notion-libextl: A C repository from jsbackus

libextl

Copyright (c) Tuomo Valkonen 2003-2005.
<tuomov at iki.fi>

Libextl is a small library for very easily extending programs with Lua
scripting. By default the library depends on my libtu available from the
same repository. However, this can be changed by redefining things in
private.h and types.h.

Libextl supports exporting functions that operate on basic data types (int,
bool, double, [const] char*) and references to Lua tables and functions
(ExtlTab, ExtlFn) simply by prefixing the function definition with the
keywords EXTL_EXPORT, EXTL_EXPORT_AS or EXTL_EXPORT_MEMBER. More complex
data must, however, either be proxied libtu objects (or objects of some
other object system with the appropriate macros redefined), or Lua tables.
The binding glue is, however, generated as painlessly as for functions that
operate on basic data types with all pointers to a type with a name that
begins with an uppercase letter considered as such objects. Libextl also
provides functions to manipulate Lua tables through references to these, and
ways to call and load Lua code.


BASIC USAGE

Include <libextl/extl.h> in your source files.

In your Makefile, process source files with libextl-mkexports to generate
exports.c. Pass it the option '-module modname' if you want non-class
functions to be put in a table "modname". (This will however introduce
naming restrictions on the C side; see below.)

Call 'exit_init' and '[modname_]register_exports' at the beginning of your
program and '[modname_]unregister_exports' and 'extl_deinit' at the end of
it.

Mark exported functions as follows in your source files:

  * Global or module functions:
  
      EXTL_EXPORT
      int foobar(const char *s)
      {
          ...
      }

  * Classes and methods:
   
      EXTL_CLASS(Foo, Obj)
   
      EXTL_EXPORT_MEMBER
      void foo_set_callback(Foo *cls, ExtlFn fn)

    'Obj' here stands for the base class of the object system (one in libtu
     by default), but more generally the second parameter to EXTL_CLASS must
     be the name of parent class of the first parameter.

  * Export in given module (plain table) or class:
  
      EXTL_EXPORT_AS(baz, somefun)
      ExtlTab just_some_name_we_dont_use_on_the_lua_side()
  

If you pass libextl-mkexport the option '-module modname', then all
EXTL_EXPORTed functions must be prefixed with 'modname_' and will be put in
the global table 'modname'. If you want to export a function directly in the
global namespace when building a module, use EXTL_EXPORT_AS(global,
funcname).


ADDITIONAL ROUTINES

'luaextl.h' lists a number of routines for accessing references to tables,
and calling Lua functions. How to use them should be fairly obvious.

'readconfig.h' lists a number of routines to load source or compiled files
from a specified path. Their usage should, again, be fairly obvious. These
lookup routines are exported to the Lua side as well in the form of the
function 'dopath'.


USING ANOTHER OBJECT SYSTEM/USING WITHOUT LIBTU

Redefine appropriate macros in private.h and types.h.


NOTES ON DATA REFERENCES

* CHAR* VS. CONST CHAR*

libextl follows the following conventions with regard to const and non-const
char* pointers:

'const char*' as parameter is considered owned by the caller, and the called
function must do nothing beyond accessing it during its execution.

'char*' as parameter is considered owned by the called function, and it must
take care of freeing the parameter when no longer needed.

'const char*' as return value is considered owned by the called function,
and caller must consider it unsafe to use after subsequent calls to related
code.

'char*' as return value is considered owned by the caller, and it must take
care of freeing the value when no longer needed.


* EXTLTAB AND EXTLFN

These references are always owned as a caller. Thus, if a called function
wants to store a reference to such a parameter, it must create a new one
with extl_ref_fn or extl_ref_table. Note that these functions do not return
the same reference as passed; there are no reference counters, just a table
of references to hold the garbage collector from claiming the object
(luaL_ref/luaL_unref).
jsbackus/notion-libextl
