Question

Suppose there’s a list of arguments stored somehow, in a array for example.

Given a function pointer, how could I make a call to it passing the stored list of arguments?

I’m not trying to pass the array as an argument ok. You got it, ok? I want to pass each of its elements as an argument. An array is just to illustrate, I could be storing the arguments in some tuple structure. Also, look that I have at hand a function pointer and may have a signature in string format. I’m not trying to just define a function that is able to deal with a variadic list.

The only way I see how to do that is by employing assembly (by __asm push et al.) or this:

void (*f)(...);

int main()
{
    f = <some function pointer>;
    int args[]; <stored in a array, just to illustrate>
    int num_args = <some value>;

    switch(num_args)
    {
        case 0:
            f();
        break;

        case 1:
            f(args[0]);
        break;

        case 2:
            f(args[0], args[1]);
        break;

        /* etc */
    }

    return 0;
}

I don’t like this approach too much…

Is there another portable and shorter form?

Several script languages are able to call C functions.

How script languages like Python or Ruby do that? How they implement it in a portable way? Does they just use assembly for several platforms or the above in the end?

Look that I’m really not asking about details of parameter marshaling and other stuff from script languages to C, I’m interested only in how, in the end, internally, the call to the C function by the script language is built.

EDIT

I’ll keep the question’s title but I think a better way for asking it is:

How to call a C function with its pointer and signature available only at runtime?

UPDATE

From Foreign Interface for PLT Scheme:

A call-out is a normal function call. In a dynamic setting,
we create a “call-interface” object which specifies (binary)
input/output types; this object can be used with an arbitrary
function pointer and an array of input values to perform a callout to the function and retrieve its result. Doing this requires
manipulating the stack and knowing how a function is called,
these are details that libffi deals with.

Thanks @AnttiHaapala for searching, finding and pointing libffi. It’s what I was looking for, it’s being used by a bunch of script languages, it’s a portable library, implemented across several architectures and compilers.

Answer 1

I am the author of libffi. It will do what you are asking.