Structures sockaddr and sockaddr$\_$in are used to handle the addresses of network traffic. In various

system calls or functions, these two structures are used whenever dealing with network addresses.

SOCKADDR uses the remaining $14$ bytes to represent Sa$\_$data, while sockaddr$\_$in splits $14$ bytes into

Sin$\_$port, sin$\_$addr, and Sin$\_$zero

Sockaddr and sockaddr$\_$in contain the same data, but they differ in their use:

Programmers should not operate SOCKADDR, as it is for the operating system.

Programmers should use sockaddr$\_$in to represent addresses, sockaddr$\_$in distinguish between

addresses and ports, and are more convenient to use.

include

struct sockaddr $\{$

unsigned short sa$\_$family; $//2$ bytes address family, AF$\_$xxx

char sa$\_$data$[14]$; $//14$ bytes of protocol address

$\}$;

$//$ IPv$4$ AF$\_$INET sockets:

struct sockaddr$\_$in $\{$

short sin$\_$family; $//2$ bytes e$.$g$.$ AF$\_$INET, AF$\_$INET$6$

unsigned short sin$\_$port; $//2$ bytes e$.$g$.$ htons$(3490)$

struct in$\_$addr sin$\_$addr; $//4$ bytes see struct in$\_$addr, below

char sin$\_$zero$[8]$; $//8$ bytes zero this if you want to

$\}$;

struct in$\_$addr $\{$

unsigned long s$\_$addr; $//4$ bytes load with inet$\_$pton$()$

$\}$;

server.c

$/*$ A simple server in the internet domain using TCP

The port number is passed as an argument $*/$

#include

#include

#include

#include

#include

#include

#include

void error$($char $*$msg$)$

$\{$

perror$($msg$)$;

exit$(1)$;

$\}$

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

$\{$

int sockfd, newsockfd, portno, clilen;

char buffer$[256]$;

struct sockaddr$\_$in serv$\_$addr, cli$\_$addr;

int n;

if $($argc $<2)\{$

fprintf$($stderr$,$"ERROR, no port provided

$")$;

exit$(1)$;

$\}$

sockfd $=$ socket$($AF$\_$INET, SOCK$\_$STREAM, $0)$;

if $($sockfd $<0)$

error$("$ERROR opening socket"$)$;

bzero$(($char $*)$ &serv$\_$addr, sizeof$($serv$\_$addr$))$;

$/*$ erases the data in the n bytes of the memory$*/$

portno $=$ atoi$($argv$[1])$;

serv$\_$addr.sin$\_$family $=$ AF$\_$INET;

serv$\_$addr.sin$\_$addr.s$\_$addr $=$ INADDR$\_$ANY;

serv$\_$addr.sin$\_$port $=$ htons$($portno$)$;

$/*$ Host to net byte order for short int$*/$

if $($bind$($sockfd$,($struct sockaddr $*)$ &serv$\_$addr, sizeof$($serv$\_$addr$))<0)$

error$("$ERROR on binding"$)$;

listen$($sockfd$,5)$;

$/*5$ outstanding connections$*/$

clilen $=$ sizeof$($cli$\_$addr$)$;

newsockfd $=$ accept$($sockfd$,($struct sockaddr $*)$ &cli$\_$addr, &clilen$)$;

if $($newsockfd $<0)$

error$("$ERROR on accept"$)$;

bzero$($buffer$,256)$;

n $=$ read$($newsockfd$,$ buffer, $255)$;

if $($n $<0)$ error$("$ERROR reading from socket"$)$;

printf$("$Here is the message: $\%$s

$",$buffer$)$;

n $=$ write$($newsockfd$,$ "I got your message", $18)$;

if $($n $<0)$ error$("$ERROR writing to socket"$)$;

return $0$;

$\}$

client.c

#include

#include

#include

#include

#include

#include

#include

#include

void error$($char $*$msg$)$

$\{$

perror$($msg$)$;

exit$(0)$;

$\}$

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

$\{$

int sockfd, portno, n;

struct sockaddr$\_$in serv$\_$addr;

struct hostent $*$server;

$/*$The hostent structure is used by functions to store information about a

given host, such as host name, IPv$4$ address, and so forth$*/$

char buffer$[256]$;

if $($argc $<3)\{$

fprintf$($stderr$,$"usage $\%$s hostname port

$",$ argv$[0])$;

exit$(0)$;

$\}$

portno $=$ atoi$($argv$[2])$;

sockfd $=$ socket$($AF$\_$INET, SOCK$\_$STREAM, $0)$;

if $($sockfd $<0)$

error$("$ERROR opening socket"$)$;

server $=$ gethostbyname$($argv$[1])$;

$/*$ gethostname$()$ system call returns a null$-$terminated hostname$*/$

if $($server $==$ NULL$)\{$

fprintf$($stderr$,$"ERROR, no such host

$")$;

exit$(0)$;

$\}$

bzero$(($char $*)$ &serv$\_$addr, sizeof$($serv$\_$addr$))$;

serv$\_$addr.sin$\_$family $=$ AF$\_$INET;

bcopy$(($char $*)$server$->$h$\_$addr,

$($char $*)$&serv$\_$addr.sin$\_$addr.s$\_$addr,

server$->$h$\_$length$)$;

serv$\_$addr.sin$\_$port $=$ htons$($portno$)$;

if $($connect$($sockfd$,($struct sockaddr $*)$&serv$\_$addr,sizeof$($serv$\_$addr$))<0)$

error$("$ERROR connecting"$)$;

printf$("$Please enter the message: $")$;

bzero$($buffer$,256)$;

fgets$($buffer$,255,$ stdin$)$;

n $=$ write$($sockfd$,$ buffer, strlen$($buffer$))$;

if $($n $<0)$

error$("$ERROR writing to socket"$)$;

bzero$($buffer$,256)$;

n $=$ read$($sockfd$,$ buffer, $255)$;

if $($n $<0)$

error$("$ERROR reading from socket"$)$;

printf$("\%$s

$",$buffer$)$;

return $0$;

$\}$

Compilation

$ gcc $-$o server server.c

$ gcc $-$o client client.c

Execution

$ $./$server $4444$

Here is the message: CIS$457$ HI SERVER!!!!!

$ $./$client $127\mathrm{.}0\mathrm{.}0\mathrm{.}14444$

Please enter the message: Hi Server, Good morning!!!!!!

I got your message