How to call C procedures from a Modula-3 program?

Calling C from Modula-3 is usually straight forward. You must declare the procedures as EXTERNAL in an interface. You must take care to ensure that the parameter types, especially their sizes, in the Modula-3 declaration and the C implementation of the procedure match exactly; otherwise your program will most likely crash or behave in other unwanted ways. It is also important to note that objects in the heap may move so that object references -- including text -- are no real good choice to pass. It is usually best to declare a local variable of known size and storage location and use this. Have a look at the M3toC module to see how to convert TEXTs to C strings etc.

Here's an example:

/* Ccode.c */

#include <stdio.h>

void age (int a) {
  printf ("%d\n", a);
};

(* Ccode.i3*)
INTERFACE Ccode;

IMPORT Ctypes;

<*EXTERNAL*> PROCEDURE age (a: Ctypes.int);

END Ccode.

(* Main.m3 *)
UNSAFE MODULE Main;
IMPORT Ccode;

BEGIN
  Ccode.age(19);
END Main.

(* m3makefile * )

import("libm3")

interface ("Ccode")
c_source ("Ccode")
implementation("Main")
program("mixed")