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macros for safe callbacks.


Rusty Russell <>


The basis of the typesafe_cb header is typesafe_cb_cast(): 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 typesafe_cb_cast() 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, void *, (cb), (arg)), \

On compilers which don't support the extensions required typesafe_cb_cast() and friend become an unconditional cast, so your code will compile but you won't get type checking.


#include <ccan/typesafe_cb/typesafe_cb.h>
#include <stdlib.h>
#include <stdio.h>

// Generic callback infrastructure.
struct callback {
        struct callback *next;
        int value;
        int (*callback)(int value, void *arg);
        void *arg;
static struct callback *callbacks;

static void _register_callback(int value, int (*cb)(int, void *),
                               void *arg)
        struct callback *new = malloc(sizeof(*new));
        new->next = callbacks;
        new->value = value;
        new->callback = cb;
        new->arg = arg;
        callbacks = new;
#define register_callback(value, cb, arg)                       \
        _register_callback(value,                               \
                           typesafe_cb_preargs(int, void *,     \
                                               (cb), (arg), int),\

static struct callback *find_callback(int value)
        struct callback *i;

        for (i = callbacks; i; i = i->next)
                if (i->value == value)
                        return i;
        return NULL;

// Define several silly callbacks.  Note they don't use void *!
#define DEF_CALLBACK(name, op)                  \
        static int name(int val, int *arg)      \
        {                                       \
                printf("%s", #op);              \
                return val op *arg;             \
DEF_CALLBACK(multiply, *);
DEF_CALLBACK(divide, /);
DEF_CALLBACK(assign, =);

// Silly game to find the longest chain of values.
int main(int argc, char *argv[])
        int i, run = 1, num = argc > 1 ? atoi(argv[1]) : 0;

        for (i = 1; i < 1024;) {
                // Since run is an int, compiler checks "add" does too.
                register_callback(i++, add, &run);
                register_callback(i++, divide, &run);
                register_callback(i++, sub, &run);
                register_callback(i++, multiply, &run);
                register_callback(i++, or, &run);
                register_callback(i++, and, &run);
                register_callback(i++, xor, &run);
                register_callback(i++, assign, &run);

        printf("%i ", num);
        while (run < 56) {
                struct callback *cb = find_callback(num % i);
                if (!cb) {
                        printf("-> STOP\n");
                        return 1;
                num = cb->callback(num, cb->arg);
                printf("->%i ", num);
        printf("-> Winner!\n");
        return 0;


CC0 (Public domain)