Dynamic FFI

ECL's unstable version (in CVS) now contains means both to call C functions and to export functions to the C world (callbacks). Furthermore, this project, initiated by M. Goffioul, has led to an implementation that, at least on the Intel platform, does not rely on the C compiler, but on assembler code generated run time. We hope to port this to the remaining architectures: PPC, Sparc, x86_64

If you want to test it, save the code below in a file called "example.lsp" and load it in the interpreter (if on x86/Windows,Linux,FreeBSD) or compile it and load it (if on other platforms).

The output should look as follows: the first column is the input value, the second and fourth column are the output of either a callback or a C function (in this case "sin"). Other columns are provided to compare accuracy of the result.

Value 1+ (cback) lisp sin(cfun) lisp

     0        1.0        1.0        0.0        0.0
     1        2.0        2.0 .841470985 0.84147096
     2        3.0        3.0 .909297427  0.9092974
     3        4.0        4.0 .141120008    0.14112
     4        5.0        5.0 -0.7568025 -0.7568025
     5        6.0        6.0 -.95892427 -0.9589243
     6        7.0        7.0 -0.2794155 -0.2794155
     7        8.0        8.0 .656986599  0.6569866
     8        9.0        9.0 .989358247 0.98935825
     9       10.0       10.0 .412118485  0.4121185

;; Notice that the following example will work on all platforms if compiled
;; as in
;; (COMPILE-FILE "example.lisp")
;; (LOAD-FILE "example")
;; However, on the x86 platform (i.e. Intel running Linux, FreeBSD, Windows,
;; etc on 32 bit modes), this example also works when loaded from the
;; interpreter. In this case, assembly code is built at run-time and there
;; is no need to have a C compiler around. Ports to other architectures are
;; being developed and any kind of help is welcome.

;; This is a "callback", i.e. a function that can be called from the C world.
;; We can retreive a UFFI pointer to this function using #'FFI:CALLBACK, as
;; shown below.
(ffi:defcallback increment :double ((x :double))
(1+ x)

;; This is a foreign function, i.e. a C function that can be called from the
;; lisp world. Here we do not need to handle pointers: a lisp function called
;; C-SIN is created that does the job of translating arguments for us. The
;; syntax here is pure UFFI.
(ffi:def-function ("sin" C-sin) ((x :double)) :returning :double :module :default)

(format t "Value 1+ (cback) lisp sin(cfun) lisp ~%")
(format t "=====================================================~%")

(dotimes (i 10)
(format t "~10D ~10f ~10f ~10f ~10f~%"
(si::call-cfun (ffi:callback 'increment) :double '(:double) (list i))
(1+ i)
(C-sin i)
(sin i)))

Posted by Juan Jose Garcia Ripoll 2012-07-25

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