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Operations on Files

Operations on files:

• Reading, writing, creating, and deleting: These are the basic operations on files. In ANSI-C [21] a file is created implicitly when a non-existing file is opened for reading. (Notice that creating and deleting files are also operations on directories,see Section 3.1.3).
• Scanning (positioning): This can be thought of as reading a file until a certain position is reached, without caring about what was read "on the way there".
• Opening and closing: In many systems, and especially those using session semantics it is impossible to read from or write to a file without having opened it (for reading or writing) first. Reads and writes to a file may be performed locally (i.e., without the rest of the system being able to notice it) before the file is closed.
• Linking and unlinking: UNIX synonyms for creating and deleting. The files are not physically removed, but they are linked to or unlinked from the directory structure, and cannot be referenced any more. The space they previously occupied can be garbage collected. (Linking and unlinking files are operations on directories, see Section 3.1.3).
• Updating: Synonym for reading and writing a file.
• Rewriting: A composite operation; rewrite $<$file$>$ = if $<$file$>$ exists then delete $<$file$>$, create $<$file$>$, write $<$file$>$.
• Copying: A composite operation on two files; copy $<$file1$>$ $<$file2$>$ = read $<$file1$>$, create $<$file1$>$, write $<$file2$>$.
• Renaming and moving: Composite operations on two (or more) files; rename/move $<$file1$>$ $<$file2$>$ = read $<$file1$>$, create $<$file2$>$, write $<$file2$>$, delete $<$file2$>$.

When a file is opened in (ANSI-)C [21] it must be specified whether the file is to be read, rewritten (written), updated (read and written), or appended to (which is just a particular form of updating; the file is "read" to the end and written from there).

Possible conflicts

• Read/write conflicts occur when something wrong is written into a file due to the reading of (a replica of) another file that is out-dated (or stale). These conflicts are not easily detected, since the system seldom knows how to relate reads and writes. It is easy to detect the possible use of stale date, but not to relate it to following updates. For this purpose transactions can be of great value; operations within the same transaction are logically bound together, and the system knows it because it has been told. The resolution of these conflicts are easy--redo the reads and thereafter the writes based upon them.
• Write/write conflicts occur when the same (logical) file is being updated concurrently in to different places (i.e., two replicas). These conflicts are (relatively) easily detected; comparing (vector) timestamps or file contents. The resolution of these conflicts are not always easy--which updates are the "correct" ones?

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