
unmapped, so this is usually not necessary. If $synchronous is true and your operating
system supports it, the flushing will be done synchronously.

* pin $lvalue

Disable paging for this map, thus locking it in physical memory. Depending on your
operating system there may be limits on pinning.

* unpin $lvalue

Unlock the map from physical memory.

* advise $lvalue, $advice

Advise a certain memory usage pattern. This is not implemented on all operating systems,
and may be a no-op. The following values for $advice are always accepted:.

• normal

Specifies that the application has no advice to give on its behavior with respect to the
mapped variable. It is the default characteristic if no advice is given.

• random

Specifies that the application expects to access the mapped variable in a random order.

• sequential

Specifies that the application expects to access the mapped variable sequentially from
start to end.

• willneed

Specifies that the application expects to access the mapped variable in the near future.

• dontneed

Specifies that the application expects that it will not access the mapped variable in
the near future.

On some systems there may be more values available, but this can not be relied on. Unknown
values for $advice will cause a warning but are further ignored.

* protect $lvalue, $mode

Change the memory protection of the mapping. $mode takes the same format as "open", but
also accepts sys_map style constants.

Locking
These locking functions provide locking for threads for the mapped region. The mapped
region has an internal lock and condition variable. The condition variable
functions("wait_until", "notify", "broadcast") can only be used inside a locked block. If
your perl has been compiled without thread support the condition functions will not be
available.

* lock_map $lvalue

Lock $lvalue until the end of the scope. If your perl does not support threads, this will
be a no-op.

* wait_until { block } $lvalue

Wait for block to become true. After every failed attempt, wait for a signal. It returns
the value returned by the block.

* notify $lvalue

This will signal to one listener that the map is available.

* broadcast $lvalue

This will signal to all listeners that the map is available.

Constants
PROT_NONE, PROT_READ, PROT_WRITE, PROT_EXEC, MAP_ANONYMOUS, MAP_SHARED, MAP_PRIVATE,
MAP_ANON, MAP_FILE
These constants are used for sys_map. If you think you need them your mmap manpage
will explain them, but in most cases you can skip sys_map altogether.

EXPORTS

       All previously mentioned functions are available for exportation, but none are exported by
       default. Some functions may not be available on your OS or your version of perl as
       specified above. A number of tags are defined to make importation easier.

       •   :map

           map_handle, map_file, map_anonymous, sys_map, unmap

       •   :extra

           remap, sync, pin, unpin, advise, protect

       •   :lock

           lock_map, wait_until, notify, broadcast

       •   :constants

           PROT_NONE, PROT_READ, PROT_WRITE, PROT_EXEC, MAP_ANONYMOUS, MAP_SHARED, MAP_PRIVATE,
           MAP_ANON, MAP_FILE

       •   :all

           All functions defined in this module.

DIAGNOSTICS

Exceptions
• Could not <function name>: this variable is not memory mapped

An attempt was made to "sync", "remap", "unmap", "pin", "unpin", "advise" or
"lock_map" an unmapped variable.

• Could not <function name>: <system error>

Your OS didn't allow File::Map to do what you asked it to do for some reason.

• Trying to <function_name> on an unlocked map

You tried to "wait_until", "notify" or "broadcast" on an unlocked variable.

• Zero length not allowed for anonymous map

A zero length anonymous map is not possible (or in any way useful).

• Can't remap a shared mapping

An attempt was made to remap a mapping that is shared among different threads, this is
not possible.

• Window (<start>, <end>) is outside the file

The offset and/or length you specified were invalid for this file.

• Can't map fake filehandle

The filehandle you provided is not real. This may mean it's a scalar string handle or
a tied handle.

• No such flag <flag_name>

The flag given for map_anonymous isn't valid, it should either be "shared" or
"private".

Warnings
• Writing directly to a memory mapped file is not recommended

Due to the way perl works internally, it's not possible to write a mapping
implementation that allows direct assignment yet performs well. As a compromise,
File::Map is capable of fixing up the mess if you do it nonetheless, but it will warn
you that you're doing something you shouldn't. This warning is only given when "use
warnings 'substr'" is in effect.

• Truncating new value to size of the memory map

This warning is additional to the previous one, warning you that you're losing data.
This warning is only given when "use warnings 'substr'" is in effect.

• Shouldn't mmap non-binary filehandle

You tried to to map a filehandle that has some encoding layer. Encoding layers are not
supported by File::Map. This warning is only given when "use warnings 'layer'" is in
effect. Note that this may become an exception in a future version.

• Unknown advice '<advice>'

You gave advise an advice it didn't know. This is either a typo or a portability
issue. This warning is only given when "use warnings 'portable'" is in effect.

• Syncing a readonly map makes no sense

"sync" flushes changes to the map to the filesystem. This obviously is of little use
when you can't change the map. This warning is only given when "use warnings 'io'" is
in effect.

• Can't overwrite an empty map

Overwriting an empty map is rather nonsensical, hence a warning is given when this is
tried. This warning is only given when "use warnings 'substr'" is in effect.

DEPENDENCIES

       This module depends on perl 5.8, Sub::Exporter::Progressive and PerlIO::Layers. Perl 5.8.8
       or higher is recommended because older versions can give spurious warnings.

       In perl versions before 5.11.5 many string functions including "substr" are limited to
       32bit logic <http://rt.perl.org/rt3//Public/Bug/Display.html?id=72784>, even on 64bit
       architectures. Effectively this means you can't use them on strings bigger than 2GB. If
       you are working with such large files, it is strongly recommended to upgrade to 5.12.

       In perl versions before 5.17.5, there is an off-by-one bug in Perl's regexp engine, as
       explained here <http://rt.perl.org/rt3//Public/Bug/Display.html?id=73542>. If the length
       of the file is an exact multiple of the page size, some regexps can trigger a segmentation
       fault.

PITFALLS

       •   This module doesn't do any encoding or newline transformation for you, and will reject
           any filehandle with such features enabled as mapping it would return a different value
           than reading it normally. Most importantly this means that on Windows you have to
           remember to use the ":raw" open mode or binmode to make your filehandles binary before
           mapping them, as by default it would do "crlf" transformation. See PerlIO for more
           information on how that works.

       •   You can map a ":utf8" filehandle, but writing to it may be tricky. Hic sunt dracones.

       •   You probably don't want to use ">" as a mode. This does not give you reading
           permissions on many architectures, resulting in segmentation faults when trying to
           read a variable (confusingly, it will work on some others like x86).

BUGS AND LIMITATIONS

       As any piece of software, bugs are likely to exist here. Bug reports are welcome.

       Please report any bugs or feature requests to "bug-file-map at rt.cpan.org", or through
       the web interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=File-Map>.  I will be
       notified, and then you'll automatically be notified of progress on your bug as I make
       changes.

       Unicode file mappings are known to be buggy on perl 5.8.7 and lower.

AUTHOR

       Leon Timmermans <fawaka@gmail.com>

COPYRIGHT AND LICENSE

       This software is copyright (c) 2008 by Leon Timmermans.

       This is free software; you can redistribute it and/or modify it under the same terms as
       the Perl 5 programming language system itself.