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Foreign interface basics

Problem

We want to use C libraries from Scheme without writing C code ourselves.

Solution

We'll use PLT Scheme's foreign module to drive libffi. Note: the foreign module is currently only available for v299.

The foreign module is part of the MzScheme core library, so we'll require it with:

(require (lib "foreign.ss"))

We can load a C library with ffi-lib:

(define glib (ffi-lib "libglib-2.0.so"))

The GLib library exports a function, g_print, that sends a formatted message to a print handler (usually a stdout printer).

Its signature is:

void g_print(const gchar* format, ...);

Let's ignore the varargs and treat the gchar* type as a constant C character string. The function type we care about is then:

g_print: string -> void

We can fetch it from the loaded library with get-ffi-obj:

(define g_print
  (get-ffi-obj "g_print" glib
     (_fun _string -> _void)))

There are three arguments to get-ffi-obj: the name of the C object we're importing, the library object to load it from, and its ffi type.

The ffi type (_fun _string -> _void) is a function that consumes a string and has no return value. Note that ffi type names start with an underscore.

Now we can use g_print as if it were a Scheme function:

(g_print "Hello, C")

We can also use Scheme functions as C callbacks. We'll make use of one right now to redirect the output from g_print. GLib allows us to install a print handler with g_set_print_handler:

GPrintFunc g_set_print_handler(GPrintFunc handler);

The GPrintFunc type is that of a function that only consumes a string. We'll define both GPrintFunc and g_set_print_handler in Scheme:

(define _GPrintFunc (_fun _string -> _void))

(define g_set_print_handler
  (get-ffi-obj "g_set_print_handler" glib
    (_fun _GPrintFunc -> _pointer)))

Now let's install a print handler and test it:

(require (lib "mred.ss" "mred"))
(g_set_print_handler (lambda (str)
                       (message-box "GLib message" str)))

(g_print "Hello from C!")

Comments about this recipe

Contributors

-- DanielSilva - 24 Sep 2004