Aucb.563
fa.editor-p
utcsrgv!utzoo!decvax!ucbvax!C70:editor-people
Mon Mar  1 15:04:20 1982
Concurrent access problem
>From Goldberg@RUTGERS Mon Mar  1 11:44:26 1982
The solution I have used in the editors I have written works as follows:

1.  Before allowing a user to edit the file with intent to change it
(i.e. not read-only mode) the editor ensures that nobody else is doing the
same and simultaneously locks the file.  On TOPS20 and TOPS10, this operation
can be performed by opening the file in READ and WRITE mode upon reading it
in.  Using this method, you also discover if the user has the priviledges
to write out the edited copy to replace the original, which is also
desirable.  The operating system does not allow more than one process to
open the file this way.  By interrogating the error flags upon a failure,
the editor program can tell if the opening failed due to simultaneous
access or for other reasons.

2.  If another user tries to edit the file with intent to change it, the
editor discovers that another user is doing the same and lets the user edit
in read-only mode, if desired.

3.  When the original user stops editing the file in whatever manner, the
file channel (JFN) that was opened at the beginning of the editing (in
READ/WRITE mode, recall) is closed so that others may now edit the file.

This method prevents two users from editing the same source and producing
two separate modified versions.  Furthermore, it prevents the problem
before any wasted editing is done.  However, the operating system under
which the editor runs must provide a file opening mode in which the
file is write-locked.  I vaguely recall that TENEX differs from TOPS20
in NOT providing this mode; and of course, other operating systems fail
to provide it as well.
                                       Bob
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