Typesafe callback infrastructure.

[ccan] / typesafe_cb / typesafe_cb.h

diff --git a/typesafe_cb/typesafe_cb.h b/typesafe_cb/typesafe_cb.h
new file mode 100644 (file)
index 0000000..cc4ea8a
--- /dev/null
+++ b/typesafe_cb/typesafe_cb.h
@@ -0,0 +1,120 @@
+#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 */