Typesafe callback infrastructure.

typesafe_cb/_info.c

+#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;
+}

typesafe_cb/test/compile_fail-cast_if_type-callback-int.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <stdlib.h>
+
+void _callback(void (*fn)(void *arg), void *arg);
+void _callback(void (*fn)(void *arg), void *arg)
+{
+	fn(arg);
+}
+
+/* Callback is set up to warn if arg isn't a pointer (since it won't
+ * pass cleanly to _callback's second arg. */
+#define callback(fn, arg)						\
+	_callback(cast_if_type((fn), void (*)(typeof(arg)), void (*)(void *)), \
+		  arg)
+
+void my_callback(int something);
+void my_callback(int something)
+{
+}
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	callback(my_callback, 100);
+#endif
+	return 0;
+}

typesafe_cb/test/compile_fail-cast_if_type-callback.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <stdlib.h>
+
+static void _callback(void (*fn)(void *arg), void *arg)
+{
+	fn(arg);
+}
+
+#define callback(fn, arg)						\
+	_callback(cast_if_type((fn), void (*)(typeof(arg)), void (*)(void *)), \
+		  arg)
+
+static void my_callback(char *p)
+{
+}
+
+int main(int argc, char *argv[])
+{
+	callback(my_callback, "hello world");
+
+#ifdef FAIL
+	/* Must be a char * */
+	callback(my_callback, my_callback);
+#endif
+	return 0;
+}

typesafe_cb/test/compile_fail-cast_if_type.c

+#include "typesafe_cb/typesafe_cb.h"
+
+void _set_some_value(void *val);
+
+void _set_some_value(void *val)
+{
+}
+
+#define set_some_value(expr)						\
+	_set_some_value(cast_if_type((expr), unsigned long, void *))
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int x = 0;
+	set_some_value(x);
+#else
+	void *p = 0;
+	set_some_value(p);
+#endif
+	return 0;
+}

typesafe_cb/test/compile_fail-typesafe_cb.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <stdlib.h>
+
+static void _register_callback(void (*cb)(void *arg), void *arg)
+{
+}
+
+#define register_callback(cb, arg)				\
+	_register_callback(typesafe_cb(void, (cb), (arg)), (arg))
+
+static void my_callback(char *p)
+{
+}
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int *p;
+#else
+	char *p;
+#endif
+	p = NULL;
+	register_callback(my_callback, p);
+	return 0;
+}

typesafe_cb/test/compile_fail-typesafe_cb_postargs.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <stdlib.h>
+
+static void _register_callback(void (*cb)(void *arg, int x), void *arg)
+{
+}
+#define register_callback(cb, arg)				\
+	_register_callback(typesafe_cb_postargs(void, (cb), (arg), int), (arg))
+
+static void my_callback(char *p, int x)
+{
+}
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int *p;
+#else
+	char *p;
+#endif
+	p = NULL;
+	register_callback(my_callback, p);
+	return 0;
+}

typesafe_cb/test/compile_fail-typesafe_cb_preargs.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <stdlib.h>
+
+static void _register_callback(void (*cb)(int x, void *arg), void *arg)
+{
+}
+
+#define register_callback(cb, arg)				\
+	_register_callback(typesafe_cb_preargs(void, (cb), (arg), int), (arg))
+
+static void my_callback(int x, char *p)
+{
+}
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int *p;
+#else
+	char *p;
+#endif
+	p = NULL;
+	register_callback(my_callback, p);
+	return 0;
+}

typesafe_cb/test/run.c

+#include "typesafe_cb/typesafe_cb.h"
+#include <string.h>
+#include "tap.h"
+
+static char dummy = 0;
+
+/* The example usage. */
+static void _set_some_value(void *val)
+{
+	ok1(val == &dummy);
+}
+
+#define set_some_value(expr)						\
+	_set_some_value(cast_if_type((expr), unsigned long, void *))
+
+static void _callback_onearg(void (*fn)(void *arg), void *arg)
+{
+	fn(arg);
+}
+
+static void _callback_preargs(void (*fn)(int a, int b, void *arg), void *arg)
+{
+	fn(1, 2, arg);
+}
+
+static void _callback_postargs(void (*fn)(void *arg, int a, int b), void *arg)
+{
+	fn(arg, 1, 2);
+}
+
+#define callback_onearg(cb, arg)					\
+	_callback_onearg(typesafe_cb(void, (cb), (arg)), (arg))
+
+#define callback_preargs(cb, arg)					\
+	_callback_preargs(typesafe_cb_preargs(void, (cb), (arg), int, int), (arg))
+
+#define callback_postargs(cb, arg)					\
+	_callback_postargs(typesafe_cb_postargs(void, (cb), (arg), int, int), (arg))
+
+static void my_callback_onearg(char *p)
+{
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_onearg_const(const char *p)
+{
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_onearg_volatile(volatile char *p)
+{
+	ok1(strcmp((char *)p, "hello world") == 0);
+}
+
+static void my_callback_preargs(int a, int b, char *p)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_preargs_const(int a, int b, const char *p)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_preargs_volatile(int a, int b, volatile char *p)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp((char *)p, "hello world") == 0);
+}
+
+static void my_callback_postargs(char *p, int a, int b)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_postargs_const(const char *p, int a, int b)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp(p, "hello world") == 0);
+}
+
+static void my_callback_postargs_volatile(volatile char *p, int a, int b)
+{
+	ok1(a == 1);
+	ok1(b == 2);
+	ok1(strcmp((char *)p, "hello world") == 0);
+}
+
+/* This is simply a compile test; we promised cast_if_type can be in a
+ * static initializer. */
+struct callback_onearg
+{
+	void (*fn)(void *arg);
+	void *arg;
+};
+
+struct callback_onearg cb_onearg
+= { typesafe_cb(void, my_callback_onearg, "hello world"), "hello world" };
+
+struct callback_preargs
+{
+	void (*fn)(int a, int b, void *arg);
+	void *arg;
+};
+
+struct callback_preargs cb_preargs
+= { typesafe_cb_preargs(void, my_callback_preargs, "hi", int, int), "hi" };
+
+struct callback_postargs
+{
+	void (*fn)(void *arg, int a, int b);
+	void *arg;
+};
+
+struct callback_postargs cb_postargs
+= { typesafe_cb_postargs(void, my_callback_postargs, "hi", int, int), "hi" };
+
+int main(int argc, char *argv[])
+{
+	void *p = &dummy;
+	unsigned long l = (unsigned long)p;
+
+	plan_tests(2 + 3 + 9 + 9);
+	set_some_value(p);
+	set_some_value(l);
+
+	callback_onearg(my_callback_onearg, "hello world");
+	callback_onearg(my_callback_onearg_const, "hello world");
+	callback_onearg(my_callback_onearg_volatile, "hello world");
+
+	callback_preargs(my_callback_preargs, "hello world");
+	callback_preargs(my_callback_preargs_const, "hello world");
+	callback_preargs(my_callback_preargs_volatile, "hello world");
+
+	callback_postargs(my_callback_postargs, "hello world");
+	callback_postargs(my_callback_postargs_const, "hello world");
+	callback_postargs(my_callback_postargs_volatile, "hello world");
+
+	return exit_status();
+}

typesafe_cb/typesafe_cb.h

+#ifndef CCAN_CAST_IF_TYPE_H
+#define CCAN_CAST_IF_TYPE_H
+#include "config.h"
+
+#if HAVE_TYPEOF && HAVE_BUILTIN_CHOOSE_EXPR && HAVE_BUILTIN_TYPES_COMPATIBLE_P
+/**
+ * cast_if_type - only cast an expression if it is of a given type
+ * @expr: the expression to cast
+ * @oktype: the type we allow
+ * @desttype: the type to cast to
+ *
+ * This macro is used to create functions which allow multiple types.
+ * The result of this macro is used somewhere that a @desttype type is
+ * expected: if @expr was of type @oktype, it will be cast to
+ * @desttype type.  As a result, if @expr is any type other than
+ * @oktype or @desttype, a compiler warning will be issued.
+ *
+ * This macro can be used in static initializers.
+ *
+ * This is merely useful for warnings: if the compiler does not
+ * support the primitives required for cast_if_type(), it becomes an
+ * unconditional cast, and the @oktype argument is not used.  In
+ * particular, this means that @oktype can be a type which uses
+ * the "typeof": it will not be evaluated if typeof is not supported.
+ *
+ * Example:
+ *	// We can take either an unsigned long or a void *.
+ *	void _set_some_value(void *val);
+ *	#define set_some_value(expr)			\
+ *		_set_some_value(cast_if_type((expr), unsigned long, void *))
+ */
+#define cast_if_type(expr, oktype, desttype)				\
+__builtin_choose_expr(__builtin_types_compatible_p(typeof(1?(expr):0), oktype), \
+			(desttype)(expr), (expr))
+#else
+#define cast_if_type(expr, oktype, desttype) ((desttype)(expr))
+#endif
+
+/**
+ * typesafe_cb - cast a callback function if it matches the arg
+ * @rettype: the return type of the callback function
+ * @fn: the callback function to cast
+ * @arg: the (pointer) argument to hand to the callback function.
+ *
+ * If a callback function takes a single argument, this macro does
+ * appropriate casts to a function which takes a single void * argument if the
+ * callback provided matches the @arg (or a const or volatile version).
+ *
+ * It is assumed that @arg is of pointer type: usually @arg is passed
+ * or assigned to a void * elsewhere anyway.
+ *
+ * Example:
+ *	void _register_callback(void (*fn)(void *arg), void *arg);
+ *	#define register_callback(fn, arg) \
+ *		_register_callback(typesafe_cb(void, (fn), (arg)), (arg))
+ */
+#define typesafe_cb(rettype, fn, arg)					\
+	cast_if_type(cast_if_type(cast_if_type((fn),			\
+					       rettype (*)(const typeof(arg)), \
+					       rettype (*)(void *)),	\
+				  rettype (*)(volatile typeof(arg)),	\
+				  rettype (*)(void *)),			\
+		     rettype (*)(typeof(arg)),				\
+		     rettype (*)(void *))
+
+/**
+ * typesafe_cb_preargs - cast a callback function if it matches the arg
+ * @rettype: the return type of the callback function
+ * @fn: the callback function to cast
+ * @arg: the (pointer) argument to hand to the callback function.
+ *
+ * This is a version of typesafe_cb() for callbacks that take other arguments
+ * before the @arg.
+ *
+ * Example:
+ *	void _register_callback(void (*fn)(int, void *arg), void *arg);
+ *	#define register_callback(fn, arg) \
+ *		_register_callback(typesafe_cb_preargs(void, (fn), (arg), int),\
+ *				   (arg))
+ */
+#define typesafe_cb_preargs(rettype, fn, arg, ...)			\
+	cast_if_type(cast_if_type(cast_if_type((fn),			\
+					       rettype (*)(__VA_ARGS__,	\
+							   const typeof(arg)), \
+					       rettype (*)(__VA_ARGS__,	\
+							   void *)),	\
+				  rettype (*)(__VA_ARGS__,		\
+					      volatile typeof(arg)),	\
+				  rettype (*)(__VA_ARGS__, void *)),	\
+		     rettype (*)(__VA_ARGS__, typeof(arg)),		\
+		     rettype (*)(__VA_ARGS__, void *))
+
+/**
+ * typesafe_cb_postargs - cast a callback function if it matches the arg
+ * @rettype: the return type of the callback function
+ * @fn: the callback function to cast
+ * @arg: the (pointer) argument to hand to the callback function.
+ *
+ * This is a version of typesafe_cb() for callbacks that take other arguments
+ * after the @arg.
+ *
+ * Example:
+ *	void _register_callback(void (*fn)(void *arg, int), void *arg);
+ *	#define register_callback(fn, arg) \
+ *		_register_callback(typesafe_cb_preargs(void, (fn), (arg), int),\
+ *				   (arg))
+ */
+#define typesafe_cb_postargs(rettype, fn, arg, ...)			\
+	cast_if_type(cast_if_type(cast_if_type((fn),			\
+					       rettype (*)(const typeof(arg), \
+							   __VA_ARGS__), \
+					       rettype (*)(void *,	\
+							   __VA_ARGS__)), \
+				  rettype (*)(volatile typeof(arg),	\
+					      __VA_ARGS__),		\
+				  rettype (*)(void *, __VA_ARGS__)),	\
+		     rettype (*)(typeof(arg), __VA_ARGS__),		\
+		     rettype (*)(void *, __VA_ARGS__))
+
+#endif /* CCAN_CAST_IF_TYPE_H */