File Handling
You've most likely had to deal with files in the past.
Go to support/simple-file-operations and run a most basic command:
cat file.txt:
student@os:~/.../lab/support/simple-file-operations$ cat file.txt
OS Rullz!
But how does cat actually "reach" the file, then read its contents, then print them to standard output?
This is part of what we're going to learn.
Reaching the File
To manipulate the file (read its contents, modify them, change its size etc.), each process must first get a handler to this file. Think of this handler as an object by which the process can identify and refer to the file.
Now take a look at the code examples in support/simple-file-operations.
Each of them reads the contents of file.txt, modifies them, and then reads the previously modified file again.
Use make to compile the C code, and make java-file-operations to compile the Java code.
Now run the programs repeatedly in whatever order you wish:
student@os:~/.../lab/support/simple-file-operations$ python3 file_operations.py
File contents are: OS Rullz!
Wrote new data to file
File contents are: Python was here!
student@os:~/.../lab/support/simple-file-operations$ ./file_operations # from the C code
File contents are: Python was here!
Wrote new data to file
File contents are: C was here!
student@os:~/.../lab/support/simple-file-operations$ java FileOperations
File contents are: Python was here!
Wrote new data to file
File contents are: Java was here!
Note that each piece of code creates a variable, which is then used as a handler.
Limitations of High-level File Handlers
Everything in Linux is a file. This statement says that the Linux OS treats every entry in a file system (regular file, directory, block device, char device, link, UNIX socket) as a file. This is very convenient for creating a unified interface that deals with all these files. We would like our file handlers to also be able to handle all types of files.
Let's try this.
Navigate to support/simple-file-operations/directory_operations.c.
Read the code.
It does something very simple:
it attempts to open the dir directory the same way file_operations.c tried to open file.txt.
Compile and run the code.
student@os:~/.../lab/support/simple-file-operations$ ./directory_operations
18:18:11 FATAL directory_operations.c:14: fopen: Is a directory
The error message is crystal clear: we cannot use fopen() on directories.
So the FILE structure is unsuited for directories.
Therefore, this handler is not generic enough for a regular Linux filesystem.
To get to the directory, we need to use a lower-level function.
Let's take a look at the syscall used by fopen().
We will use a simpler syscall, called open().
In fact, open() is a wrapper over openat().
Navigate to support/file-descriptors/directory_open.c.
Inspect, compile and run the code.
student@os:~/.../lab/support/file-descriptors$ ./open_directory
Directory file descriptor is: 3
We can now see that the open() syscall is capable of also handling directories, so it's closer to what we want.
Note that it is rather uncommon to use open() for directories.
Most of the time, opendir() is used instead.
But what does it return?
Find out in the "File Descriptors" section.