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.. index::
   pair: structure signatures; design
   single: signatures

.. _design-sig:


Signatures in the MPS
=====================

.. mps:prefix:: design.mps.sig
   pair: structure signatures; design
   single: signatures


Introduction
------------
Integrity of data structures is absolutely critical to the cost of
deploying the Memory Pool System.  Memory corruption and memory
management bugs are incredibly hard to detect and debug, often
manifesting themselves hours or days after they occur.  One of the key
ways the MPS detects corruption or the passing of illegal data is using
*signatures*.  This simple technique has proved invaluable at catching
defects early.


Overview
--------
Signatures are `magic numbers`_ which are written into structures when
they are created and invalidated (by overwriting with ``SigInvalid``)
when they are destroyed. They provide a limited form of run-time type
checking and dynamic scope checking. They are a simplified form of
"Structure Marking", a technique used in the Multics filesystem
[THVV_1995]_.

.. _`magic numbers`: http://en.wikipedia.org/wiki/Magic_number_(programming)


Definitions
-----------
Nearly every structure should start with a field of type :c:type:`Sig` with the name
``sig``.  For example::

    typedef struct mps_message_s {
      Sig sig;                      /* <design/sig/> */
      Arena arena;                  /* owning arena */
      MessageClass class;           /* Message Class Structure */
      Clock postedClock;            /* mps_clock() at post time, or 0 */
      RingStruct queueRing;         /* Message queue ring */
    } MessageStruct;

There must also be a definition for the valid value for that signature::

    #define MessageSig      ((Sig)0x5193e559) /* SIG MESSaGe */

This is a 32-bit hex constant, spelled according to guide.hex.trans_::

    ABCDEFGHIJKLMNOPQRSTUVWXYZ
    ABCDEF9811C7340BC6520F3812

.. _guide.hex.trans: guide.hex.trans

This allows the structure to be recognised when looking at memory in a hex
dump or memory window, or found using memory searches.


Init and Finish
---------------
When the structure is initialised, the signature is initialised as the
*last* action, just before validating it.  (Think of it as putting your
signature at the bottom of a document to say it's done.)  This ensures
that the structure will appear invalid until it is completely initialized
and ready to use.  For example::

    void MessageInit(...) {
      ... 
      message->arena = arena;
      message->class = class;
      RingInit(&message->queueRing);
      message->postedClock = 0;
      message->sig = MessageSig;
      AVERT(Message, message);
    }

When the structure is finished, the signature is invalidated as the
*first* action, ensuring that the structure appears invalid while it is
being torn down.  For example::

    void MessageFinish(Message message)
    {
      AVERT(Message, message);
      AVER(RingIsSingle(&message->queueRing));

      message->sig = SigInvalid;
      RingFinish(&message->queueRing);
    }

Do not do anything else with signatures.  See :mps:ref:`.rule.purpose`.


Checking
--------
The signature is checked in various ways.  Every function that takes a
(pointer to) a signed structure should check its argument using the
:c:macro:`AVERT` macro.  This macro has different definitions depending on how
the MPS is compiled (see design.mps.config.def.var_).  It may simply check
the signature directly, or call the full checking function for the
structure.

.. _design.mps.config.def.var: config#def-var

The checking function for the structure should also validate the
signature as its first step using the :c:func:`CHECKS()` macro (see
check.h_). For example::

    Bool MessageCheck(Message message)
    {
      CHECKS(Message, message);
      CHECKU(Arena, message->arena);
      CHECKD(MessageClass, message->class);
      ...

This combination makes it extremely difficult to get an object of the
wrong type, an uninitialized object, or a dead object, or a random
pointer into a function.

.. _check.h: ../code/check.h

Rules
-----
:mps:tag:`rule.purpose` **Do not** use signatures for any other purpose.  For
example, don't use them to make any actual decisions within the code. 
They must not be used to discriminate between structure variants (or
union members). They must not be used to try to detect *whether* a
structure has been initialised or finished.  They are there to
double-check whether these facts are true. They lose their value as a
consistency check if the code uses them as well.


Tools
-----
:mps:tag:`test.uniq` The Unix command::

    sed -n '/^#define [a-zA-Z]*Sig/s/[^(]*(/(/p' code/*.[ch] | sort | uniq -c

will display all signatures defined in the MPS along with a count of how
many times they are defined.  If any counts are greater than 1, then the
same signature value is being used for different signatures.  This is
undesirable and the problem should be investigated.


References
----------
.. [RB_1995-08-25] Richard Brooksby. Harlequin. 1995-08-25. "`design.mps.sig: The design of the Memory Pool System Signature System <https://info.ravenbrook.com/project/mps/doc/2002-06-18/obsolete-mminfo/mminfo/design/mps/sig/>`__".

.. [THVV_1995] Tom Van Vleck. 1995. "`Structure Marking <http://www.multicians.org/thvv/marking.html>`__".