A number of operations can be
carried out on a file. However, there are 6 basic file operations. As we know a
file is an ADT or abstract data type. Certain operations need to be considered
for defining a file. Operating systems makes calls to these operations.
Following are the six basic operations:
1. Creation of a file:
- This operation involves two steps.
- Firstly, a sufficient space has to be
found for storing the file in the file system. - Secondly, for this new file
an entry should be made in the directory.
2. Writing to a file:
- For
writing data to a file, a system call has to be made with name and the
data to be written as its arguments.
- A writer pointer is kept by the
system at the location in the file where the next write operation is to be
carried out.
- This pointer is updated whenever a write operation occurs.
3. Reading from a file:
- Just like the write operation, in order to read information from a file, a
system call has to be generated along with the name of the file and the
location of the content to be read as its arguments.
- Here, instead of a
write pointer there is a read pointer that will indicate the location
where the next read operation is to take place.
- The location at which the
current operation is being carried out is kept as a “per – process current
– file – position” pointer since the process is either writing to or
reading from the file.
- The same pointer can be used by both the read and
write operations in order to reduce the complexity of the system as well
as for saving space.
4. Re-positioning within
a file:
- System carries out search in the directory looking for the
appropriate entry.
- When found, the current file position pointer is
re-pointed to this position.
- This file operation does not require carrying
out any input or output operation in actual.
- Another name for this file
operation is the file seek.
5. Deletion of a file:
- For deletion of the file, the system searches through the directory to
find the appropriate entry.
- When found, the space held by this file is released
and the entry in the directory is destroyed so that this space can be
reused by other files.
6. Truncating a file:
- Sometimes you may require deleting only the contents of a file while
keeping it attributes.
- Deleting the file and recreating it is not an
efficient solution.
- This file operation lets you to erase the contents of
the file but save its attributes.
- But here the length attribute of the
file will be changed to zero after truncation.
- The file space is released
after truncating.
The above mentioned six basic file
operations constitute the minimal file operations set. These operations are
primary ones and if combined can perform some other secondary file operations
such as copying. A table known as the open file table is maintained by the
operating system that stores all the information about the files that are
currently open. When the file is closed, its entry is deleted from the open
file table. Some files have to be opened explicitly with the function open()
before using it. The name of the file is passed as an argument to this
function. Then it looks in the directory for this file and an entry is made in
the open file table. Each file has some
access rights. It is in these access modes that a process uses the file. A
process can perform only those operations which are permitted by the access
rights of the file.
