Thread: How do I use this function?

I'm reading this wonderful book called The C Programming Language 2nd Edition and I'm having a hard time understanding how to use this function that is on page 97. Here it is:

Code:

int getint(int *pn)
{
    int c, sign;

    while (isspace(c = getch()))
        ;
    if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
        ungetch(c);
        return 0;
    }
    sign = (c == '-') ? -1 : 1;
    if (c == '+' || c == '-')
        c = getch();
    for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');
    *pn *= sign;
    if (c != EOF)
        ungetch(c);
    return c;
}

Can someone show me an example of this function in action and also explain what is up with

Code:

for (*pn = 0; isdigit(c); c = getch())
        *pn = 10 * *pn + (c - '0');

? I just don't get these two lines. Thanks a lot.