--- /dev/null
+#include <stdio.h>
+#include <string.h>
+#include "config.h"
+
+/**
+ * typesafe_cb - macros for safe callbacks.
+ *
+ * The basis of the typesafe_cb header is cast_if_type(): a
+ * conditional cast macro. If an expression exactly matches a given
+ * type, it is cast to the target type, otherwise it is left alone.
+ *
+ * This allows us to create functions which take a small number of
+ * specific types, rather than being forced to use a void *. In
+ * particular, it is useful for creating typesafe callbacks as the
+ * helpers typesafe_cb(), typesafe_cb_preargs() and
+ * typesafe_cb_postargs() demonstrate.
+ *
+ * The standard way of passing arguments to callback functions in C is
+ * to use a void pointer, which the callback then casts back to the
+ * expected type. This unfortunately subverts the type checking the
+ * compiler would perform if it were a direct call. Here's an example:
+ *
+ * static void my_callback(void *_obj)
+ * {
+ * struct obj *obj = _obj;
+ * ...
+ * }
+ * ...
+ * register_callback(my_callback, &my_obj);
+ *
+ * If we wanted to use the natural type for my_callback (ie. "void
+ * my_callback(struct obj *obj)"), we could make register_callback()
+ * take a void * as its first argument, but this would subvert all
+ * type checking. We really want register_callback() to accept only
+ * the exactly correct function type to match the argument, or a
+ * function which takes a void *.
+ *
+ * This is where typesafe_cb() comes in: it uses cast_if_type() to
+ * cast the callback function if it matches the argument type:
+ *
+ * void _register_callback(void (*cb)(void *arg), void *arg);
+ * #define register_callback(cb, arg) \
+ * _register_callback(typesafe_cb(void, (cb), (arg)), (arg))
+ *
+ * On compilers which don't support the extensions required
+ * cast_if_type() and friend become an unconditional cast, so your
+ * code will compile but you won't get type checking.
+ */
+int main(int argc, char *argv[])
+{
+ if (argc != 2)
+ return 1;
+
+ if (strcmp(argv[1], "depends") == 0) {
+ return 0;
+ }
+
+ return 1;
+}