fundamentals of socket programming

8/11/2019 Fundamentals of Socket Programming

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Chapter 6

Fundamentals of Socket Programming(Recommended reading: Comer, Vol , Chapter !"

Re#ie$ of C Programming

% Some &ata 'pes

'he follo$ing data tpes are often used in socketprogramming:

u_char

u_short

u_long

unsigned )*+it character

unsigned 16*+it integer

unsigned *+it integer

- structure is a group of different tpes of data.

/0ample:struct info {

char name[20] ;

};

int ID ;

struct info student[60] ;

student[0].name = "chan" ;11

student[0].ID = 1234 ;

student[1].name = "cheung" ;

student[1].ID = 5678 ;

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% Casting of &ata 'pes

Casting is to change the data tpe of a

#aria+le or e0pression to the designated one.

/0ample:

% 4f i is an int, then the data tpe of the

follo$ing e0pression is dou+le:

(double) (i+1)

% Pointers

/#er memor location has an address

% 2e can use this address to access the

content in this memor location.

- pointer is a #aria+le that stores the

memory address(not the #alue" of a data

item.

&eclaration of pointer #aria+le:

data-type *pointer-variable;

&ereference operator 5:

%*pvgi#es the content stored in the

memor location $ith addresspv.

-ddress operator :

% 'he address storing the first memor

+te of the #aria+levis&v.

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%

%

/0ample

% 0 and are declared to +e of tpe

7float7, and p is declared to +e of tpe7pointer to float7:

float x, y, *p;

'he follo$ing statements

p = &x ;

y = *p ;

are e8ui#alent to

y = x ;Pointer and -rra

'he name of an arra is actuall a pointer

to the first element in the arra.

4fxis a one*dimensional arra,

% the address of the 1st arra element is

&x[0]orx;

% the address of the nd arra element is

&x[1]or(x+1);

Pointer 'pes

'he )90)6 memor sstem is di#ided into

segments, and each segment has 6 ;. -n address consists of t$o parts:segment

andoffset.

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Pointer tpes:

% -near pointeronl stores the offset $hich is

relati#e to the data segment.

% -far pointerstores +oth the segment and offset.

*+it integer (calledhost byte

order).

% /0amples


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!

% 'CP>4P specifies anetwork byte order$hich

is thebig-endian+te order.

% For some 2inSock functions, their arguments

(i.e., the parameters to +e passed to thesefunctions" must +e stored in net$ork +te order.

% 2inSock pro#ides functions to con#ert +et$een

host +te order and net$ork +te order:

% Prototpes for these con#ersion functions:

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6

/ndpoint -ddress% @eneric /ndpoint -ddress

'he socket a+straction accommodates man

protocol families.

% 4t supports man address families.

% 4t defines the follo$ing generic

endpoint address:

%( address family, endpoint address in

that family ) &ata tpe for generic endpoint address:

% 'CP>4P /ndpoint -ddress

For 'CP>4P, an endpoint address is

composed of the follo$ing items:

% -ddress famil isAF_INET(-ddress

FamilyforInterNET).

% /ndpoint address in that famil is

composed of anIP addressand aportnmber.

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A

'he 4P address identifies a particularcomputer, $hile the port num+er identifies aparticular application running on thatcomputer.

'he 'CP>4P endpoint address is a special

instance of the generic one:

Port Bum+er

% - port num+er identifies an application

running on a computer.

% 2hen a client program is e0ecuted,

2inSock randoml chooses an unusedport num+er for it.

% /ach ser#er program must ha#e a pre*

specified port num+er, so that the clientcan contact the ser#er.

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)

% 'he port num+er is composed of 16 +its,

and its possi+le #alues are used in the

follo$ing manner:

9 * 19: For $ell*kno$n ser#erapplications.

19 * D1!1: For user*defined

ser#er applications (tpical range to

+e used is 19 * !999".

D1! * 6!!!: For client programs.% Port num+ers for some $ell*kno$n ser#er

applications:

222 ser#er using 'CP: )9

'elnet ser#er using 'CP:

S?'P (email" ser#er using 'CP: !

SB?P ser#er using E&P: 161.

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D

% &ata tpes for 'CP>4P endpoint address:

struct in_addr {

u_long

address */

s_addr ; /* IP

};

'he 4P address is stored in a structure

instead of an unsigned long ($ith +its" +ecause of a historical reason.

%so!addrandso!addr"inare compati+le:

% 4f ou onl use 'CP>4P, ou can useso!addr"in$ithout usingso!addr.

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% /0ample 1

'he 4P address of a ser#er is

1!).1).A.6. 4ts decimal #alue is

66AD!!. 2e can specif theendpoint address for this ser#er asfollo$s:struct sockaddr_in ServerAddr;

ServerAddr.sin_family =

AF_INET;

ServerAddr.sin_port = 2000;

ServerAddr.sin_addr.s_addr =

htonl (2662729535);

% /0ample

2e specif the endpoint address for a

ser#er as follo$s:

struct sockaddr_in ServerAddr;

ServerAddr.sin_family =

AF_INET;

ServerAddr.sin_port = 2000;


htonl(INADDR_ANY);

$here the sm+olic constant

INA!!"_AN#represents a$ildcard address that matches an ofthe computers 4P address(es".

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Sketch of a 'CP Client and a 'CP Ser#er

% Esing 'CP, +oth the client and the ser#er use

three maGor steps for communication:

4nitialiHe sockets.

Communicate +et$een sockets.

Close the sockets.

% 4n this section, $e stud the a+o#e three steps in

detail.

% Remark

Bet$ork communication ma +e considered

as net$ork 4>=. 'he a+o#e steps are similar

to that for file 4>=:

% =pen a file.

% Read and $rite data.

% Close the file.

% 4nitialiHe Sockets

'here are three initialiHation steps:

% 4nitialiHe 2inSock &33.% Create a socket.

% -ssign an endpoint address to a socket.

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4nitialiHe 2inSock &33

%


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% /0ample#define WSVERS MAKEWORD(2, 0)

WSADATA wsadata ;

WSAStartup ( WSVERS, &wsadata );

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Create a Socket

% - program callsso!et()to create a ne$

socket, $hich can then act as the endpoint

of the 'CP connection.% 'he functionso!et()re8uires three

arguments:

'he 1st argument is the the protocol

famil. For 'CP>4P, specif PFI4B/'

(Protocol Famil for the 4nterB/'". 'he nd argument is the tpe of socket

to +e created. For stream ('CP"

sockets, specif S=C;IS'R/-?.

'he rd argument is simpl 9.

% 'he call toso!et()returns a socketdescriptor for the ne$l created socket.

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% /0ample

SOCKET s;

s = socket( PF_INET, SOCK_STREAM, 0 );

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-ssign an /ndpoint -ddress to a Socket

% -fter creating a socket, a ser#er must assign

its endpoint address to this socket.

'hen the client can identif the ser#ers

socket and send re8uests to this ser#er.

% Steps

'he ser#er specifies its endpoint

address. 4n other $ords, it assigns the

follo$ing three attri+ute #alues to a

structure of tpe sockaddrIin:

protocol famil (-FI4B/' for 'CP>4P",

4P address,

port num+er.

'he ser#er calls'ind()to +ind its

endpoint address to the ne$l created

socket.

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% /0amplestruct sockaddr_in ServerAddr;

/* Specify the server's endpoint

address in ServerAddr here */

bind ( s, (struct sockaddr *)

&ServerAddr, sizeof(ServerAddr)

);

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% Communicate


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);1D

% /0ample

struct sockaddr_in ServerAddr;

/* Specify the server's endpoint

address in ServerAddr here */

connect ( s, (struct sockaddr

*)&ServerAddr, sizeof(ServerAddr)

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Ser#er 3istens to Connection Re8uests

% -fter creating a socket, the ser#er calls

listen()to place this socket in passi#e

mode.

'hen the socket listens and accepts

incoming connection re8uests from

the clients.

% ?ultiple clients ma send re8uests to a

ser#er.

'he functionlisten()tells the =S to

8ueue the connection re8uests for the

ser#ers socket.

% 'he functionlisten()has t$o arguments:

the ser#ers socketJ

the ma0imum siHe of the 8ueue.

% 'he functionlisten()applies onl to

sockets used $ith 'CP.

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% /0ample

listen(s, 1);

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Ser#er -ccepts a Connection Re8uest

% 'he ser#er callsaept()to

e0tract the ne0t incoming

connection re8uest from the 8ueue.

'hen the ser#er creates a ne$

socket for this connection re8uest

and returns the descriptor of this

ne$ socket.

% Remarks

'he ser#er uses the ne$ socket for

the ne$ connection onl. 2hen

this connection ends, the ser#er

closes this socket.

'he ser#er still uses the originalsocket to accept additional

connection re8uests.

'he functionaept()applies onl

to stream ('CP" sockets.

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% /0ample

struct sockaddr_in ClientAddr;

int len;

SOCKET nsock;

len = sizeof ( ClientAddr );

nsock = accept( s, (struct

sockaddr *)&ClientAddr, &len );

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Send &ata

%


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!

% /0ample

char *message="Hello world!";

send(s, message, strlen(message),

0);

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Recei#e &ata

%


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% /0ample: recei#e a small message $ithat most ! characters

char buf[5], *bptr;

int buflen;

bptr = buf;

buflen = 5;

recv(s, bptr, buflen, 0);

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)

% Close the Sockets

Close a Socket

% =nce a client or ser#er finishes using a

socket, it callsloseso!et()to

terminate the 'CP connection

associated $ith this socket, and

deallocate this socket.

% /0ample

closesocket(s);

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Clean Ep

% 2hen an application finishes using

sockets, it must call#$%leanp()to

deallocate all data structures and socket+indings.

% /0ample

WSACleanup();

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% Se8uence of /0ecution of 2inSock Functions

2e assume that 'CP is used and the ser#er

ser#es the clientsK re8uests one after the

other.

Se8uence of e0ecution of 2inSock

functions:

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% Remarks on 2inSock Functions

2inSock has defined some data structures

that can +e used in 2inSock programs.

% /0amples

so!addr

so!addr"in

2inSock has defined some sm+olic

constants that can +e used as arguments to2inSock functions.

% /0amples

%"-

"-

%//"%$23"$-%4(stream socket"

$23"/5%4(datagram socket"

'o use these sm+olic constants and

declarations, include the header file

6inso!.hor6inso!7.h8

or

#include

#include

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{

- Simple and Complete /0ample

% 4n this section, $e implement the follo$ing client*

ser#er application using 'CP:

'he client sends the message 7Lello7 to the

ser#er.

'he ser#er sends this message +ack to the client.

'hen +oth the client and ser#er are terminated.

% Client Program

#include

#include

#define

main()

WSVERS MAKEWORD(2,0)

WSADATA

SOCKET

struct sockaddr_in

char

buf[5],int

wsadata;

s;

ServerAddr;

*message="Hello",

*bptr;i, buflen, count;

/* call WSAStartup() and socket() */

WSAStartup ( WSVERS , &wsadata ) ;

s = socket ( PF_INET , SOCK_STREAM , 0 );

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/* call connect() to connect to the server */

ServerAddr.sin_family = AF_INET ;

ServerAddr.sin_port = htons(2000) ;


htonl(2662729535);connect(s, (struct sockaddr *)

&ServerAddr, sizeof(ServerAddr));

/* call send() to send a message to the

server */

send(s, message, strlen(message), 0);

/* call recv() to receive a message from

the server */

bptr = buf; buflen = 5;

recv(s, bptr, buflen, 0);

/* Echo the received message from theserver */

for (i=0; i


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% Ser#er Program

#include

#include

#define

main()

{

WSVERS MAKEWORD(2,0)

WSADATA

SOCKET

struct sockaddr_in

char

int

wsadata;

s, nsock;

ServerAddr,

ClientAddr;

buf[5], *bptr;

i, buflen, count;

/* call WSAStartup() */

WSAStartup ( WSVERS , &wsadata ) ;

/* call socket() */s = socket ( PF_INET , SOCK_STREAM , 0 );

/* call bind() */

ServerAddr.sin_family = AF_INET;

ServerAddr.sin_port = htons(2000);


htonl(INADDR_ANY);

bind ( s, (struct sockaddr *) &ServerAddr

, sizeof(ServerAddr) );

/* call listen() */

listen(s,1);

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/* call accept() */

i = sizeof ( ClientAddr );

nsock = accept (s, (struct sockaddr

*) &ClientAddr , &i );

/* call recv() to receive a message from

the client */

bptr = buf; buflen = 5;

recv ( nsock , bptr , buflen , 0 );

/* call send() to send the message

back to the client */

send ( nsock, buf, strlen(buf), 0);

/* call closesocket() */

closesocket ( nsock );

closesocket ( s );

/* call WSACleanup() */

WSACleanup();

}

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% Remarks

For clarit, the a+o#e t$o programs do not

check an possi+le error in net$ork

communication (e.g., $hether the remote

ser#er can +e reached".

% 4n practice, a sophisticated program

should check all the possi+le errors.

4f a program has to recei#e a large message,

it should callrev()repeatedl until the

entire message has +een recei#ed. 'he

details are e0plained in the ne0t chapter.

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'utorial Pro+lems

% Esing the net$ork +te order, ho$ is the 4P

address 1!).1).A.1 stored in the memorM

% State the items in a generic endpoint address.

% State the items in a 'CP>4P endpoint address.

% 2hat is the relationship +et$een the a+o#e t$o

addressesM

% - 2inSock application has to call +oth

#$%$tartp()andso!et()for initialiHation.

2hat is the difference +et$een these t$o

functionsM

% 2hen should an application callloseso!et()9

% 2hen should an application call

#$%leanp()9

% 2hat is the main purpose of'ind()M

% Should a 'CP client call'ind()9

% Should a 'CP ser#er call'ind()9

% Consider a 'CP client*ser#er application.

Should the client callonnet()M

Should the ser#er callonnet()M

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% 2hat are the main purposes oflisten()M

% 2hat are the main purposes ofaept()M

% 4n different e0ecutions of a ser#er, is its port

num+er al$as the sameM

% 4n different e0ecutions of a client, is its port

num+er al$as the sameM

% 2hen ou $rite a ser#er program, should ou

assign a port num+er to itM

% 2hen ou $rite a client program, should ou

assign a port num+er to itM

% - program callssend()to send a message.

2hen the call returns, does it mean that the

entire message has +een sent outM

% - program callsrev()to recei#e a message.

2hen the call returns, does it mean that the

entire message has +een recei#edM

% 4f ou $ant to de#elop a ser#er that can store

199 $aiting re8uests, ho$ $ould ou doM

fundamentals of socket programming

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