File I/O

Question 1

6 basic I/O system calls

Answer 1

1. open()
2. write()
3. read()
4. close()
5. lseek()
6. fcntl()

Question 2

Flags in open()

Answer 2

Specifies access and creation mode, i.e.
O_WRONLY, O_RDWR, O_CREAT, etc.

Question 3

Mode in open()

Answer 3

Specifies file permission if file is created using O_CREAT or O_TMPFILE

Question 4

File descriptor

Answer 4

A small, non-negative integer used as a handle for a particular file to read/write to
Can change each time the file is opened

Question 5

File offset

Answer 5

The file offset is a pointer to a specific byte in a file we are reading/writing to
read() automatically increments the file offset, write() starts at the end and increments to end each time

Question 6

lseek()

Answer 6

Used to manually adjust the file offset, 3 parameters: fd, offset, whence - adjusts file offset by offset bytes, specified by ‘whence’ which can be:
SEEK_SET - beginning of file
SEEK_CUR - current offset
SEEK_END - end of the file

Question 7

Stream vs. File Descriptor

Answer 7

A stream is a “wrapper” around a file descriptor that also has a buffer attached to it

Question 8

Buffered I/O vs. system calls

Answer 8

Buffered I/O writes to a buffer before making a system call because system calls are costly

Question 9

fflush()

Answer 9

immediately sends buffered data to the kernel - setbuf(NULL) does the same thing

Question 10

Blocking vs Nonblocking

Answer 10

Blocking call doesn’t return until the operation can be done, a nonblocking call will return anyway (usually w/ an error)

Question 11

Everything is a File model

Answer 11

Everything is represented as a file including hardware like keyboard, mouse, and others like the terminal

Question 12

Disk Partitions

Answer 12

A disk is divided into partitions - can be found at /proc/partitions - usually used as a file system, also used as swap space

Question 13

File System

Answer 13

a system that manages files/directories, each system has a file tree that starts with root directory /

Question 14

i-node

Answer 14

An i-node contains metadata for every file, i.e. type, permissions, owner, etc. - identified by a number

Question 15

Hard link

Answer 15

Giving another name to an existing file - will have the same metadata and content

Hard links can’t be made on directories or outside the same file system

Question 16

Soft/symbolic link

Answer 16

A soft link is an actual file where the content is the path to the original file
Can be used on directories and between file systems

Question 17

Setuid Bit

Answer 17

If set, the user that runs the program can act as the owner of the program

Question 18

Setgid Bit

Answer 18

If set, the user that runs the program can act as if the user belonged to the group of the program

Question 19

Sticky bit

Answer 19

If set on a directory, a user can delete a file under it only

Question 20

VFS (virtual file system)

Answer 20

Defines an interface that different file systems can implement such as open, read, write, etc.

Question 21

Mounting/Unmounting

Answer 21

Different file systems can be mounted to others to create a single file tree