Use the built-in chmod command rather than system calls. The first element in the list must be un numerical mode and must be an octal number (and not a string containing the number). Also, contrary to what you would do at the shell prompt under U*ix, don't forget the comma between the mode and the list of files (I once wasted half an hour before I figured out that).

In brief, the typical syntax should be something like this:

Check also for the chown function.