5 - System Calls

Question 1

What are system calls used for?

Answer 1

When the application wants to access a system service/resource

Question 2

What is a trap?

Answer 2

A special CPU instruction
Switches from user to kernel mode
Invokes a trap handler, registered by kernel

Question 3

What happens inside a trap handler?

Answer 3

1. OS saves application state
2. OS does the requested operation, eg. involving some hardware
3. OS switches back to user mode and restores application state

Question 4

How many system calls could a typical OS be executing per second?

Answer 4

1000s

Question 5

What language are system calls typically implemented in and why?

Answer 5

Assembly - Optimized for performance

Question 6

Are system calls usually made directly by the application?

Answer 6

No - Usually wrapped in libraries

Question 7

Why are system calls typically made using library wrapper functions?

Answer 7

Hides the implementation details
Easier to port to other systems
that support the same APIs
Still works if system call is deprecated or changes

Question 8

List some common system call APIs

Answer 8

POSIX for Unix, Linux, Mac OS X
Win32 for Windows
Java APIs for JAVA VM

Question 9

What is a system call table?

Answer 9

A table of all system
calls, indexed by a
unique number
associated with each
system call.

Question 10

What system call can you use to trace system calls?

Answer 10

strace

Question 11

How do you get more information about system calls?

Answer 11

man pages

e.g., man strace

Question 12

What is a process?

Answer 12

A program in execution

A container of information needed by an OS to run a program

Question 13

What is a process tree?

Answer 13

The idea that processes can create new child processes and so in in a tree like way (Only one parent)

Question 14

What are Pipes?

Answer 14

A mechanism for letting two processes communicate with eachother.

Question 15

What does IPC stand for?

Answer 15

Inter-process communication

Question 16

What are sockets used for?

Answer 16

Networking

Question 17

What are devices - from a UNIX file API perspective?

Answer 17

Under UNIX, every piece of hardware is a file. Device files allow access to hardware.

Question 18

What are block devices?

Answer 18

Devices which are read from in discrete units (e.g., 1024 units at a time)

Question 19

What are character devices?

Answer 19

Devices, where a specific piece of data can only be written/read once (e.g., mouse click)