Question

Is there any way to create callbacks and signals in C?

I can use function pointer of course, but I want create a signal that can hold multiple functions i.e can bind to multiple events. Something like boost.signals but in C . Is there any way?

There are many gui libraries in C that use callbacks, how do they achieve it?

Answer 1

A simple solution is to keep a list of functions to call, and in the signal handler just iterate over that list calling every function.

Something like this:

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

typedef void (*event_callback_t)();

struct event_handler_node
{
    event_callback_t callback;
    struct event_handler_node *next;
};

void add_event_handler(struct event_handler_node **list_head,
                       event_callback_t callback)
{
    struct event_handler_node *node = malloc(sizeof(struct event_handler_node));

    node->callback = callback;
    if (*list_head != NULL)
        node->next = *list_head;
    *list_head = node;
}

void call_all_event_handlers(struct event_handler_node *list_head)
{
    struct event_handler_node *node;

    for (node = list_head; node != NULL; node = node->next)
    {
        (*node->callback)();
    }
}

struct event_handler_node *signal_callbacks = NULL;
void signal_handler(int signum)
{
    call_all_event_handlers(signal_callbacks);
}

void my_callback()
{
    printf("Hello from callback!\n");
}

int main()
{
    /* Add a callback to the list */
    add_event_handler(&signal_callbacks, my_callback);

    /* Set signal handler for a signal */
    signal(SIGUSR1, signal_handler);

    /* Invoke the signal handler */
    kill(getpid(), SIGUSR1);

    return 1;
}

Note: There is no error handling, and no freeing of allocated memory. That’s left as an exercise for the reader. 🙂