When you start dealing with large software projects composed of multiple source files, or even multiple libraries and programs, you will almost inevitably run up against the problem of revision management. This is especially true if you are not the only person working on a piece of software. Unless you use some kind of revision management software you may find that modifying and maintaining a large project, especially with several active programmers, is at the least difficult, and probably next to impossible for any project which could really be called "large".
Fortunately there are at least two free revision management packages available. One is RCS, (Revision Control System) and the other is CVS. I won't go into the details here, but here is documentation for CVS from the Free Software Foundation.