# 2023-10-10

> **Summary**

> **keywords**

> **TODO**

> **HW**

> **Exercise**\*

> **Next time**

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## Recap

### Socket programming

Why do we learn this?

If you do not denote the serverPort, it is a loopback, also called localhost.

decode is required to make binary sentences into strings.

Accept will accept the connection requested by client call `clientSocket.send()`.

### difference in TCP & UDP

* There is connection establishment in TCP. (shown in accept call)

\#todo : add System theory practice book, Programmer's Perspective![](https://3863748873-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F7lhTZlN8P2dz9Om0Ykpg%2Fuploads%2Fgit-blob-6f0c0dc35e3c42efbfefa62f7aeecbc588ff51b3%2F20231010131807.png?alt=media)

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## Multiplexing using Sockets

> Let's support multiple clients.\
> How one-process one-server can support multiple clients?

We will focus on...

* one process
* sequential manner
* First-come-first-served manner

#### Multiplexing

Many people are sharing a particular media.

> all sockets are files.

### Three system calls for implementing multiplexing.

> `socket()`,`accept()`,`select()`

#### `socket()`, `accept()`

1. Creating Master Socket `Socket()`

* \*`Socket()` will create and return Master Socket(Master file Descriptor, like a pointer).
* `serverSocket = socekt(AF_INET,SOCK_STREAM)`

2. Master Socket is responsible for connection request.`accept()`

* For each acceptance of request, master socket will create communication socket (Communication file descriptors).
* \*`serverSocket.accept()` returns client address(IP+port), and the communication socket
* If second client requests, Master socket will accept, and create another communication socket. with, *this* socket, further communication is made.
* `accept()` is for 3-way handshake.

3. Each Communication Socket is responsible for Service request(Further communications).

* Further communication will be handled by each communication socket.

#### `select()`

* a waiting(blocking) system call.
* for monitoring all client activity.
* monitoring all socket File Descriptor list (Master socket, communication sockets)
* in the argument, list of sockets are requested.
* monitoring all connection request, service request.
* `select()` unblocks when
  * New connection request arrives
  * New Service request from existing client arrives.
* will return a particular **socket** that needs an activity.

### Example codes: Chat application

listen 5 connections\
prepare a list `inputs[]` of socket FDs.\
While there is a element in `inputs[]`,

if s is server : if s is a master socket