harbour-core/doc/statics.txt

  I just started implementing Classes and objects creation when I realized
Harbour is not managing static variables yet (Harbour recognizes them but
does not generate the proper pcode for them).

  So I would like to make an introduction to static variables management as
it is a sophisticated system that Harbour is going to implement.

  It is something publicly known that Clipper static variables are
located at the bottom of the data segment. This has caused all kinds of
troubles. This is why when I designed Five I did it in a way that could not
cause any trouble in the future.

  In Harbour all static variables (and I mean on all PRGs) are stored in
just one Harbour array (a Clipper language array), this guarantees that we
may have as many static variables as desired without limits (just limited
by the available memory). This aStatics array is not visible from the
application (PRG level).

  Basically what happens when a function is called and that function uses
static variables, is that the stack sets a pointer to that array section
where our statics are, so from that moment on, accessing a static1, static2,
... is accessing some elements on that array:

    static1 = statics[ statics_base_for_this_function + 1 ]

    ...

    staticn = statics[ statics_base_for_this_function + n ]

  In order to implement this we just use two new pcode opcodes: _STATICS,
_SFRAME. _STATICS makes the global statics array grow enough to hold our new
defined statics:

   _STATICS <n> -->  ASize( aStatics, Len( aStatics ) + <n> )

   _SFRAME --> tell the stack from what location into aStatics are ours.

  _STATICS is just called once for an entire PRG from an init function
named _INITSTATICS (STATICS$ and SINIT in Clipper). That function stores in
a tricky place (its own function pointer in the symbol table!) our statics
base, and later on _SFRAME simply takes it from there and sets it in the
stack. That _INITSTATICS function will perform whatever initialization our
global statics may have defined in that PRG).

  You are going to see the code for all these. I just wanted to provide a
clear idea about how Harbour does its magic :-)


Antonio