--- /dev/null
+#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 */