Just because a function returns a value, doesn't mean you need to use it. The in-place functions return pointers for convenience -- so you can do printf("%s\n", strrev(string)); instead of strrev(string); printf("%s\n", string); But the pointer itself should not change (they'd hardly be "in place" if they did).

Notably, it means you cannot do this:

Code:

char buf[]="slartibartfast";
buf=strrev(buf);

But this will work just fine for a function that works in-place:

Code:

char buf[]="slartibartfast";
strrev(buf);

That other function on the other hand actually does allocate new memory which should be freed later. Take a good look at line 57. What, exactly would it do? You need a free() somewhere, but that's not really a useful place to put it. the return() causes the function to end before its freed. ...and if you put it first, you'd be returning invalid memory -- memory that'd already been freed. That's no good either. So where does it need to go?

Code:

char *newpointer;
printf("%s\n", newpointer=copyreverse(oldpointer));
free(newpointer);

As for why your program didn't work, I'd have to see it to guess. You piled several problems in your example for brevity but they interacted with each other and obscured the point.