Page 449 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes sort would be set (by the shell) to write to the pipe and file descriptor 0 (standard input) in the process
running head would be set to read from the pipe. In this way, sort just reads from file descriptor
0 (set to the file in) and writes to file descriptor 1 (the pipe) without even being aware that these
have been redirected. If they have not been redirected, sort will automatically read from the
keyboard and write to the screen.

Figure 14.28: The Fields Returned by the Stat System Call

Device the file is on

I-node number (which file on the device)
File mode (includes protection information)
Number of links to the file
Identify of the file’s owner

Group the file belongs to
File size (in bytes)
Creation time
Time of last access

Time of last modification

Similarly, when head reads from file descriptor 0, it is reading the data sort put into the pipe
buffer without even knowing that a pipe is in use. This is a clear example where a simple concept
(redirection) with a simple implementation (file descriptors 0 and 1) leads to a powerful tool
(connecting program in arbitrary ways without having to modify them at all). The last system
call in Figure 14.28 is fcntl. It is used to lock and unlock files, apply shared or exclusive locks,
and perform a few other file-specific operations. Now let us look at some system calls that
relate more to directories or the file system as a whole, rather than just to one specific file. Some
common ones are listed in Figure 14.29. Directories are created and destroyed using mkdir and
rmdir, respectively. A directory can only be removed if it is empty.

Figure 14.29: Some System Calls Relating to Directories

System call Description
s = mkdir(path, mode) Create a new directory

s = rmdir(path) Remove a directory
s = link(oldpath, newpath) Create a link to an existing file
s = unlink(path) Unlink a file

s = chdir(path) Change the working directory
dir = opendir(path) Open a directory for reading

s = closedir(dir) Close a directory
dirent = readdir(dir) Read one directory entry
Rewind a directory so it can be
rewinddir(dir)
reread

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