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Socket Programming (Cont.)
Networking
CS 3470, Section 1
Sarah Diesburg
Control Flow
socket()
bind()
TCP Client
Socket()
connect() Connection establishment
send()
TCP Server
Well-known port
listen()
accept()
blocks until connection from client
recv()
process request
recv()
close()
send()
read()
close()
Byte Ordering
Increasing memory address
Address A+1
Little-endian
byte order
High-order byte
MSB
Big-endian
byte order
Address A
Low-order byte
16-bit value
Low-order byte
Address A+1
LSB
High-order byte
Address A
Increasing memory address
Implications of Byte Order



Unfortunately there is no standard between
these two byte orderings and we encounter
systems that use both formats
We refer to the byte ordering used by a given
system as host byte order
The sender and the receiver must agree on the
order in which the bytes of these multi-byte field
transmitted: specify network byte order, which
is big-endian byte ordering
Byte Order Functions
#include <netinet.h>
/* Host to network */
uint16_t htons(uint16_t host16bitvalue)
Converts a 16-bit integer from host to network byte order
uint32_t htonl(uint32_t host32bitvalue)
Converts a 32-bit integer from host to network byte order
Both return: value in network byte order
/* Network to host */
uint16_t ntohs(uint16_t net16bitvalue)
uint32_t ntohl(uint32_t net32bitvalue)
Both return: value in host byte order
When do we use hton/ntoh
functions?

Use hton the port number in struct
sockaddr_in

If we create a custom struct to hold our
headers and data


Sending our data through send() and recv()
functions
E.g., if our first struct member is a 2-byte header,
and sender/receiver have different memory
orderings, number would look very different to
each machine
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Socket Address Structures
#include <netinet/in.h>
// Pointers to socket address structures are often cast to pointers
// to this type before use in various functions and system calls:
struct sockaddr {
unsigned short
char
};
sa_family;
sa_data[14];
// address family, AF_xxx
// 14 bytes of protocol address
// IPv4 AF_INET sockets:
struct sockaddr_in {
short
sin_family;
unsigned short
sin_port;
struct in_addr
sin_addr;
char
sin_zero[8];
};
//
//
//
//
struct in_addr {
uint32_t s_addr;
};
// load with inet_pton()
e.g. AF_INET, AF_INET6
e.g. htons(3490)
see struct in_addr, below
pad to fit into sockaddr
Address Conversion Functions
#include <arpa/inet.h>
int inet_pton(int af, const char *src, void *dst); /*
Returns 1 on success, < 1 on error */
Converts the character string src into a network address
structure, then copies the network address structure to
dst.
int inet_aton(const char *strptr, struct in_addr
*addrptr); /* return 1 if string was valid,0 error */
Convert an IP address in string format (x.x.x.x) to the
32-bit packed binary format used in low-level network
functions
Address Conversion Functions
#include <arpa/inet.h>
in_addr_t inet_addr(const char *strptr);
/* return 32-bit binary network byte ordered IPv4
address; INADDR_NONE if error, deprecated and replaced
by inet_aton() */
char *inet_ntoa(struct in_addr inaddr);
/* returns: pointer to dotted-decimal string */
Example
struct sockaddr_in ip4addr;
int s;
ip4addr.sin_family = AF_INET;
ip4addr.sin_port = htons(3490);
inet_pton(AF_INET, "10.0.0.1", &ip4addr.sin_addr);
s = socket(PF_INET, SOCK_STREAM, 0);
bind(s, (struct sockaddr*)&ip4addr, sizeof ip4addr);
Man Pages

Use man pages to look up useful information

$> man cat


$> man bind



Get information about shell commands
Get information about C library/system calls
Also tells you which header files to include
$> man man

Get information about man pages
Man Pages

Sometimes you need to specify a man
section


E.g., printf is both a shell command and a C
library call
Use either



$>man 1 printf
$>man 3 printf
See
http://www.cs.uni.edu/~diesburg/courses/cs3470_
fa13/resources/man_page_levels.htm
C Header Files

Located at /usr/include

Can find out what functions are available for
you to use and struct definitions

(Hint: check out <string.h> and <strings.h>)
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Byte Manipulation Functions
#include <strings.h>
/* Berkeley-derived functions */
void bzero(void *dest, size_t nbytes)
Set the first part of an object to null bytes
void bcopy(const void *src, void *dest, size_t nbytes);
int bcmp(const void *ptr1, const void *ptr2, size_t nbytes)
/* return:0 if equal, nonzero if unequal */
#include <string.h>
/* ANSI C defined functions */
void *memset(void *dest,int c,size_t len)
Sets the first len bytes in memory dest to the value of c
void *memcpy(void *dest,const void *src, size_t nbytes)
void memcmp(const void *ptr1, const void *ptr2, size_t nbytes)
Demo of Program
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