NAME

mkVFunc - Support for "nested" dynamic loading

SYNOPSIS

mkVFunc xxx.h

DESCRIPTION

perl/Tk is designed so that Tk can be dynamically loaded 'on top of' perl. That is the easy bit. What it also does is allow Tk::Xxxx to be dynamically loaded 'on top of' the perl/Tk composite. Thus when you 'require Tk::HList' the shared object .../HList.so needs to be able to call functions defined in perl and functions defined in loadable .../Tk.so . Now functions in 'base executable' are a well known problem, and are solved by DynaLoader. However most of dynamic loading schemes cannot handle one loadable calling another loadable.

Thus what Tk does is build a table of functions that should be callable. This table is auto-generated from the .h file by looking for 'extern' (and EXTERN which is #defined to 'extern'). Thus any function marked as 'extern' is 'referenced' by the table. The address of the table is then stored in a perl variable when Tk is loaded. When HList is loaded it looks in the perl variable (via functions in perl - the 'base executable') to get the address of the table.

The same utility that builds the table also builds a set of #define's. HList.c (and any other .c files which comprise HList) #include these #define's. So that

Tk_SomeFunc(x,y,z)

Is actually compiled as

(*TkVptr->V_Tk_SomeFunc)(x,y,z)

Where Tk_ptr is pointer to the table.

See:

Tk-b*/pTk/mkVFunc - perl script that produces tables
         /tk.h        - basis from which table is generated
         /tk.m        - #define's to include in sub-extension
         /tk_f.h      - #included both sides.
         /tk_f.c      - Actual table definition.
         /tk.t        - 'shared' set of macros which produce table
                        included in tk_f.c and tk_f.h
         /tkVMacro.h  - Wrapper to include *.m files

In addition to /tk* there are /tkInt*, /Lang* and /tix*