The solution in the link only uses pointers to the string. "In place" as I understood it means not allocating additional memory for the string. His code does

The solution in the link only uses pointers to the string. "In place" as I understood it means not allocating additional memory for the string. His code does that, but it does need two pointers (the one passed in, and the one created to keep track of secondary position) in order to traverse the string. Because the resulting string will only be shorter (never longer), this works. You simply keep track of where you're copying to and where you're copying from, and move character by character until you reach the end. The resulting string may have extra stuff at the end, but since it's null terminated the standard C string functions will not see that cruft as they will stop at the first null.