MLton’s ForeignFunctionInterface allows an SML program to import C functions. Suppose you would like to import from C a function with the following prototype:
int foo (double d, char c);
MLton extends the syntax of SML to allow expressions like the following:
_import "foo": real * char -> int;
This expression denotes a function of type real * char -> int whose behavior is implemented by calling the C function whose name is foo. Thinking in terms of C, imagine that there are C variables d of type double, c of type unsigned char, and i of type int. Then, the C statement i = foo (d, c) is executed and i is returned.
The general form of an _import expression is:
_import "C function name" attr... : cFuncTy;
The type and the semicolon are not optional.
The function name is followed by a (possibly empty) sequence of attributes, analogous to C __attribute__ specifiers. For now, the only attributes supported are cdecl and stdcall. These specify the calling convention of the C function on Cygwin/Windows, and are ignored on all other platforms. The default is cdecl. You must use stdcall in order to correctly call Windows API functions.
Example
import.sml imports the C function ffi and the C variable FFI_INT as follows.
(* main.sml *)
(* Declare ffi to be implemented by calling the C function ffi. *)
val ffi = _import "ffi" public: real array * int * int ref * char ref * int -> char;
open Array
val size = 10
val a = tabulate (size, fn i => real i)
val ri = ref 0
val rc = ref #"0"
val n = 17
(* Call the C function *)
val c = ffi (a, Array.length a, ri, rc, n)
(* FFI_INT is declared as public in ffi-import.c *)
val (nGet, nSet) = _symbol "FFI_INT" public: (unit -> int) * (int -> unit);
val _ = print (concat [Int.toString (nGet ()), "\n"])
val _ =
print (if c = #"c" andalso !ri = 45 andalso !rc = c
then "success\n"
else "fail\n")
ffi-import.c is
#include "export.h"
Int32 FFI_INT = 13;
Word32 FFI_WORD = 0xFF;
Bool FFI_BOOL = 1;
Real64 FFI_REAL = 3.14159;
Char8 ffi (Pointer a1, Int32 a1len, Pointer a2, Pointer a3, Int32 n) {
double *ds = (double*)a1;
int *pi = (int*)a2;
char *pc = (char*)a3;
int i;
double sum;
sum = 0.0;
for (i = 0; i < a1len; ++i) {
sum += ds[i];
ds[i] += n;
}
*pi = (int)sum;
*pc = 'c';
return 'c';
}
Compile and run the program.
% mlton -default-ann 'allowFFI true' import.sml ffi-import.c % ./import 13 success