Socket Programming Basics

8/9/2019 Socket Programming Basics

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Sockets Part 2


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struct sockaddr

{

unsigned short sa_family; /* Address family (e.g., AF_IN!" */

char sa_data#$%&; /* 'rotocols)ecific address information */

;

struct sockaddr_in

{

unsigned short sin_family; /* Internet )rotocol (AF_IN!" */

unsigned short sin_)ort; /* 'ort ($+its" */

struct in_addr sin_addr; /* Internet address (-its" */

char sin_ero#0&; /* Not used */;

struct in_addr

{

unsigned long s_addr; /* Internet address (-its" */

;

Generic

IP

Specifc

sockaddr

sockaddr_in

Family

Family Port

Blob

Internet address Not used

2 bytes 2 bytes 4 bytes 8 bytes


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A socket address structures is alays passed by re!erence "enpassed as an ar#ument to any socket !unctions$ But any socket!unction t"at takes one o! t"ese pointers as an ar#ument mustdeal it" socket address structures !rom any o! t"e supportedprotocol !amilies$


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%"e IP&' socket address is defned by includin# t"e (netinet)in$"*

"eader


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+%"e SIN',-.N constant must be defned i! t"e system supports t"e

len#t" member !or socket address structures$

+%"e IP&' !amily is AF,IN.%'/ "ereas t"e IP&4 !amily is AF,IN.%$

+%"e members in t"is structure are ordered so t"at i! t"esockaddr,in' structure is '40bit ali#ned/ so is t"e 1280bit sin',addr

member$ n some '40bit processors/ data accesses o! '40bit &aluesare optimi3ed i! stored on a '40bit boundary$

+%"e sin',oin!o member is di&ided into to felds5+%"e lo0order 26 bits are t"e o label+%"e "i#"0order 12 bits are reser&ed

+%"e sin',scope,id identifes t"e scope 3one in "ic" a scopedaddress is meanin#!ul/ most commonly an inter!ace inde7 !or a link0local address


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A ne #eneric socket address structure as defned as part o! t"e IP&'

sockets API/ to o&ercome some o! t"e s"ortcomin#s o! t"e e7istin# structsockaddr$ nlike t"e struct sockaddr/ t"e ne struct sockaddr,stora#e islar#e enou#" to "old any socket address type supported by t"e system$

%"e sockaddr,stora#e structure is defned by includin# t"e (netinet)in$"*"eader


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%"e sockaddr,stora#e type pro&ides a #eneric socket addressstructure t"at is di9erent !rom struct sockaddr in to ays5

a$I! any socket address structures t"at t"e system supports "a&eali#nment re:uirements/ t"e sockaddr,stora#e pro&ides t"estrictest ali#nment re:uirement$

b$%"e sockaddr,stora#e is lar#e enou#" to contain any socketaddress structure t"at t"e system supports$


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11

Net1ork 'rogramming issues

Byte Ordering Naming

Addressing


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12

2yte 3rdering of Integers

Different CPU architectures have different byte

ordering

D3

high-order byte low-order byte

memory

address A

memory

address A !

Stored at little0endiancomputer

Stored at bi#0endiancomputer

low-order byte high-order byte

"#Inte#er representation ;2byteessa#e in >emory o!o! bi#0endian?omputer

>essa#e is sentacross Netork 48 4@ 4? 4? 'F 61

66

>essa#e is5ello/1C

>essa#e is5ello/@12C

Pro c e s s i

n#

48 4@ 4? 4? 'F 61

66

>essa#e in >emory o!little0endian?omputer P

ro c e s s

in#

Anser5

Not"in# i! t"ey do not e7c"an#e inte#ersD

But5 I! t"ey e7c"an#e inte#ers/ t"ey ould #et t"eron# order o! bytes/ t"ere!ore/ t"e ron# &alueD

.7ample5


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14

2yte 3rdering 4olution )here are two solutions if com'uters with different byte

ordering system want to communicate )hey must *now the *ind of architectureof the sending

com'uter+bad solution, it has not been im'lemented

.ntroduction of a networ* byte order +big-endian/ )he functions

are%uint16_t htons(uint16_thost16bitvalue)

uint32_t htonl(uint32_t host32bitvalue)

uint16_t ntohs(uint16_t net16bitvalue)uint32_t ntohs(uint32_t net32bitvalue) Note% use for all integers +short and long, which

are sent across the networ*

.ncluding 'ort numbers and .P addresses


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Networ* Byte Order

# if __BYTE_ORDER == __BIG_ENDIAN

/* The host byte order is the same as netor! byte order"

so these fn$tions are a%% &st identity' */

# define ntoh%() ()

# define ntohs() ()

# define hton%() ()

# define htons() ()

# e%se# if __BYTE_ORDER == __+ITT+E_ENDIAN

# define ntoh%() __bsa,_-. ()

# define ntohs() __bsa,_0 ()

# define hton%() __bsa,_-. ()

# define htons() __bsa,_0 ()

# endif

# endif

#in$%de 1netinet/in'h2

int0_t htons(int0_t host0bit3a%e4int-._t hton%(int-._t host-.bit3a%e4int0_t ntohs(int0_t net0bit3a%e4int-._t ntoh%(int-._t net-.bit3a%e4


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Program to determine host byte order


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Byte 0ani'ulation "unctions# grou's of functions o'erate on multibyte fields

can1t rely on C convention of null-terminated string, since

data can contain 2eros +e/g .P addresses

operate on multibyte felds/ it"out interpretin# t"edata/ and it"out assumin# t"at t"e data is a null0

terminated ? strin#need t"ese types o! !unctions "en dealin# it"socket address structures because e need tomanipulate felds suc" as IP addresses


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Byte 0ani'ulation functions4#include

void bzero(void *dest, size_t nbtes)! "*zeros nbtes

void bco(const void *src, void *dest, size_tnbtes)!

int bc$(const void *tr1, const void *tr2,

size_t nbtes)!5eturns% 6 if e7ual, non2ero if une7ual

void* $e$set(void *dest, int c, size_t len)!

void* $e$c(void *dest, const void *src, size_tnbtes)!

int $e$c$(const void *tr1, const void *tr2,size_t nbtes)!

5eturns% 6 if e7ual, 86 or 96 if une7ual


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1E

Naming and Addressing

:ost name identifies a single host +Domain Name ;ystem variable length string+e/g/ www/ber*eley/edu is ma''ed to one or more .P addresses

.P Address written as dotted octets +e/g/ !6/6/6/! 3# bits/ Not a number< But often needs to be

converted to a 3#-bit to use/ Port number

identifies a 'rocess on a host != bit number


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inet>aton, inet>addr and

inet>ntoa Convert internet addresses between

A;C.. strings and networ* byte ordered

binary values inet>aton, inet>ntoa and inet>addr convert

an .'v? address from a dotted-decimal

string +e/g/ @#6=/!=/!!#/= to its 3#-bit

networ* byte ordered binary value

Newer functions inet>'ton and inet>nto'

handle both .Pv? and .Pv= addresses/


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IPv4 Address Conversion Functions

include (arpa)inet$"*

int inet,aton;const c"ar strptr/ struct in,addr addrptr


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IPv4 IPv! Address Conversion Functions

inet_"tonand inet_nto"Functionsinclude (arpa)inet$"*

int inet,pton;int !amily/ const c"ar strptr/ &oid addrptr


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#ummary o0 address conversion 0unctions


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#ock_nto" Function

inet,ntopis t"at it re:uires t"e caller to pass a pointer to a binary addret"e caller to kno t"e !ormat o! t"e structure and t"e address !amily

sock,ntopt"at takes a pointer to a socket address structure/ looks inside t"e struct

and calls t"e appropriate !unction to return t"e presentation !ormat o! t"e address$

include Lunp$"L

c"ar sock,ntop;const struct sockaddr sockaddr/ socklen,taddrlen


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readn/ riten/ and readlineFunctions

include Lunp$"L

ssi3e,t readn;int fledes/ &oid bu9/ si3e,t nbytes


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2'

Other useful functions

gethostname+char name, int len%gets the name of thecurrent host gethostbyaddr+char addr, int len, int ty'e%converts .P

hostname to structure containing long integer gethostbyname+const char name);

getaddrinfo() & getnameinfo() work with IPv6 as well better to use these

int getsoc*name+int socket, struct soc*addr restrictaddress, soc*len>t restrict address_lenE

&arning% chec* function assum'tions about byte-ordering

+host or networ*/ Often, they assume 'arameters F returnsolutions in networ* byte-order


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2M

Issues in ?lient)Ser&er

Pro#rammin#


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.ssues in Client Programming .dentifying the ;erver/

Goo*ing u' a .P address/

Goo*ing u' a well *nown 'ort name/

;'ecifying a local .P address/

UDP client design/ )CP client design/


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2E

.dentifying the ;erver O'tions%

hard-coded into the client 'rogram/

re7uire that the user identify the server/

read from a configuration file/

use a se'arate 'rotocolFnetwor* service to

loo*u' the identity of the server/


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=1

;ervices and Ports 0any services are available via @well

*nown addresses +names/

)here is a ma''ing of service names to

'ort numbers%struct *servent getservbna$e( char *service,

char *rotocol )!

servent-9s>'ort is the 'ort number in

networ* byte order/


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=2

;'ecifying a Gocal Address &hen a client creates and binds a

soc*et it must s'ecify a local 'ort and

.P address/ )y'ically a client doesnHt care what 'ort

it is on%

haddr%>ort & htons(')!

give me any avai2ab2e "ort give me any avai2ab2e "ort


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==

Gocal .P address A client can also as* the o'erating system

to ta*e care of s'ecifying the local .P

address%

haddr%>sin_addr.s_addr&

htonl(++_-)!

5ive me the a""ro"riate address5ive me the a""ro"riate address


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=4

UDP Client Design Istablish server address +.P and 'ort/

Allocate a soc*et/

;'ecify that any valid local 'ort and .P

address can be used/

Communicate with server +send, recv

Close the soc*et/


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=@

Connected mode UDP A UDP client can call connect+ to

establish the address of the server/

)he UDP client can then use read+ andwrite+ or send+ and recv+/

A UDP client using a connected mode

soc*et can only tal* to one server+using the connected-mode soc*et/


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='

)CP Client Design Istablish server address +.P and 'ort/

Allocate a soc*et/

;'ecify that any valid local 'ort and .P

address can be used/

Call connect+

Communicate with server +read,write/

Close the connection/


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=M

Closing a )CP soc*et 0any )CP based a''lication 'rotocols

su''ort multi'le re7uests andFor

variable length re7uests over a single)CP connection/

:ow does the server *now when the

client is done +and it is OJ to close thesoc*et (


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=8

Partial Close One solution is for the client to shut

down only itHs writing end of the soc*et/

)he shutdown+ system call 'rovidesthis function/

shutdown( int s, int direction);

direction can be 6 to close the reading endor ! to close the writing end/

shutdown sends info to the other 'rocess.ASO % PGA>

+transport protocol "andles t"e tou#"transport protocol "andles t"e tou#"stu9$stu9$

+re:uires separate socket !or eac"re:uires separate socket !or eac"connection$connection$?onnectionless?onnectionless

+less o&er"eadless o&er"ead+no limitation on number o! clientsno limitation on number o! clients


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Connectionless .terative

server Uses connectionless 'rotocol% UDP/ ;erver 'rocesses one re7uest at a time/

;erver uses one single 'ort Lwell-*nown 'ortM


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Connection oriented concurrent server


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Connection-oriented concurrent server

Uses connection-oriented% )CP At first connection is established/ Uses one well-*nown 'ort and many e'hemeral 'orts

;erver issues 'assive-o'en at well-*nown 'ort/ Client initially a''roaches this 'ort/ After connection is made, server assigns a tem'orary 'ort to free

the well-*nown 'ort/ Data transfer via e'hemeral 'ort/


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4E

Concurrent ;erver

Design AlternativesOne child 'er client

;'awn one thread 'er client

Prefor*ing multi'le 'rocesses

Prethreaded ;erver


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@6

One child 'er client )raditional UniK server% )CP% after call to accet(),call or/().

UDP% after readro$(),call or/(). Iach 'rocess needs only a few soc*ets/

;mall re7uests can be serviced in a small

amount of time/

Parent 'rocess needs to clean u' after

children


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"or* and eKec functions

Need to create a new child 'rocess tohandle each incoming client

re7uestFtransaction

for* function is the only way in UniK to

create a new 'rocess%

#in$%de 1nistd'h2

,id_t for!(3oid4

5eturns% 6 in child, 'rocess .D of child in 'arent, -! on error Called once but returns !5I6

3nce in the )arent )rocess (returns child )rocess id", and once in the child )rocess (return of 7"


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0ore "or*ing

All descri'tors o'en in the 'arent before

the call to for*+ are shared with the

child after for* returns/ .ncluding the connected soc*et file

descri'tion returned by acce't


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IKam'le


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@@

One thread 'er client Almost li*e using for*+ - ust call

'thread>create instead/

Using threads ma*es it easier +lessoverhead to have sibling 'rocesses

share information/

;haring information must be donecarefully +use 'thread>muteK


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@'

Preor/()Hd ;erver Creating a new 'rocess for each client

is eK'ensive/

&e can create a bunch of 'rocesses,each of which can ta*e care of a client/

Iach child 'rocess is an iterative

server/


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@M

Preor/()Hd )CP ;erver .nitial 'rocess creates soc*et and binds

to well *nown address/

Process now calls or/()a bunch oftimes/

All children call accet(). )he neKt incoming connection will be

handed to one child/


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@8

Prefor*ing :aving too many 'refor*ed children can

be bad/

Using dynamic 'rocess allocationinstead of a hard-coded number of

children can avoid 'roblems/

)he 'arent 'rocess ust manages thechildren, doesnHt worry about clients/


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@E

;oc*ets library vs/ system call

A 'refor*ed )CP server wonHt usually

wor* the way we want if socketsis not

'art of the *ernel% calling acce't+ is a library call, not an

atomic o'eration/

&e can get around this by ma*ing sureonly one child calls acce't+ at a time

using some loc*ing scheme/


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'6

Prethreaded ;erver

;ame benefits as 'refor*ing/

Can also have the main thread do all

the calls to acce't+ and hand off eachclient to an eKisting thread/


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'1

&hatHs the best server design

for my a''lication( 0any factors%

eK'ected number of simultaneous clients/

)ransaction si2e +time to com'ute orloo*u' the answer

ariability in transaction si2e/

Available system resources +'erha's whatresources can be re7uired in order to run

the service/


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'2

;tatelessness

tate' .nformation that a server

maintains about the status of ongoing

client interactions/ Connectionless servers that *ee' state

information must be designed carefully";close(listen_sd";e?it($";else)rintf(>Iser8er ind(" is 3:@n>";


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/* 4et the listen acklog */rc 9 listen(listen_sd, E";if(rc = 7"{)error(>Iser8er listen(" error>";

close(listen_sd";e?it($";else)rintf(>Iser8er listen(" is 3:@n>";

/* Inform the user that the ser8er is ready */)rintf(>!he Iser8er is ready@n>";

/* Go through the loo) once for each connection */

for(i97; i = num; iHH"{/* 5ait for an incoming connection */)rintf(>Iteration JKd@n>, iH$";)rintf(> 1aiting on acce)t("@n>";acce)t_sd 9 acce)t(listen_sd, NLMM, NLMM";if(acce)t_sd = 7"{)error(>Iser8er acce)t(" error>";

close(listen_sd";e?it($";else)rintf(>acce)t(" is 3: and com)leted successfully@n>";


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/* ecei8e a message from the client */)rintf(>I am 1aiting client(s" to send message(s" to me...@n>";rc 9 rec8(acce)t_sd, uffer, sieof(uffer", 7";if(rc =9 7"{)error(>Iser8er rec8(" error>";

close(listen_sd";close(acce)t_sd";e?it($";else)rintf(>!he message from client @>Ks@>@n>, uffer";/* cho the data ack to the client */)rintf(>choing it ack to client...@n>";len 9 rc;rc 9 send(acce)t_sd, uffer, len, 7";

if(rc =9 7"{)error(>Iser8er send(" error>";close(listen_sd";close(acce)t_sd";e?it($";else)rintf(>Iser8er send(" is 3:.@n>";/* 6lose the incoming connection */close(acce)t_sd";/* 6lose the listen socket */close(listen_sd";return 7;

Concurrent server +using for*


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Concurrent server +using for*

inet_ntoa=remoteaddr(sin_addr>nebu0 ''do any other tasks as needede3it=

KK


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Socket Programming Basics

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