This page is part of the web mail archives of SRFI 50 from before July 7th, 2015. The new archives for SRFI 50 contain all messages, not just those from before July 7th, 2015.
> Marc> To avoid the tedium of writing by hand the associated Scheme-to-C and > Marc> C-to-Scheme conversion functions (i.e. "FILENAME_FROM_SCHEME" and > Marc> "FILENAME_TO_SCHEME", "STREAM_FROM_SCHEME", ...), a way to define type > Marc> aliases on the C side of the FFI is useful so that one can write in a > Marc> C header file something like: > > Marc> /* file: "my_types.h" */ > Marc> #include "basic_types.h" > Marc> #ifdef UNICODE > Marc> #define FILENAME NONNULL_UCS2STAR > Marc> #define FILENAME_FROM_SCHEME NONNULL_UCS2STAR_FROM_SCHEME > Marc> #define FILENAME_TO_SCHEME NONNULL_UCS2STAR_TO_SCHEME > Marc> #else > Marc> #define FILENAME NONNULL_CHARSTAR > Marc> #define FILENAME_FROM_SCHEME NONNULL_CHARSTAR_FROM_SCHEME > Marc> #define FILENAME_TO_SCHEME NONNULL_CHARSTAR_TO_SCHEME > Marc> #endif > > I think I'm missing a crucial bit here: Where do > NONNULL_UCS2STAR_FROM_SCHEME, NONNULL_UCS2STAR_TO_SCHEME and > associates come from? What's "a way to define type aliases on the C > side of the FFI"? These are conversion functions (in C) defined by the FFI (they take a bunch of parameters that are irrelevant to this discussion). The C code generated by the Gambit compiler contains calls to these conversion functions at the appropriate places. What I am saying is that the C code generated to interface to the function defined by (define-c-function (open-path FILENAME) STREAM "open_path") contains a call to FILENAME_FROM_SCHEME (and STREAM_TO_SCHEME). By defining FILENAME_FROM_SCHEME to be synonymous to NONNULL_CHARSTAR_FROM_SCHEME we get a conversion to a "char*" string. By defining FILENAME_FROM_SCHEME to be synonymous to NONNULL_UCS2STAR_FROM_SCHEME we get a conversion to a "wchar_t*" string. Does that make it clearer? [Note: this is not exactly the syntax or names that Gambit uses... I have simplified things to ease comprehension of the general approach.] Marc