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sockets

Socket Programming in C/C++

c Mani Radhakrishnan and Jon Solworth

September 24, 2004

c Mani Radhakrishnan and Jon Solworth Socket Programming in C/C++
http://goforward/http://find/http://goback/


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sockets

Contact Info

Mani Radhakrishnan

Office

4224 SEL

emailmradhakr @ cs . uic . edu

Office HoursTuesday 1 - 4 PM



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sockets TCPUDP

Introduction

Sockets are a protocol independent method of creating aconnection between processes. Sockets can be either

connection based or connectionless: Is a connection

established before communication or does each packetdescribe the destination?packet based or streams based : Are there message boundariesor is it one stream?

reliable or unreliable. Can messages be lost, duplicated,reordered, or corrupted?



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sockets TCPUDP

Socket characteristics

Socket are characterized by their domain, type and transportprotocol. Common domains are:

AF UNIX: address format is UNIX pathnameAF INET: address format is host and port number

Common types are:virtual circuit: received in order transmitted and reliably

datagram: arbitrary order, unreliable



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sockets TCPUDP

Socket characteristics (contd)

Each socket type has one or more protocols. Ex:TCP/IP (virtual circuits)UDP (datagram)

Use of sockets:Connectionbased sockets communicate client-server: theserver waits for a connection from the clientConnectionless sockets are peer-to-peer: each process issymmetric.


TCP


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sockets TCPUDP

Socket APIs

socket : creates a socket of a given domain, type, protocol(buy a phone)bind : assigns a name to the socket (get a telephone number)listen : species the number of pending connections that

can be queued for a server socket. (call waiting allowance)accept : server accepts a connection request from a client(answer phone)connect : client requests a connection request to a server

(call)send, sendto : write to connection (speak)recv, recvfrom : read from connection (listen)shutdown : end the call


TCP


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sockets TCPUDP

Connection-based communication

Server performs the following actionssocket : create the socketbind : give the address of the socket on the server

listen : species the maximum number of connectionrequests that can be pending for this processaccept : establish the connection with a specic clientsend,recv : stream-based equivalents of read and write(repeated)shutdown : end reading or writingclose : release kernel data structures


TCP


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sockets TCPUDP

TCP client

Client performs the following actionssocket : create the socket

connect : connect to a serversend,recv : (repeated)shutdown

close


TCP


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TCP-based sockets

bind

listen

accept close

send/recv

shutdown

close

socket

connect

send/recv

shutdown

close

server client

socket


TCP


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socket API# i n c l u d e < s y s / t y p e s . h >

2 # i n c l u d e < s y s / s o c k e t . h >

4 i n t sock e t ( i n t domain , i n t type , i n t p r o t o c o l ) ;

Returns a le descriptor (called a socket ID) if successful, -1otherwise. Note that the socket returns a socket descriptor whichis the same as a le descriptor.The domain is AF INET.The type argument can be:

SOCK STREAM: Establishes a virtual circuit for streamSOCK DGRAM: Establishes a datagram for communicationSOCK SEQPACKET: Establishes a reliable, connection based,two way communication with maximum message size. (This isnot available on most machines.)

protocol is usually zero, so that type denes the connection

within domain .c Mani Radhakrishnan and Jon Solworth Socket Programming in C/C++

k TCP


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bind

# i n c l u d e < s y s / t y p e s . h >2 # i n c l u d e < s y s / s o c k e t . h >

4 i n t bind ( i n t s i d , s t r u c t s o c k a d d r addrPtr , i n t l e n )

Where

sid : is the socket idaddrPtr : is a pointer to the address family dependentaddress structurelen : is the size of *addrPtr

Associates a socket id with an address to which other processescan connect. In internet protocol the address is [ipNumber,portNumber]


k t TCP


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sockets UDP

sockaddr

For the internet family:s t r u c t s o c k a d d r i n {

2 s a f a m i l y t s i n f a m i l y ; / / = AF INET i n p o r t t s i n p o r t ; // i s a p o r t n umber

4 s t r u c t i n a d d r s i n a d d r ; // an I P a d dr e ss }

For unix sockets (only works between processes on the samemachine)s t r u c t s o c ka d d r u n {

2 u i n t 8 t s u n l e n g t h ; // s h o r t s u n f a m i l y ; // = AF LOCAL

4 char s u n p a t h [ 1 0 0 ] ; // n u l l t e r m i n a t e d p a th na me // ( 10 0 i s p o s i x 1 . g minimum )

6 }

When using internet sockets, the second parameter of bind (of type sockaddr in * ) must be cast to (sockaddr *) .


sockets TCP


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sockets UDP

listen


4 i n t l i s t e n ( i n t s i d , i n t s i z e ) ;

Where size it the number of pending connection requests allowed(typically limited by Unix kernels to 5).Returns the 0 on success, or -1 if failure.


sockets TCP


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sockets UDP

accept


4 i n t accep t ( i n t s i d , s t r u c t s o c k a d d r addrPtr , i n t l e n P t r )

Returns the socketId and address of client connecting to socket.if lenPtr or addrPtr equal zero, no address structure is returned.lenPtr is the maximum size of address structure that can becalled, returns the actual value.Waits for an incoming request, and when received creates a socketfor it.


sockets TCP


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sockets UDP

accept styles

There are basically three styles of using accept:Iterating server: Only one socket is opened at a time. When the

processing on that connection is completed, thesocket is closed, and next connection can be

accepted.Forking server: After an accept, a child process is forked off to

handle the connection. Variation: the child processesare preforked and are passed the socketId.

Concurrent single server: use select to simultaneously wait on allopen socketIds, and waking up the process only whennew data arrives.


sockets TCPUDP


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sockets UDP

Pro and Con of Accept styles

Iterating server is basically a low performance technique sinceonly one connection is open at a time.Forking servers enable using multiple processors. But they

make sharing state difficult, unless performed with threads.Threads, however present a very fragile programmingenvironment.Concurrent single server: reduces context switches relative to

forking processes and complexity relative to threads. But doesnot benet from multiprocessors.


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UDP

send


4 i n t sen d ( i n t s i d , c o ns t c h ar b u f f e r P t r ,i n t l en , i n t f l a g )

Send a message. Returns the number of bytes sent or -1 if failure.(Must be a bound socket).ag is either

0: default

MSG OOB: Out-of-band high priority communication


sockets TCPUDP


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UDP

recv


4 i n t r ec v ( i n t s i d , cha r b u f f e r P t r ,i n t l en , i n t f l a g s )

Receive up to len bytes in bufferPtr . Returns the number of bytes received or -1 on failure.ags can be either

0: default

MSG OOB: out-of-bound messageMSG PEEK: look at message without removing


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UDP

shutdown


4 i n t shutdown( i n t s i d , i n t how)

Disables sending (how=1 or how=2) or receiving (how=0 orhow=2). Returns -1 on failure.acts as a partial close.


sockets TCPUDP


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UDP

connect

this is the rst of the client calls


4 i n t conne ct ( i n t s i d , s t r u c t s o c k a d d r addrPtr , i n t l e n )

Species the destination to form a connection with (addrPtr), andreturns a 0 if successful, -1 otherwise.


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U

Denoting Connections

Note that a connection is denoted by a 5-tuple:from IPfrom portprotocolto IPto port

So that multiple connections can share the same IP and port.


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Port usage

Note that the initiator of communications needs a xed port totarget communications.This means that some ports must be reserved for these well

known ports.Port usage:0-1023: These ports can only be binded to by root1024-5000: well known ports

5001-64K-1: ephemeral ports


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APIs for managing names and IP addresses

We next consider a number of auxiliary APIs:The hostent structure: describes IP, hostname pairs

gethostbyname : hostent of a specied machinehtons, htonl, ntohs, ntohl : byte orderinginet pton, inet ntop : conversion of IP numbers betweenpresentation and strings


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gethostname

# i n c l u d e < u n i s t d . h >2

i n t gethostname ( cha r h os tn am e , s i z e t n am eL en gt h )

Returns the hostname of the machine on which this commandexecutes (What host am i?). Returns -1 on failure, 0 on success.MAXHOSTNAMELENis dened in .


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hostent structure

s t r u c t h o s t e n t {2 char h name ; / / o f f i c i a l ( c a n o n i c a l ) name o f t h e h o s t

char h a l i a s e s ; // n u l l t er mi na te d a r ra y o f a l t e r n a t i v e hostnames 4 i n t h a d d r t y p e ; // h o s t a d d r e s s t y p e AF INET o r AF INET6

i n t h l e n g t h ; // 4 o r 16 b yt es 6 char h a d d r l i s t ; // I Pv4 o r I Pv6 l i s t o f a d d r es s e s

}

Error is return through h error which can be:HOST NOT FOUND

TRY AGAIN

NO RECOVERYNO DATA


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Gethostbyname

Auxiliary functions# i n c l u d e < ne tdb . h >

2s t r u c t h o s t e n t gethostbyname ( c o n st c h ar hostname)

Translates a DNS name into a hostent.Example:s t r u c t h o s t e n t h o s t E n t i t y =

2 g e t ho s tb y n am e ( b e r t . c s . u i c . e du ) ;memcpy( soc ke tAd dr > s i n a d d r ,

4 h o s t E n t i t y > h a d d r l i s t [ 0 ] ,

h o s t E n t i t y > h l e n g t h ) ;


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Network byte ordering

Network ordering in big endian. (Sparc is big endian, Intel is littleendian).// Host t o n et wo rk b yt e o r de r f o r s h o r t s ( 16 b i t )

2 u i n t 1 6 t h to ns ( u i n t 1 6 t v ) ;

4 // Host t o n et wo rk b yt e o r de r f o r l on g ( 32 b i t )u i n t 3 2 t h t on l ( u i n t 3 2 t v ) ;

6// Network t o h os t b yt e o r de r f o r l on g ( 16 b i t )

8 u i n t 1 6 t n to hs ( u i n t 1 6 t v ) ;

10 // Network t o h os t b yt e o r de r f o r l on g ( 32 b i t )u i n t 3 2 t n t oh l ( u i n t 3 2 t v ) ;


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IP Number translation

IP address strings to 32 bit numberIn what follows, p stands for presentation.Hence, these routines translate between the address as a string andthe address as the number.Hence, we have 4 representations:

IP number in host orderIP number in network orderPresentation (eg. dotted decimal)

Fully qualied domain nameOnly the last needs an outside lookup to convert to one of theother formats.


sockets TCPUDP


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inet pton

# i n c l u d e < a r p a / i n e t . h >2

i n t i n e t p t o n ( i n t f a m i l y , c o ns t c h ar s t r P t r ,4 v o i d a d d r P t r ) ;

returns 1 if OK, 0 if presentation error, -1 errorWhere family is either AF INET or AF INET6.The strPtr is the IP address as a dotted string.Finally, addrPtr points to either the 32 bit result (AF INET) or

128 bit result (AF INET6

).


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inet ntop

# i n c l u d e < a r p a / i n e t . h >2

i n t i n e t n t o p ( i n t f a m i l y , c o ns t c h ar addrPtr ,4 cha r s tr Pt r , s i z e t l en ) ;

returns 1 if OK, 0 if presentation error, -1 errorWhere family is eitherAF INET or AF INET6.The strPtr is the return IP address as a dotted string.Finally, addrPtr points to either the 32 bit (AF INET) or 128 bit(AF INET6).Length is the size of destination.


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Example: TCP/IP Server CodeWithout error checking.

s o c k a d d r i n s e r v er A d d r ;2 s o c ka d d r & s e r v e r A d d r C a st = ( s o c ka d d r &) s e r v e r A d d r ;

4 // g et a t cp / i p s oc ke t i n t l i s t en Fd = soc ke t (AF INET , SOCK STREAM, 0) ;

6bze ro (&serve rAddr , s i z e o f ( s e r v e r A d d r ) ) ;

8 s e r v e r A d d r . s i n f a m i l y = A F INET ;// any i n t e rn e t i n t e r f a c e on t h i s s e rv e r .

10 se rv e rA ddr . s i n ad dr . s add r = h to n l ( INADDR ANY) ;s e r v e r A d d r . s i n p o r t = h t o ns ( 1 3 ) ;12

b i n d ( l i s t e n F d , &s e r v e r A d d r C a s t , s i z e o f ( s e r v e r A d d r ) ) ;14

l i s t e n ( l i s t en Fd , 5 ) ;16

f o r ( ; ; ) {18 i n t connec tFd =

a c c e p t ( l i s t e n F d , ( s o c k a d d r ) NULL, NULL ) ;20 // . . r ea d and w r i t e o p e r a t io n s on c on ne ct Fd . .

shutdown( connec tFd , 2 ) ;22 c l os e ( connec tFd ) ;

}

Note that the above is an iterative server, which means that it

serves one connection at a time.c Mani Radhakrishnan and Jon Solworth Socket Programming in C/C++

sockets TCPUDP


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Concurrent Server

To build a concurrent server:a fork is performed after the accept.

The child process closes listenFd, and communicates usingconnectFd.The parent process closses connectFd, and then loops back tothe accept to wait for another connection request.


sockets TCPUDP


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Example: TCP/IP Client code

s o c k a d d r i n s e r v er A d d r ;2 s o c ka d d r & s e r v e r A d d r C a st = ( s o c ka d d r &) s e r v e r A d d r ;

4 // g et a t cp / i p s oc ke t i n t sock Fd = so c ke t (AF INET , SOCK STREAM, 0 ) ;

6

bze ro (&serve rAddr , s i z e o f ( s e r v e r A d d r ) ) ;8 s e r v e r A d d r . s i n f a m i l y = A F INET ;// h o st I P # i n d o tt e d d e ci m a l f or m at !

10 i n e t p t o n ( AF INET , s e rv er N am e , s e r v e r A d d r . s i n a d d r ) ;s e r v e r A d d r . s i n p o r t = h t o ns ( 1 3 ) ;

12connec t ( sockFd , s e rve r AddrCa s t , s i z e o f ( s e r v e r A d d r ) ) ;

14 // . . r ea d and w r i t e o p e r a t io n s on so ck Fd . .shutdown( sockFd , 2 ) ;

16 c l os e ( sockFd ) ;


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Connectionless communication

Communication is symmetric (peer-to-peer)socket

bind : bind is optional for initiatorsendto, recvfrom (repeated)shutdown

close


sockets TCPUDP


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Connectionless communication

socket

bind

sendto/recvfrom

shutdown

close

client


sockets TCPUDP


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UDP variations

It is not necessary for both sockets to bind

The receiver gets the address of the sender

It is possible for a UDP socket toconnectIn this case, send/recv (or write/read) must be used insteadof sendto/recvfrom .Asynchronous errors can be returned (using ICMP)


sockets TCPUDP


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sendto

for connectionless protocols# i n c l u d e < s y s / t y p e s . h >


4 i n t send to ( i n t s i d , c o ns t v o i d b u f f e r P t r ,s i z e t b u ff e rL e ng th , i n t f l a g ,

6 s t r u c t s o c k a d d r a d dr P tr , s o c k l e n t a d dr L en g th )

Send a buffer, bufferPtr , of length bufferLength to addressspecied by addrPtr of size addrLength . Returns number of bytes sent or -1 on error.


sockets TCPUDP


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recvfrom

for connectionless protocols# i n c l u d e < s y s / t y p e s . h >


4 i n t r ecvf r om ( i n t s i d , v o i d b u f f e r P t r , i n t b u f f e r L e n g t h ,i n t f l a g , s o ck a dd r addrPtr , i n t addrLeng thP t r )

Receive a buffer inbufferPtr of maximum length bufferLengthfrom an unspecied sender.Sender address returned in addrPtr , of size *addrLengthPtr .

Returns number of bytes receive or -1 on error.


sockets TCPUDP


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Example: UDPserver

i n t so ck et Id = so ck et (AF INET , SOCK DGRAM, 0) ;

2 s o c k a d d r i n s e rv e rA d dr , c l i e n t A d d r ;4 s o ck a dd r &s e r v er A d d rC a s t = ( s oc k ad d r &) s e r v e rA d d r ;

s o c k ad d r &c l i e n t A d d r C a s t = ( s o c ka d d r &) c l i e n t A d d r ;6

// a l l ow c o nn e ct i on t o any a dd r o n h os t 8 // f o r h o st s w it h m u l t i p l e n et wo rk c o nn e c ti o ns

// and a s t s e r v e r p o rt .10 s e r v er A d d r . s i n f a m i l y = AF INET ;

s e r v er A d d r . s i n p o r t = h t o ns ( s e r v e r P o r t ) ;12 s e r v e r A d d r . s i n a d d r . s a d d r = INADDR ANY;

14 // a s s o ci a t e p r oc e ss w it h p or t b i n d ( s o c k e t I d , &s e r v e r A d d r C a s t , s i z e o f ( addr ) ) ;

16// r e c e iv e from a c l i e n t

18 i n t s i z e = s i z e o f ( c l i e n t A d d r ) ;r e c vf r o m ( s o c ke t I d , b u f f e r , b u f f e r S i z e ,

20 0 , c l i e n t A d d r C a s t , &s i z e ) ;

22 // r e p l y to th e c l i e n t j u s t r e ce i ve d f roms e nd t o ( s o c ke t I d , b u f f e r , b u f f e r S i z e ,

24 0 , c l i e n t A d d r C a s t , s i z e ) ;

26 c l o se ( s o ck e t I d ) ;


sockets TCPUDP


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Example: UDPclient

i n t so ck et Id = so ck et (AF INET , SOCK DGRAM, 0) ;2

s o c k a d d r i n s e rv e rA d dr , c l i e n t A d d r ;4 s o ck a dd r &s e r v er A d d rC a s t = ( s oc k ad d r &) s e r v e rA d d r ;

s o c k ad d r &c l i e n t A d d r C a s t = ( s o c ka d d r &) c l i e n t A d d r ;6

// s p e c i f y s e r v e r a dd re ss , p or t 8 s e rv e rA d dr . s i n f a m i l y = AF INET ;

s e r v er A d d r . s i n p o r t = h t o ns ( s e r v e r P o r t ) ;10 s t r u c t h o s t e n t hp = gethostbyname ( hostName ) ;

memcpy(( cha r )& s e r v e r A d d r . s i n a d d r ,12 ( cha r )hp> h a d d r , hp> h l e n g t h ) ;

14 // no n ee d t o b in d i f n ot p ee r to p e e r i n t s i z e = s i z e o f ( s e r v e r A d d r ) ;

16 s en dt o ( s o ck e tI d , b u f fe r , b u f f e r S i ze , 0 ,s e r v e rA d d r C as t , s i z e ) ;

18

r e c vf r o m ( s o c ke t I d , b u f f e r , b u f f e r S i z e , 0 ,20 se r v e r A d d r C a s t , &s i z e ) ;

22 c l o se ( s o ck e t I d ) ;


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