For example, consider two developers, dangermouse and penfold, who are both working on whizprog.c. They each start with version 1.4 of the file. dangermouse commits his changes, creating version 1.5. Before penfold can commit his changes, the source code management system notices that the file has changed in the repos- itory. penfold must first merge dangermouse's changes into his working copy. If there are no conflicts, he can then commit his changes, creating version 1.6. On the other hand, if there are conflicts, he must first resolve them (they'll be marked in the working copy), and only then may he commit his version. The combination of the "copy, modify, merge" model with a networked client/server facility creates a powerful environment for doing distributed development. Devel- opers no longer have to worry about file locks. Because the source code management system enforces serialization (making sure that new changes are based on the latest version in the repository), development can move more smoothly, with little danger of miscommunication or that successive changes will be lost. Source Code Management Systems There are several source code management systems used in the Unix community: SCCS The Source Code Control System. SCCS is the original Unix source code management system. It was developed in the late 1970s for the Programmer's Workbench (PWB) Unix systems within Bell Labs. It is still in use at a few large longtime Unix sites. However, for a long time it was not available as a standard part of most commercial or BSD Unix systems, and it did not achieve the widespread popularity of other, later systems. (It is still available with Solaris.) SCCS uses a file storage format that allows it to retrieve any version of a source file in constant time. RCS The Revision Control System. RCS was developed in the early 1980s at Purdue University by Walter F. Tichy. It became popular in the Unix world when it was shipped with 4.2 BSD in 1983. At the time, Berkeley Unix was the most widely used Unix variant, even though to get it, a site had to have a Unix license from AT&T. RCS is easier to use than SCCS. Although it has a number of related commands, only three or four are needed for day-to-day use, and they are quickly mastered. A central repository is easy to use: you first create a direc- tory for the sandbox. In the sandbox, you make a symbolic link to the repository named RCS, and then all the developers can share the repository. RCS uses a file format that is optimized for retrieving the most recent version of a file. CVS The Concurrent Versions System. CVS was initially built as a series of shell scripts sitting atop RCS. Later it was rewritten in C for robustness, although still using RCS commands to manage the storage of files. However, for quite some time, CVS has had the RCS functionality built into it, and it no longer requires that RCS be available. The file format continues to be the same. CVS Source Code Management Source Code Management Systems | This is the Title of the Book, eMatter Edition Copyright © 2006 O'Reilly & Associates, Inc. All rights reserved. 795