Linux kernel function Doesn't return correct value?

Question

I have change linux kernul to add this function (after adding new faild called my_field of type long):

asmlinkage long sys_set_weight(long weight) {
    if (weight < 0)
    {
        return -EINVAL; // -22
    }
    current->my_field = weight;
    return 0;
}

in syscall_64.tbl I have:

334 common  set_weight      sys_set_weight

and in syscalls.h I have:

asmlinkage long sys_set_weight(long weight);

in C when I wrote:

int set_weight(int weight) {
    long r = syscall(334, weight);
    return r;
}

int main () {
    int x = set_weight(-3);
    cout << "set_weight returns: " << x << endl;
    return 0;
}

But I see the following printed:

set_weight returns: 0

why is that? I am retuning -EINVAL not 0

Answer 1

When you call syscall , you are passing through ... . You pass an int through ... , but the function expects a long .

I'm not sure why you made set_weight accept an int when the call expects a long , but you have no prototype that converts it to a long before passing it.