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XXCOPY TECHNICAL BULLETIN #44
From: Kan Yabumoto firstname.lastname@example.org
To: XXCOPY user
Subject: Concurrent access of a file
In a multi-tasking operating system such as Windows, many
processes (executing programs) are running at any given moment.
Inevitably, there will be instances where a file is accessed
by two or more processes at the same time.
Ideally speaking, if all programs completely avoid accessing
the same file, there will be no problems except every program
needs to wait for its turn for access. Therefore, if the
programs coordinate their accesses to a common file whenever
the simultaneous access can be safely performed, the system
performance can be increased. For example, when all programs
that access the file in read-only mode, then, sharing the files
should be done and such file sharing takes place all the time.
In the Windows system, files are available for access on a
first-come-first-served basis. That is, when a file is currently
not opened by any program, it is available to any program (provided
that the log-in user has the necessary access permission) in any
share mode it wants. The programmer who writes the application
has discression in the determining the file share setting. The
application declares the file share mode setting at the time of
opening the file. The default mode of opening a file is an
exclusive access (not to share with any other program until the
file is closed). Fair-minded programs often share the file they
open for read-only purposes.
The file share control switch:
XXCOPY (starting with ver 2.96.0) now allows the user to
select how the file sharing is performed with the following
/SHR // Enables read-share (default)
/SHW // Enables write-share
/SHRW or /SHWR // Enables rd/wr-share (old default)
/SH0 // Disables file share (most conservative)
The great majority of file-copy operations can be safely carried
out with the default (/SHR) setting. Since XXCOPY does not alter
the contents of the source file, using the read-share (/SHR)
setting allows it to share a file that has been opened by another
program that declared its willingness to share the file on a
When a program modifies the contents of a file, it typically opens
the file for read-write purposes with no share allowed. In this
case, XXCOPY will have to wait for the file to become accessible
(XXCOPY's /CR switch determines the length of the retry period).
But, there are rare cases where a program opens a file for
read/write purposes, yet allows other programs such as XXCOPY
to concurrently access the file by allowing a read-write share.
Then, XXCOPY with the /SHRW switch can access the file and make
its copy. In such a case, there is an implicit "trust" by XXCOPY
to the other program that first opened the file to behave
reasonably in order to maintain the integrity of the shared file.
If the other program modifies part of the file in such a way that
the snapshot of the given file (read by XXCOPY) loses the
coherency, then, the copy of the file may become "corrupt" even
though XXCOPY completes the copy operation "without error".
We believe there are rare but useful occasions where copying
a file regardless of the integrity of the file contents is
beneficial. XXCOPY allows for the use of /SHRW (or even /SHW)
when the particular situation calls for it.
Then, there are cases where the user does not want to copy a
file which is being opened by another program even though the
conservative sharing mode (/SHR) makes a perfectly safe copy.
In such a case, /SH0 disallows XXCOPY to access any file that
is being opened by another program.
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