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Fig. 7.26. Two ways to perform file operations The second way in the file system
introduces two special system call: open-opening the file and close-closing the
file. The system call to open it is open before any order transactions file, and
close the file challenge to the closure-after working with the file. The main
task is to transform call open character of the filename of its unique numeric
name, copy of the file from the disk into the buffer area of memory and
verification of the user to perform the requested operation. Close Call release
buffer with the characteristics file, and makes it impossible to continue
operations in the file without opening it again. Operation opening and closing a
file in one form or another utverdilis operating systems long ago. Even in the "old
" operating system, as OS/360, a macro OPEN, in which a special buffer, called
DCB (Data Control Block), collected from various sources all desired
characteristics of the data set (a concept close to the modern concept of a file)
used later in the performance of read and write operations. Next basic system
calls file operations are discussed in greater detail in the case into the UNIX
operating system, in which they acquired the type, which today supports
virtually all operating systems.
Opening the file System open challenge to UNIX OS works with two arguments: a
character named opened a file and open it. The regime said the opening of the
system, which operations will be carried out over the file in the sequence of
operations before the closure of the file systems close call, for example: read
only, write only, or read and write. When you open the file first OS able to
convert the first argument systemic challenge, which is a symbolic name of the
file in its unique numeric name, which in the traditional UNIX file system is
the descriptor index number. The procedure was considered above in the
description of the file system s5. On descriptor index number inode file system
is a record on disk and copies of her performance file into RAM. To keep copies
of the index buffer descriptor used for the system virtual space. Specifications
descriptor index transferred into RAM, housed in the structure of the so-called
virtual descriptor vnode (virtual node). Vnode structure of the index file
descriptor fields inode, and several of the following additional fields useful
in carrying out operations with the file. index descriptor memory, reflecting:
Do locked file; Is waiting to unlock the file any process; Do different
characteristics of the file in memory of its disk copies from changing the
contents of the index descriptor; is whether the file in memory of its disk
copies from changing the contents of the file; Is the file of the mount.
Logical device number file system containing the file. descriptor index number.
In disk descriptor index this field is absent, as determined by the number of
descriptor for the start of index descriptors. Count references to the vnode
structure. In the same file in a period of time can be various processes, but
the operating system does not create a separate process for each copy vnode
structure, and for each file, which is currently working at least one process,
stores exactly one copy virtual descriptor. When the file is opened OS verifies
whether in system memory vnode structure opened file (the number of logical
devices and descriptor index number, which identifies with the transformation of
the symbolic name), and, if available, links to a count increased by one. When
the closure of the file reference count is reduced by one, and if it is on an
equal, the buffer that keeps this vnode is free. Using only a copy of the file,
and some of the characteristics of file operations (for example, sign blocking),
common to all workers file processes, saves system memory. Nevertheless, there
are characteristics of each individual process, performing a sequence of
operations with a particular file. For storage structure used in the UNIX file
type, which, like vnode is stored in the system memory area. Each opening of the
file OS checks the user process to perform the requested operation with the file
and, if the tests are successful, in a system of the new memory structure I f i
e, which explains how to open a file, and operations that process is going to
produce the file (such as reading). Structure file contains fields such as:
Token regime opening (read-only, read-write, etc.); vnode pointer to the
structure; The ongoing shift in the file (variable offset) in the operation of
reading / writing; About count references to the structure; pointer to the
structure containing the right process, opening the file (this structure is in
the process descriptor); pointers to the previous and subsequent file
structure, linking all such structures in the double list. Partly offset stored
in the file, allows OS remember position a pointer to byte file. When you open a
file, the variable contains the start or end of file in bytes of a given regime
opening. After the read / write operations pointer shifted to the number of
bytes, which was read or written as a result of the operation. Next comes
operations pointer in the condition in which it left the previous operation.
Application software can manage the situation clearly pointer with System lseek
call, which will be considered below. With each new opening any file OS creates
a new file structure and places it in the linked list twice (Figure 7.27).
Usually, under the custody of the file system area are limited area, so the
total number of open files all processes at any time limit. After an operating
system file structure pointer puts it in the table of open files, which is in
the process. If the process several times opens the same file, the file
structure is created for each operation opening. As a parent in the context of
Trial UNIX inherited protsessom- descendant, a descendant of inherited and
pointers to open all files parent to get them to perform the operation. System
call open returns to a user process descriptor file, which is a record number in
the table of open files. The file descriptor is locally significant only for the
process that opened the file to the different processes the same meaning
descriptor pointed to the different operations in general on different files.
After opening its file descriptor is used in all future transactions with the
file up to the apparent closure of the file. Thus, the descriptor file is a
temporary unique name, but did not file, a sequence of operations with this file.
To open a file / bin/prog1. Ehe mode read-only application programmer can use
that expression in C: fd = open ( " '/ bin / progl.exe." 0_RDONLY): Here is fd
integer variable holds the importance of open file descriptor. Its importance
should be used in the operation data from the file / bin/prog1. Exe. In an
unsuccessful attempt to open a file (no rights for zatrebovannoy operations
wrong filename) fd variable the values are -1, which is an indicator of error
for all UNIX system calls.
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