Open The File Table
The process table also has a list with all
the files that are opened
Each open file descriptor entry contain a
pointer to an open file object that contains
all the information about the open file.
Both the Open File Table and the Open
File Objects are stored in the kernel.
The Open File Table
The system calls like write/read refer to
the open files with an integer value called
file descriptor or fd that is an index into
the table.
The maximum number of files descriptor
per process is 32 by default but but it can
be changed with the command ulimit up
to 1024.
The Open File Table
Open File
Table
0
1
2
3
4
.
.
31
Open File Object
I-NODE
Open Mode
Offset
Reference
Count
Open File Object
An Open File Object contains the state of an
open file.
I-Node
Major number Determines the devices
It uniquely identifies a file in the computer. An I-nodes is
made of two parts:
Minor number determines It what file it refers to inside the
device.
Open Mode How the file was opened:
Read Only
Read Write
Append
Open File Object
Offset
The next read or write operation will start at this offset
in the file.
Each read/write operation increases the offset by the
number of bytes read/written.
Reference Count
It is increased by the number of file descriptors that
point to this Open File Object.
When the reference count reaches 0 the Open File
Object is removed.
The reference count is initially 1 and it is increased
after fork() or calls like dup and dup2.
Default Open Files
When a process is created, there are three files
opened by default:
0 Default Standard Input
1 Default Standard Output
2 Default Standard Error
Write(1, Hello, 5) Sends Hello to stdout
Write(2, Hello, 5) Sends Hello to stderr
Stdin, stdout, and stderr are inherited from the
parent process.