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Introduction to Unix Network

Programming

Reference: Stevens Unix

Network Programming

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How do we Communicate? Send a mail from Alice to Bob

Alice in Champaign, Bob in Hollywood

Example:

US Postal Service

Bob

Champaign, Illinois

Hollywood, California

Alice

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What does Alice do?

Bobs address (to a mailbox)

Bobs name in case people share mailbox

Postage have to pay!

Alices own name and address in case Bob wants to return amessage

Bob

100 Santa Monica Blvd.

Hollywood, CA 90028

Alice

200 Cornfield Rd.

Champaign, IL 61820

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What does Bob do?

Install a mailbox

Receive the mail

Get rid of envelope

Read the message

Bob

100 Santa Monica Blvd.

Hollywood, CA 90028

Alice

200 Cornfield Rd.

Champaign, IL 61820

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What about sending a TCP/IP

packet?

Very similar to Alice-mailing-to-Bob

Different terminologies veryconfusing

We have to remember

Different technologies

Suppose to be better (faster, more

reliable, cheaper, )

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Two simplest networking

programs Alice

the sending process

Alices address:128.174.246.177 (IP addr)

Alices name: 12345 (port #)

Bob the receiving process

Bobs address:216.52.167.132 (IP addr)

Bobs name: 23456 (port #)

int main () {

int sockfd;

struct sockaddr_in bob_addr, alice_addr;

bzero(&bob_addr, sizeof(bob_addr));

bob_addr.sin_family = AF_INET;bob_addr.sin_addr.s_addr = 0xD834A784;

bob_addr.sin_port = 23456;

// do the same for alice_addr

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

sendto(sockfd, hi, strlen(hi), 0,

&bob_addr, sizeof(bob_addr));

}

int main () {

int sockfd, n;

struct sockaddr_in bob_addr, alice_addr;

char mesg[100];

bzero(&bob_addr, sizeof(bob_addr));bob_addr.sin_family = AF_INET;

bob_addr.sin_addr.s_addr = 0xD834A784;bob_addr.sin_port = 23456;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

bind(sockfd, &bob_addr, sizeof(bob_addr));

n= recvfrom(sockfd, mesg, 100, 0,

&alice_addr, sizeof(alice_addr));

}

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What are the problems?

Message may be lost

Network is congested

Receiver is congested

Message may be duplicated, corrupted

Multiple messages: re-ordered

Concurrent connections

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Direction and Principles

Physical

Transport

Data Link

Network

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Physical

Transport

Data Link

Network

Programming

learn to use Internet forcommunication (with focus on

implementation of networking

concepts)

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Physical

Transport

Data Link

Network

Programming

learn to use Internet forcommunication (with focus on

implementation of networking

concepts)

Principles and

Concepts

learn to build network from ground

up

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Sockets

process

TCP with

buffers,

variables

socket

host or

server

process

TCP with

buffers,

variables

socket

host or

server

Internet

controlled by

app developer

process sends/receivesmessages to/from itssocket

socket analogous tomailbox

sending process relieson transportinfrastructure whichbrings message tosocket at receivingprocess

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Network Programming with

Sockets

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Sockets

Reading: Stevens 2nd ed., Ch. 1-6 or 1st ed., Ch. 1-3, 6

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Sockets

Reading: Stevens 2nd ed., Ch. 1-6 or 1st ed., Ch. 1-3, 6

Sockets API: A transport layer service interface

Introduced in 1981 by BSD 4.1

Implemented as library and/or system calls

Similar interfaces to TCP and UDP

Can also serve as interface to IP (for super-user);known as raw sockets

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Outline

Client-Sever Model

TCP/UDP Overview

Addresses and Data

Sockets API

Example

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Client-Server Model Asymmetric Communication

Client sends requests

Server sends replies

Server/Daemon Well-known name (e.g., IP address +

port)

Waits for contact

Processes requests, sends replies

Client

Initiates contact Waits for response

Client

Server

Client

Client

Client

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Socket Communication

Client process Server process

Client

socket

3-way

handshakingListening

socket

Connection

socket

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Client-Server Communication

Model Service Model

Concurrent: Server processes multiple clients requests

simultaneously

Sequential: Server processes only one clients requests at a time

Hybrid: Server maintains multiple connections, but processes

responses sequentially

Client and server categories are not disjoint A server can be a client of another server

A server can be a client of its own client

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TCP Connections

Transmission Control Protocol (TCP)

Service

OSI Transport Layer Service Model

Reliable byte stream (interpreted by application)

16-bit port space allows multiple connections on a

single host

Connection-oriented

Set up connection before communicating

Tear down connection when done

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TCP Service Reliable Data Transfer

Guarantees delivery of all data

Exactly once if no catastrophic failures

Sequenced Data Transfer Guarantees in-order delivery of data

If A sends M1 followed by M2 to B, B never receives M2before M1

Regulated Data Flow Monitors network and adjusts transmission appropriately

Prevents senders from wasting bandwidth

Reduces global congestion problems

Data Transmission Full-Duplex byte stream

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UDP Services

Unit of Transfer Datagram (variable length packet)

Unreliable No guaranteed delivery

Drops packets silently

Unordered

No guarantee of maintained order of delivery Unlimited Transmission

No flow control

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Addresses and Data

Internet domain names Human readable

Variable length Ex: sal.cs.uiuc.edu

IP addresses Easily handled by routers/computers

Fixed length Somewhat geographical

Ex: 128.174.252.217

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Byte Ordering

Big Endian vs. Little Endian Little Endian (Intel, DEC):

Least significant byte of word is stored in the lowestmemory address

Big Endian (Sun, SGI, HP): Most significant byte of word is stored in the lowest

memory address

Network Byte Order = Big Endian Allows both sides to communicate

Must be used for some data (i.e. IP Addresses)

Good form for all binary data

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Byte Ordering Functions 16- and 32-bit conversion functions (for platform

independence)

Examples:

int m, n;short int s,t;

m = ntohl (n) net-to-host long (32-bit) translations = ntohs (t) net-to-host short (16-bit) translation

n = htonl (m) host-to-net long (32-bit) translationt = htons (s) host-to-net short (16-bit) translation

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Socket Address Structure IP address:

struct in_addr {in_addr_t s_addr; /* 32-bit IP address */

};

TCP or UDP address:struct sockaddr_in {

short sin_family; /* e.g., AF_INET */ushort sin_port; /* TCP/UDP port */

struct in_addr; /* IP address */};

all but sin_family in network byte order

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Structure: hostent The hostentdata structure (from

/usr/include/netdb.h)

canonical domain name and aliases

list of addresses associated with machine

also address type and length informationstruct hostent {

char* h_name; /* official name of host */

char** h_aliases; /* NULL-terminated alias list */

int h_addrtype /* address type (AF_INET) */

int h_length; /* length of addresses (4B) */char** h_addr_list; /* NULL-terminated address list */

#define h_addr h_addr_list[0];/* backward-compatibility */

};

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Address Access/Conversion

Functionsin_addr_t inet_addr (const char* strptr);

Translate dotted-decimal notation to IP address; returns -1 onfailure, thus cannot handle broadcast value 255.255.255.255

int inet_aton (const char *strptr, structin_addr *inaddr); Translate dotted-decimal notation to IP address; returns 1 on

success, 0 on failure

int gethostname (char* name, size_tnamelen); Read hosts name (use with gethostbyname to find local IP)

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UDP Connection Example

client server

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client server

socket

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client server

socket

bind

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UDP Connection Exampleclient server

socket

socket

bind

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socket

socket

sendto

bind

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socket

socket

sendto

bind

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socket

socket

sendto

bind

recvfrom

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socket

socket

sendto

bind

recvfrom

sendto

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socket

socket

sendto

bind

recvfrom

sendto

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socket

socket

sendto

bind

recvfrom

sendto

recvfrom

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socket

socket

sendto

bind

recvfrom

sendto

recvfrom

close

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Functions: sendtoint sendto (int sockfd, char* buf, size_t nbytes,

int flags, struct sockaddr* destaddr, intaddrlen);

Send a datagram to another UDP socket. Returns number of bytes written or -1. Also sets errnoon

failure.

sockfd: socket file descriptor (returned from socket)

buf: data buffer

nbytes: number of bytes to try to read

flags: see man page for details; typically use 0 destaddr: IP address and port number of destination socket

addrlen: length of address structure = sizeof (struct sockaddr_in)

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Functions: recvfromint recvfrom (int sockfd, char* buf, size_t

nbytes, int flags, struct sockaddr* srcaddr,int* addrlen);

Read a datagram from a UDP socket. Returns number of bytes read (0 is valid) or -1. Also sets errno

on failure. sockfd: socket file descriptor (returned from socket) buf: data buffer nbytes: number of bytes to try to read flags: see man page for details; typically use 0

srcaddr: IP address and port number of sending socket

(returned from call) addrlen: length of address structure = pointer to int set to

sizeof (struct sockaddr_in)

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Socket FunctionsTCP Client

TCP Server

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Socket Functions socket()TCP Client

TCP Server

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Socket Functions socket()

bind()Well-known

port

TCP Client

TCP Server

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listen()

bind()Well-known

port

TCP Client

TCP Server

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listen()

accept()

bind()Well-known

port

TCP Client

TCP Server

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listen()

accept()

bind()Well-known

port

blocks until connection

from client

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port


from client

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port


from clientconnect()

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port



TCP three-way handshaking

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port




read()

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port



write()


read()

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port



write()


data (request)read()

TCP Client

TCP Server

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listen()

accept()

socket()

bind()Well-known

port



write()



process request

TCP Client

TCP Server

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Socket Functionssocket()blocks until connection


write()



process request

TCP ServerTCP Client

write()

read()

data (reply)

close() read()

close()

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TCP Connection Exampleclient server

socket

socket

connect

bind

listenaccept

write

read

write

read

close

close

read

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Socket Creation and Setup Include file

Create a socket int socket (int family, int type, int protocol);

Returns file descriptor or -1.

Bind a socket to a local IP address and port number int bind (int sockfd, struct sockaddr* myaddr, int

addrlen);

Put socket into passive state (wait for connections rather thaninitiate a connection). int listen (int sockfd, int backlog);

Accept connections int accept (int sockfd, struct sockaddr*

cliaddr, int* addrlen);

Returns file descriptor or -1.

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Function: bindint bind (int sockfd, struct sockaddr*myaddr, int addrlen);

Bind a socket to a local IP address and port

number. Returns 0 on success, -1 and sets errnoon failure. sockfd: socket file descriptor (returned from socket) myaddr: includes IP address and port number

IP address: set by kernel if value passed is INADDR_ANY,else set by caller

port number: set by kernel if value passed is 0, else set bycaller

addrlen: length of address structure = sizeof (struct sockaddr_in)

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TCP and UDP Ports Allocated and assigned by the Internet Assigned

Numbers Authority see RFC 1700 orftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers

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TCP and UDP Ports Allocated and assigned by the Internet Assigned

Numbers Authority see RFC 1700 orftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers

private/ephemeral ports49152-65535

registered services/ephemeral ports1024-49151

registered and controlled, also used for identity

verification

super-user only

513-1023

standard services (see /etc/services)

super-user only

1-512

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Functions: listenint listen (int sockfd, int backlog);

Put socket into passive state (wait for connections

rather than initiate a connection). Returns 0 on success, -1 and sets errnoon failure.


backlog: bound on length of unaccepted connection

queue (connection backlog); kernel will cap, thus better to

set high

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Functions: acceptint accept (int sockfd, struct sockaddr* cliaddr,

int* addrlen);

Accept a new connection. Returns file descriptor or -1. Also sets

errnoon failure.


cliaddr: IP address and port number of client (returned fromcall)

addrlen: length of address structure = pointer to int set tosizeof (struct sockaddr_in)

addrlenis a value-result argument: the caller passes the size of the address structure, the kernel

returns the size of the clients address (the number of byteswritten)

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server#include

#include

#include

#include #include

#include

#include

#include

#define PORT 3490 /* well-known port */#define BACKLOG 10 /* how many pending

connections queue

will hold */

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servermain()

{

int sockfd, new_fd; /* listen on sock_fd, newconnection on new_fd */

struct sockaddr_in my_addr; /* my address */

struct sockaddr_in their_addr; /* connector addr */

int sin_size;

if ((sockfd = socket(AF_INET, SOCK_STREAM, 0))==-1){

perror("socket");exit(1);

}

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server

bzero(&my_addr, sizeof(struct sockaddr));

/* zero the struct */

my_addr.sin_family = AF_INET; /* host byte order */

my_addr.sin_port = htons(MYPORT); /* short, networkbyte order */

my_addr.sin_addr.s_addr = htonl(INADDR_ANY);

if (bind(sockfd, (struct sockaddr *)&my_addr,

sizeof(struct sockaddr)) == -1) {

perror("bind");

exit(1);

}

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server

if (listen(sockfd, BACKLOG) == -1) {

perror("listen");

exit(1);

}

while(1) { /* main accept() loop */

sin_size = sizeof(struct sockaddr_in);

if ((new_fd = accept(sockfd, (struct sockaddr*)

&their_addr,&sin_size)) == -1){

perror("accept");

continue;

}

printf("server: got connection from %s\n",

inet_ntoa(their_addr.sin_addr));

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Establishing a Connection

Include file

int connect (int sockfd, structsockaddr* servaddr, int addrlen);

Connect to another socket.

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Functions: connect

int connect (int sockfd, structsockaddr* servaddr, int addrlen);

Connect to another socket. Returns 0 on success, -1 and sets errnoon failure.


servaddr: IP address and port number of server

addrlen: length of address structure

= sizeof (struct sockaddr_in)

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client

if ((sockfd = socket (AF_INET, SOCK_STREAM, 0)) == -1) {

perror (socket);

exit (1);

}

their_addr.sin_family = AF_INET; /* interpd by host */

their_addr.sin_port = htons (PORT);

their_addr.sin_addr = *((struct in_addr*)he->h_addr);

bzero (&(their_addr.sin_zero), 8);

/* zero rest of struct */

if (connect (sockfd, (struct sockaddr*)&their_addr,

sizeof (struct sockaddr)) == -1) {

perror (connect);

exit (1);

}

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Sending and Receiving Data

int write (int sockfd, char* buf, size_tnbytes);

Write data to a stream (TCP) or connected

datagram (UDP) socket. Returns number of bytes written or -1.

int read (int sockfd, char* buf, size_tnbytes);

Read data from a stream (TCP) or connecteddatagram (UDP) socket. Returns number of bytes read or -1.

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Sending and Receiving Data

int sendto (int sockfd, char* buf,size_t nbytes, int flags, structsockaddr* destaddr, int addrlen);

Send a datagram to another UDP socket. Returns number of bytes written or -1.

int recvfrom (int sockfd, char* buf,size_t nbytes, int flags, struct

sockaddr* srcaddr, int* addrlen); Read a datagram from a UDP socket.

Returns number of bytes read or -1.

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Functions: write

int write (int sockfd, char* buf, size_tnbytes);

Write data to a stream (TCP) or connected

datagram (UDP) socket. Returns number of bytes written or -1. Also sets errnoonfailure.

sockfd: socket file descriptor (returned from socket) buf: data buffer nbytes: number of bytes to try to write

Some reasons for failure or partial writes: process received interrupt or signal

kernel resources unavailable (e.g., buffers)

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Functions: read

int read (int sockfd, char* buf, size_tnbytes);

Read data from a stream (TCP) or connected

datagram (UDP) socket. Returns number of bytes read or -1. Also sets errnoon

failure.

Returns 0 if socket closed.


buf: data buffer

nbytes: number of bytes to try to read

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Tearing Down a Connection

int close (int sockfd);

Close a socket.

Returns 0 on success, -1 and sets errnoon failure.

int shutdown (int sockfd, int howto);

Force termination of communication across a socket in

one or both directions.

Returns 0 on success, -1 and sets errnoon failure.

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Functions: shutdown

int shutdown (int sockfd, int howto);

Force termination of communication across a socket in one orboth directions. Returns 0 on success, -1 and sets errnoon failure.


howto: SHUT_RD to stop reading

SHUT_WRto stop writing

SHUT_RDWRto stop both

shutdownoverrides the usual rules regarding duplicatedsockets, in which TCP teardown does not occur until all copieshave closed the socket.

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Advanced Sockets

Managing Multiple Connections fork/exec: multiple server processes

pthread_create: multi-threaded serverprocess

(no calls): event-based server process

Detecting Data Arrival select andpoll functions

Synchronous vs. AsynchronousConnections

Other Socket Options

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server

#include

#include

#include

#include #include

#include

#include

#include

#define PORT 3490 /* well-known port */

#define BACKLOG 10 /* how many pending

connections queue

will hold */

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main()

{

int sockfd, new_fd; /* listen on sock_fd, newconnection on new_fd */

struct sockaddr_in my_addr; /* my address */

struct sockaddr_in their_addr; /* connector addr */

int sin_size;

if ((sockfd = socket(AF_INET, SOCK_STREAM, 0))==-

1){perror("socket");

exit(1);

}

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my_addr.sin_family = AF_INET; /* host byte order */

my_addr.sin_port = htons(MYPORT); /* short, network

byte order */

my_addr.sin_addr.s_addr = htonl(INADDR_ANY);/* automatically fill with my IP (w/o Beejs bug)*/

bzero(&(my_addr.sin_zero), 8); /* zero the struct*/

if (bind(sockfd, (struct sockaddr *)&my_addr,sizeof(struct sockaddr)) == -1) {

perror("bind");

exit(1);

}

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server

if (listen(sockfd, BACKLOG) == -1) {

perror("listen");

exit(1);

}

while(1) { /* main accept() loop */

sin_size = sizeof(struct sockaddr_in);

if ((new_fd = accept(sockfd, (struct sockaddr*)

&their_addr,&sin_size)) == -1){

perror("accept");

continue;

}

printf("server: got connection from %s\n",

inet_ntoa(their_addr.sin_addr));

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server

if (!fork()) { /* this is the child process */

if (send(new_fd,"Hello, world!\n", 14, 0)

== -1)

perror("send");close(new_fd);

exit(0);

}

close(new_fd); /* parent doesn't need this */

/* clean up all child processes */

while(waitpid(-1,NULL,WNOHANG) > 0);}

}

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client

#include

#include

#include

#include #include

#include

#include

#define PORT 3490 /* well-known port */

#define MAXDATASIZE 100 /* max number of bytes we

can get at once */

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int main (int argc, char* argv[]){

int sockfd, numbytes;

char buf[MAXDATASIZE + 1];

struct hostent* he;

struct sockaddr_in their_addr;/* connectors address information */

if (argc != 2) {

fprintf (stderr, usage: client hostname\n);

exit (1);

}

if ((he = gethostbyname (argv[1])) == NULL) {

/* get the host info */perror (gethostbyname);

exit (1);

}

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if ((sockfd = socket (AF_INET, SOCK_STREAM, 0)) == -1) {

perror (socket);

exit (1);

}

their_addr.sin_family = AF_INET; /* interpd by host */

their_addr.sin_port = htons (PORT);

their_addr.sin_addr = *((struct in_addr*)he->h_addr);

bzero (&(their_addr.sin_zero), 8);

/* zero rest of struct */

if (connect (sockfd, (struct sockaddr*)&their_addr,

sizeof (struct sockaddr)) == -1) {perror (connect);

exit (1);

}

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if ((numbytes = recv (sockfd, buf, MAXDATASIZE, 0))

== -1) {

perror (recv);

exit (1);}

buf[numbytes] = \0;

printf (Received: %s, buf);

close (sockfd);

return 0;

}

CS438 02.Sockets

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