What is the best way to return an error from a function when I'm already returning a value?

Question

I wrote a function in C that converts a string to an integer and returns the integer. When I call the function I also want it to let me know if the string is not a valid num

Answer 1

a common way is to pass a pointer to a success flag like this:

int my_function(int *ok) {
    /* whatever */
    if(ok) {
        *ok = success;
    }
    return ret_val;
}

call it like this:

int ok;
int ret = my_function(&ok);
if(ok) {
    /* use ret safely here */
}

EDIT: example implementation here:

s32 intval(const char *string, int *ok) {
    bool negative = false;
    u32 current_char = 0;

    if (string[0] == '-') {
        negative = true;
        current_char = 1;
    }

    s32 num = 0;
    while (string[current_char]) {
        if (string[current_char] < '0' || string[current_char] > '9') {
                // Return an error here.. but how?
                if(ok) { *ok = 0; }
        }

        num *= 10;
        num += string[current_char] - '0';
        current_char++;
    }

    if (negative) {
        num = -num;
    }
    if(ok) { *ok = 1; }
    return num;
}

int ok;
s32 val = intval("123a", &ok);
if(ok) {
    printf("conversion successful\n");
}