Experimental dune version of TSDL

(SDL2 bindings for OCaml with runtime dynamic loading)

original: https://github.com/dbuenzli/tsdl

In order to test the experimental version:

opam pin https://github.com/sanette/tsdl.git
opam install tsdl.1.2.0-a2386d5.dune

Back to the official version:

opam unpin https://github.com/sanette/tsdl.git

Why ?

• This version has been reported to work on Windows-Cygwin (see below) (and of course in Linux and Macos too).

• It works with any version of SDL >= 2.0.7. (Of course, if you have a more recent version, more functions will be available)

SDL and Windows

• As far as I understand, SDL+Windows has some peculiarities that make the use of pkg-config for finding C-compile flags inappropriate. In particular, a problem arises because of the way the Main entry point is handled. In principle, you may decide (or not) that SDL takes care of your Main function, so that it can inject OS specific code (for Windows). In this case, you should link with SDL2main and use the -mwindows to tell Windows to use a specific GUI startup. (This is what pkg-config suggests.)

  However, most SDL bindings to other languages (tsdl for OCaml, but also, apparently, bindings for Rust, Zig, Python, Lua) decided not to rely on this and instead call SDL_SetMainReady at init, to tell SDL "we keep our main entry point". In this case, Windows should not add its GUI startup, so one should not link with SDL2main, nor use the -mwindows. Instead, the -mconsole flag would be more appropriate.

  => In this version, I have removed the call to pkg-config. The library uses runtime dynamic loading to access the SDL2 library. The SDL2 shared library is not linked at build time; instead it is located and loaded at runtime using dlopen (or the platform equivalent).

• Another Windows specificity: the "function" SDL_RWclose, which on Linux/MacOS can be bound (by tsdl) via a foreign call, is just a Macro in the Windows version. Hence the foreign call is not recognized, and compilation fails with

  Fatal error: exception
  Dl.DL_error("dlsym: The specified procedure could not be found.")

=> In this version SDL_RWclose is implemented in OCaml by accessing the rw_ops_close entry of the rw_ops structure.

Thanks

Thanks @digitallysane for his patience with numerous tests! sanette/tsdl-ttf#9