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.
<!IncludeSVN(mlton/trunk/doc/examples/ffi/import.sml, sml)>
ffi-import.c is
<!IncludeSVN(mlton/trunk/doc/examples/ffi/ffi-import.c, c)>
Compile and run the program.
% mlton -default-ann 'allowFFI true' import.sml ffi-import.c % ./import 13 success