Question

I have to use the clone() system call in the main-function to get 2 threads. (I know, there are other options, but in this case, it has to be clone()).

The system call works and both threads arrive in the designated function (foo). But in this function I need them to call another function with this signature:

void increment(int* a, int b)

(Sidenote: It adds b * 1 to a. (= a+b))

The important thing is, that both, a and b, are declared in the main-function and I don’t know how to pass them to foo.

I already tried different things, but without success. I’ve gotten a hint: Use an adapter.
But I have no clue how to do this. (I also dont know how to use the args in clone with int.)

Any suggestions?

Answer 1

One of the arguments to clone() is a void* arg. This lets you pass a void pointer to your function. In order to pass an int pointer and an int instead, you have to create a struct with an int pointer and int assigned to a and b respectively, then cast a pointer to that struct into a void pointer. Then inside the function you reverse the process.

My C is a little rusty and I haven’t compiled this, so don’t quote me on it, but it should look roughly like this:

struct clone_args {
    int* a;
    int b
};

int main(int argc, char* argv[])
{
    struct clone_args args;
    args.a = a;
    args.b = b;
    void* arg = (void*)&args;
    clone(fn, ..., arg, ...);
}

int fn(void* arg)
{
    struct clone_args *args = (struct clone_args*)arg;
    int* a = args->a;
    int b = args->b;
}

Note: take care that the struct you create is still in scope when fn is called, as it isn’t copied. You might have to malloc it.