1. An Intro to Concurrent Versions System (CVS)
ECE 417/617: Elements of Software Engineering
Stan Birchfield
Clemson University

2. What is CVS? CVS What does it do? What is it not?
is a version (or revision) control system
maintains entire history of source file (for each change: who, when, and why)
is open-source and free (unlike SourceSafe)
What does it do?
stores entire history of each file efficiently
allows multiple people to work simultaneously
enables retrieval of old systems
helps to manage different versions, releases
works well over WAN (server/client)
works for any ASCII file, and limited support for binary
What is it not?
a build system
a substitute for management or communication

3. Why CVS?
Problem: how to coordinate source file changes from multiple developers?
Solution #1: Manual merging and coordination
Solution #2: lock-modify-unlock (RCS, SCCS)
Solution #3: concurrent development (CVS)

4. How CVS manages changes
Scenarios:
People work on different files  trivial
People work on different parts of the same file  CVS essentially runs ‘diff’
People work on the same part of the same file  CVS alerts user, requires manual merging of changes

5. CVS model Server (Unix/Linux) Local machine (any O/S)
checkout, update, diff
Respository: central, official copy, as well as entire history
Local copy: used for editing
add, remove, commit, tag
Note: checkout is a one-time operation

6. CVS directory structure
Repository on server
Copy on local machine
/pub/cvsprojects/ece417 |
+--CVSROOT (admin. files)
+--3dmm
| +-- data
| main.cpp,v
| database.cpp,v
| +-- ui
| MainFrame.cpp,v
| SettingsDlg.cpp,v |
+--klt-ui
+--bibtex-manager
...
C:/user/me/mycode |
+--CVS (admin. files) |
+--data
+-- CVS (admin. files)
+-- main.cpp
+-- database.cpp
+--ui
+-- MainFrame.cpp
+-- SettingsDlg.cpp
Note: All information stored on a per-file basis

7. Pserver authentication
Each CVS command needs to authenticate you
Instead of a password for each command, first login
Steps:
cvs login (server checks its passwd file; if no passwd, then falls back to Unix system authentication)
now your password is stored locally in ~/.cvspass
local password is automatically used in the future as long as it’s there
CVS_PASSFILE can be used to change location of .cvspass
Note: pserver authentication provides minimal security; do NOT use an important password

8. CVS commands Syntax: cvs [goptions] command [coptions] [arguments]
Example: cvs -d login
Common commands:
cvs checkout modules ; create private copy
cvs update ; incorporate others’ changes into private copy
cvs add file ; add private file to repository
cvs remove file ; remove file from repository
cvs commit file ; publish changes to others
Customizing:
.cvsrc file contains default options, both global and command-specific

9. Create / checkout module
To put a directory under CVS control,
cvs import –m “New file” foo me start
All files in current directory are stored in foo subdirectory of repository
-m “New file” specifies the log comment
foo is directory name in repository
me is vendor tag (can be anything)
start is start tag (can be anything)
To checkout a directory,
cvs checkout foo
All files in foo subdirectory of repository are copied to current directory

10. Changing files To add a file to the repository,
cvs add file ; takes effect at the next commit
To remove a file from the repository,
cvs remove file ; takes effect at the next commit
To commit changes you’ve made to a file,
cvs commit files
To tag a file revision,
cvs tag –c MyProject-v files
(-c warns and aborts if source files are not up-to-date)

11. Bringing files up to date
To bring local files up to date,
cvs update
All files in current directory are updated using repository
For each file, prints one-letter code:
U (update): your file has been updated with someone’s changes
M (merge): same, but you had made changes to the file, too
C (conflict): same, but CVS could not merge the changes
? (unknown): file is not under CVS control
Note: You cannot commit a file unless it’s up-to-date

12. Other useful commands To get information about a file,
cvs status file
To examine a file’s log,
cvs log file
To see who made what change,
cvs annotate file

13. Conflicts
If two people modify the same part of a file, there is a conflict
CVS retains both sets of changes in the file, separated by <<<< (changes1) ==== (changes2) >>>>
Example:
main() { <<<<<<< hello.c puts("Hello, World!"); ======= printf("Hello World"); >>>>>>> 1.2 }

14. Setting up server Create a CVS repository if there is not already one
Add this to /etc/services: cvs 2401/tcp # remote cvs server
Add this to /etc/inetd.conf: cvs stream tcp nowait root /path/to/cvs cvs pserver
Restart inetd
To encrypt password for CVSHOME/passwd file,
/usr/local/bin/perl e 'print crypt("MyPassword","St") . "\n";‘
copy and paste into passwd file as name:epswd
To use RSH instead, use :ext instead of :pserver for CVSROOT. Might want to add set CVS_RSH to ssh which makes CVS use SSH and encryption for remote access.
Initialize CVS repository,
cvs –d /usr/local/cvsrep init ; sets up CVSROOT directory

15. Versions and revisions
Each file has a unique revision number
Even number of dot-separated integers (start with 1.1)
Successive revisions increment the rightmost number
Each branch has a unique branch number
Number appended to revision number
Correspond to user versions

16. Suggested CVS policies
Each file has an owner
You may
completely modify your own file (but don’t change class interface)
make minor/obvious bug fixes to others’ files, but only if 100% confident (also notify them, just in case)
ask for permission before making bigger changes, or request the change from the developer
Alternative: You are never allowed to modify anyone else’s file
Good: no chance of conflict
Bad: extra overhead, not taking advantage of the “C” in CVS (might as well be using lock-modify-unlock)

17. Other suggestions Make sure system clock reflects real time
Frequently update (stay in sync with repository)
Frequently commit changes (once compiled, completed, tested, reviewed)
Do NOT share code except via repository (e.g., s)
Build early, build often (BEBO)
Tag early, tag often
Save executables associated with tagged builds

18. Obtaining CVS
CvsGui (WinCVS/MacCvs,gCvs): Download and install; this is a GUI front-end and also includes CVS (latest version: 1.2)
CVS: If you just want the command-line version; documentation is here, too (latest version: 1.11)