Calling a bound function

Question

I'm trying to achieve something I've easily done in dynamic languages, but have no idea how to do in C.

I have a program that needs to call different functions depending on a given string by the user. Each function has a C file of its own.

The easy way to do this is just do if-else-elseif along with strcmp and call each function if it matches.

I want do to something nicer: at the end of each C file holding each significant function, add a "binding" call, a bit like this:

int my_action(char smth[]);

bind_function("myaction", my_action);

Then, in the main program, be able to look up the function for "myaction" string, and if it exists, call it.

I reckon it has to do with mapping a string to a pointer to a function. But, honestly, I have no idea if it's possible or where to start.

Is such a thing possible in C? If it is, then, how can I do it?

Answer 1

It is possible and the Linux Kernel implements this a lot. See the MACHINE_START macro for example. However, it is very platform, compiler and linker specific. You can't do this in plain, standard C.

Under gcc, I can write something like:

struct function_binding {
    const char *name;
    int (*func)(char *);
};

#define bind_function(binding_name, func_name)          \
    static                          \
    __attribute__((section(".funcbind")))           \
    __attribute__((used))                   \
    struct function_binding __##func_name##_binding = { \
        .name = binding_name, .func = func_name     \
    }

int test_func(char *x) {}

bind_function("test_func", test_func);

Which will put all your function bindings into a section called .funcbind. You'll then want to edit your linker script to add something like this:

function_binding_list = .;
*(.funcbind);
LONG(0); /* Terminate list */

In the file you want to traverse the list in, you can simply use a loop:

extern struct function_binding function_binding_list[];
struct function_binding *curr;
for (curr = function_binding_list; curr->name != NULL; curr++) {
    if (!strcmp(curr->name, name)) {
        curr->func(args);
        break;
    }
}