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Michelle Sullivan (change cpan to cpanreal on the front of the email address)


Locked::Storage - A locked in RAM memory region


    use Locked::Storage;

    $a = new Locked::Storage [$nPages];

    print $a->pagesize;

    $bytesAllocated = $a->initialize();

    $bytesAllocated = $a->set_pages($nPages);
    $bytesAllocated = $a->set_size($nBytes);

    print "memory region locked" if $a->is_locked();
    print "process locked in memory" if $a->process_locked();

    $a->store($data, $length);
    print $a->get();




Locked::Storage implements a set of calls to mlock(), munlock() mlockall() and munlockall().; On new() It allocates memory pages specified and will lock them, into RAM (preventing them from going to swap memory.).

This module was written for secure(ish) storage purposes like you would use in cryptographic routines particularly those manipulating private keys.

lockall/unlockall is available to lock the entire process instead of just a memory region however it could easily fail due to system constraints so locking the region in the constructor is always enabled. unlockall will unlock the process and immediately relock the memory reserved in the constructor.


$a = new Locked::Storage [$nPages]

Creates and returns a new Locked::Storage object. The optional $nPages specifies the number of pages to be allocated and locked on return.


Will allocate the storage and lock it in memory if $nPages was not specified as part of the contructor. Returns the number of bytes available.


Will return 1 if the process has been locked in RAM by a successful mlockall() call.


Will return 1 if the allocated memory cannot be paged to swap (ie is locked in RAM)


Will return the page size on this system.


Will set thelocked storage region to be of size $bytes.

If this is called after bytes are written, it will reallocate the storage to the new size and copy over all the data from the old memory region before clearing and releasing it.

If the new size is smaller than the data stored, the data will be truncated to the new length.


Similar to set_size it will set the allocation to bytes that are a multiple of $pages.

$a->store($data, $length)

Stores the data in the allocated storage of length. $data can be of any type, however it will be truncated at $length if the length is longer. If the storage is insufficient 0 (zero) will be returned, otherwise 1 (one) is returned.


Returns the data as a scalar string.


Will lock the entire process in RAM, on error will croak.


Will unlock the process from RAM (if locked) and immediately relock the preallocted memory.


Will return a hexdump of the memory allocated.


Users on systems may have restrictions on the amount of memory that may be locked. This may cause lockall() to fail with ENOMEM which is not caught and will cause a fatal error. Similarly if you attempt to set_size or set_pages and the combined total of the original and new regions exceed the limit on the user a fatal error will occur.

unlockall() knows nothing of other mlock() calls except those in its own constructor, so if you have multiple instances and you call unlockall() it will unlock the regions in those instances and they will not be relocked. It is recommended that you either rely on lockall()/unlockall() or the internal locked storage but not both.

When using this module for cryptography you should undef everything in the same function if possible and overwrite each scalar immediately to prevent the memory being put back into the pool unwiped and therefore defeating the whole purpose of locking the sensitive data in memory.


Various failures in the C libraries are not checked. Particularly ENOMEM where there isn't enough system memory to allow the process or pages to be locked to RAM. This is particularly noted on systems such as linux and freebsd which restrict users (non root) to disallowing the calls either totally or based on the memory required to complete the lock (Thanks to Slaven Rezic for noting that a user can only lock 64kB by defaul on debian/jessie machines by default.)


Michelle Sullivan, cpan@sorbs.net


perl(1), mlock(2), munlock(2), munlockall(2), mlockall(2)