An Intro to Concurrent Versions System (CVS)

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An Intro to Concurrent Versions System (CVS)
ECE 417/617Elements of Software Engineering
Stan Birchfield Clemson University
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What is CVS?

• CVS
• 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

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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)

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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

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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
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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 -- CVS
(admin. files) -- MainFrame.cpp --
SettingsDlg.cpp
Note All information stored on a per-file basis
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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 its there
• CVS_PASSFILE can be used to change location of
  .cvspass
• Note pserver authentication provides minimal
  security do NOT use an important password

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CVS commands

• Syntaxcvs goptions command coptions
  arguments
• Examplecvs -d pserverme_at_cvs.ces.clemson.edu/p
  ub/cvsprojects/ece417 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

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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

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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 youve made to a file,
• cvs commit files
• To tag a file revision,
• cvs tag c MyProject-v1.0.0.0 files
• (-c warns and aborts if source files are not
  up-to-date)

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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
  someones 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 its
  up-to-date

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Other useful commands

• To get information about a file,
• cvs status file
• To examine a files log,
• cvs log file
• To see who made what change,
• cvs annotate file

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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 ltltltlt (changes1) (changes2) gtgtgtgt
• Example
• main() ltltltltltltlt hello.c puts("Hello,
  World!") printf("Hello World")
  gtgtgtgtgtgtgt 1.2

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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 nameepswd
• 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

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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

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Suggested CVS policies

• Each file has an owner
• You may
• completely modify your own file (but dont
  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 elses 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)

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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.,
  emails)
• Build early, build often (BEBO)
• Tag early, tag often
• Save executables associated with tagged builds

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Obtaining CVS

• CvsGui (WinCVS/MacCvs,gCvs) http//www.wincvs.or
  gDownload and install this is a GUI front-end
  and also includes CVSCVS http//www.cvshome.org
  If you just want the command-line version
  documentation is here, too