How do I wrap C consts in a Ruby FFI module?

Question

I have a (somewhat exorbitant) number of these:

extern TCODLIB_API const TCOD_color_t TCOD_red;
extern TCODLIB_API const TCOD_color_t TCOD_flame;
extern TCODLIB_API const TCOD_color_t TCOD_orange;
extern TCODLIB_API const TCOD_color_t TCOD_amber;
extern TCODLIB_API const TCOD_color_t TCOD_yellow;
extern TCODLIB_API const TCOD_color_t TCOD_lime;

How can I get at the values on the Ruby side nicely? It feels like there should be an attach_const equivalent to attach_function, as used below:

module TCOD
  extend FFI::Library
  ffi_lib File.join(APP_ROOT, "libtcod-1.5.1/libtcod.so")
  attach_function :color_RGB, 'TCOD_color_RGB', [:uchar, :uchar, :uchar], Color.val
end

I'd prefer not to have to redefine everything from lightest_sepia to desaturated_chartreuse if at all possible...

Asherah · Accepted Answer · 2013-06-01 11:31:50Z

4

There is, attach_variable is what you're looking for.

This will work for anything that is actually a global variable (which looks to be your case) but not if the constants are just #define macros.

Quoting the FFI::Library documentation for examples:

module Bar
  extend FFI::Library
  ffi_lib 'my_lib'
  attach_variable :c_myvar, :myvar, :long
end
# now callable via Bar.c_myvar

and:

module Bar
  extend FFI::Library
  ffi_lib 'my_lib'
  attach_variable :myvar, :long
end
# now callable via Bar.myvar

edited Jun 1, 2013 at 11:31

Asherah

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answered Jun 1, 2013 at 10:11

Frederick Cheung

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3 Comments

Mispy Over a year ago

Pity there isn't a solution for #define constants too, but this helps :D

womble Over a year ago

@Mispy: There is a solution for #define constants; see FFI::ConstGenerator.

womble Over a year ago

@stopanko questions go in questions, not comments.

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