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Week 06 Notes for CST8165
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-Ian! D. Allen - idallen@idallen.ca

Remember - knowing how to find out an answer is more important than
memorizing the answer.  Learn to fish!  RTFM!  (Read The Fine Manual)

New in Notes:
    buffer_overflows.txt
    deep_indentation.txt
    header_files.txt

Domain Name System (DNS)
------------------------

DNS turns names into IP addresses.  Not essential for a raw Internet
connection; but, very, very useful.

Important Fact: DNS uses (mostly) UDP, not TCP, for queries and responses.

Q: What type of IP protocol does DNS use (most of the time)?

Q: T/F The choice of DNS transport protocol means a DNS request is
   automatically retried if it fails.


Multiple simultaneous connections
---------------------------------

How does Unix/Linux handle multiple simultaneous connections?
- if your server process is blocked on an accept() or read(), it can't
  process other client connections
- if your client process is reading your keyboard, it can't display
  input coming from the server
- need to listen to multiple file descriptors at the same time

Solutions:

A.  fork() separate processes to handle each file descriptor
    - server forks a new child process for each new client connection
    - client forks into one process for keyboard, one for server socket

B.  use select() to listen to multiple file descriptors at the same time
    - no need to fork()
    
C.  separate threads

fork() and select() are the traditional designs; threads are new

Using select():

- the select() system call listens to multiple I/O descriptors all
  at the same time and unblocks when I/O is possible on any of them
- select() I/O descriptors are represented in bit vectors:
    * FD_ZERO(fd_set *set) -- clears a file descriptor set
    * FD_SET(int fd, fd_set *set) -- adds fd to the set
    * FD_CLR(int fd, fd_set *set) -- removes fd from the set
    * FD_ISSET(int fd, fd_set *set) -- tests to see if fd is in the set

http://beej.us/guide/bgnet/output/htmlsingle/bgnet.html#select

Write the PDL for this program.

In Week 6:

1) I reviewed section on "6.2. select()--Synchronous I/O Multiplexing"
   in http://beej.us/guide/bgnet/output/htmlsingle/bgnet.html
   and did a read-through of code that uses the select() syscall.

2) I wrote the PDL for selectserver.c and reworked it (and the code)
   to reduce the number of indentation levels (as per Lab #04).

If you missed the class, more descriptions of how select() works are here:

  http://www.mksxserver.com/docs/man3/select.3.asp
  http://www.opengroup.org/onlinepubs/007908799/xsh/select.html
  http://www.lowtek.com/sockets/select.html
  http://swoolley.homeip.net/man.cgi/select_tut
  http://www.developerweb.net/forum/showthread.php?t=2933