Differentiate between return value and error in C

Question

I am trying to write a function that takes a linked list, an index and returns the value at the index+1 position.

int getData(list *l, int index) {
    if(...) { 
        ...
        return result; // this could be -1
    }

    else {
        return -1;
    }
}

In C you return a value of -1 when the function fails (as a way to let the user know that the call failed because of bad input). However, because the return type of this function is int, it includes the value of -1 (-1 as a number, as well ass -2, -3 etc)

My question is: Is there a mechanism in which you can return from your function and make the user of the function aware that the returned value is in fact an error and not a value?

Answer 1

Return the value by passing a pointer and make the return value an error statement:

int getData(list *l, int inIndex, int* outIndex) {
    if(...) { 
        ...
        *outIndex = result;
        return 0;
    }

    else {
        return -1;
    }
}