32.1.3.1 Basic Version Control with Merging

When your version control system is merging-based (the default for CVS and all newer version control systems), work files are always writable; you need not do anything special to begin editing a file. The status indicator on the mode line is ‘-’ if the file is unmodified; it flips to ‘:’ as soon as you save any changes (see VC Mode Line).

Here is what C-x v v does when using a merging-based system:

• If the work file is in a directory that is not controlled by any version control system, prompt for a repository type. Then, create a version control repository of that type and register the file with it.
• If the work file is in a directory that is controlled by a version control system but not registered with it, register the file.
• If the work file is the same as in the repository, do nothing.
• If you have not changed the work file, but some other user has checked in changes to the repository, merge those changes into the work file.
• If you have made modifications to the work file, attempt to commit the changes. To do this, Emacs first reads the log entry for the new revision (see Log Buffer). If some other user has committed changes to the repository since you last checked it out, the checkin fails. In that case, type C-x v v again to merge those changes into your own work file; this puts the work file into a “conflicted” state. Type C-x v v to clear the “conflicted” state; VC then regards the file as up-to-date and modified, and you can try to check it in again.

  To pick up any recent changes from the repository without trying to commit your own changes, type C-x v m <RET>. See Merging.

These rules also apply when you use RCS in its “non-locking” mode, except that changes are not automatically merged from the repository. Nothing informs you if another user has committed changes in the same file since you began editing it; when you commit your revision, his changes are removed (however, they remain in the repository and are thus not irrevocably lost). Therefore, you must verify that the current revision is unchanged before checking in your changes. In addition, locking is possible with RCS even in this mode: C-x v v with an unmodified file locks the file, just as it does with RCS in its normal locking mode (see VC With A Locking VCS).