File Descriptor

Depends on	File Access • Filing System Implementation

The descriptor will be created when a file is opened and is associated with a particular file within a particular process. It is used for immediate file access. It will hold information such as:

• the access permissions.
  • e.g. whether you can currently read or write; different from the file access rights.
• the current position (index) in the file.
• the error status.
• control flags.

The details are deliberately kept opaque – loosely the equivalent of having private variables. A sketch (not 100% guaranteed!) is shown below. Do not try and memorise (or even understand!) all the details here – mostly marvel at the complexity!

A couple of points:

• Unix maintains a simple table of files for each process.
• One file may be opened several times, simultaneously; each ‘opening’ has its own descriptor.
• Opened files can be shared: several processes can read the same file at the same time, for example.
• Eventually there is a pointer to the appropriate i-node.

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File Descriptor vs. File Attributes

Like the file attributes, a file descriptor holds some metadata about a file. However these are not the same thing.

• The file attributes are ‘permanent’, in that they exist with the file; the file descriptor only exists whilst the file is in use.
• There is exactly one set of file attributes per file; there can be several file descriptors for the same file if it has been opened multiple times concurrently. Each descriptor will have (for example) its own file position.

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Context Dependencies Dependants

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Articles on Filing System

"Everything is a File" • FAT • File Access • File Attributes • File Descriptor • File Locking • File Permissions • File Systems • File Types • Files • Filing System • Filing System Implementation • Fragmentation • I-nodes • Journalling File System • Links • Network File System (NFS) • Resources