Programming with UDP – I
Covered Subjects:
• IPv4 Socket Address Structure
• Byte Ordering Functions
Address Access/Conversion Functions
• Functions:
•
1.
2.
3.
4.
5.
socket()
bind()
sendto()
recvfrom()
close()
Socket Address Structures

Most socket functions require a pointer
to a socket address structure as an
argument.

Each supported protocol defines its own
socket address structure.

The names of these structures begin with
sockaddr_ and end with a unique suffix
for each protocol suite.
IPv4 Socket Address Structure


Internet socket address structure is named
sockaddr_in and defined in <netinet/in.h>
IP address:
struct in_addr {
in_addr_t s_addr;
};

/* 32-bit IP address */
/* network byte ordered */
TCP or UDP Address:
struct sockaddr_in {
uint8_t sin_len;
/* length of structure*/
short sin_family; /* e.g., AF_INET */
ushort sin_port;
/* TCP/UDP port */
struct in_addr;
/* IP address */
char
sin_zero[8]; /* unused */
};
All but
sin_family
in network
byte order
Note: Having sin_len simplifies handling variable length socket address
structures.
Generic Socket Address Structures
Problem:
 A socket address is always passed by
reference when passed as an argument to
socket functions.
 But any socket call that takes one of these
pointers as an argument must deal with
socket address structures from ANY of the
supported protocol families.
 A problem arises in how to declare the type
of pointer that is passed.
continued..
Solution:
 is to define a generic socket structure
defined in <sys/socket.h>
struct sockaddr{
uint8_t
sa_len;
sa_family_t sa_family; /* AF_INET, AF_INET6 .. */
char
sa_data[14] /* protocol specific addr */
};
Example:
int bind(int, struct sockaddr *, socklen_t);
struct sockaddr_in serv; /* IPv4 address */
bind(sockfd, (struct sockaddr *)&serv,sizeof(serv));
If we omit this warning comes!
Q: Why not do (void *)?
Byte Ordering

Big Endian vs. Little Endian
◦ Little Endian (Intel, DEC):
 Least significant byte of word is stored in the
lowest memory address
◦ Big Endian (Sun, SGI, HP):
 Most significant byte of word is stored in the lowest
memory address
◦ Network Byte Order = Big Endian
 Allows both sides to communicate
 Must be used for some data (i.e. IP Addresses)
 Good form for all binary data
Byte Ordering Functions
16- and 32-bit conversion functions (for
platform independence)
 Examples:

int m, n;
short int s,t;
m = ntohl (n)
net-to-host long (32-bit) translation
s = ntohs (t)
net-to-host short (16-bit) translation
n = htonl (m)
host-to-net long (32-bit) translation
t = htons (s)
host-to-net short (16-bit) translation
Address Access/Conversion
Functions
in_addr_t inet_addr(const char* strptr);
◦ Translate dotted-decimal notation to IP address; returns -1 on failure,
thus cannot handle broadcast value “255.255.255.255”
int inet_aton(const char *strptr, struct
in_addr *inaddr);
◦ Translate dotted-decimal notation to IP address; returns 1 on success, 0
on failure.
char* inet_ntoa(struct in_addr inaddr);
◦ Translate IP address to ASCII dotted-decimal notation (e.g.,
“128.32.36.37”)
UDP Client-Server Interaction
Model
socket() Function

Create a socket
◦ int socket(int family, int type, int protocol);
◦ Returns file descriptor or -1.

Fields:
◦ int family
 AF_INET
 AF_INET6
 IPv4 protocols
 IPv6 protocols
◦ int type
 SOCK_DGRAM
 SOCK_STREAM
 for UDP datagrams
 for TCP streams
◦ int protocol (for AF_INET & AF_INET6 family sockets)
 IPPROTO_UDP
 IPPROTO_TCP
 UDP Transport
 TCP Transport
bind() Function

Bind a socket to a local IP address and port number
◦ int bind (int sockfd, struct sockaddr* myaddr,
int addrlen);
◦ Returns 0 if OK or -1 on error.
◦ sockfd: socket file descriptor (returned from socket)
◦ myaddr: includes IP address and port number
 IP address: set by kernel if value passed is INADDR_ANY, else set by
callerSpecifies
Process
Result
 port number: set by kernel if value passed is 0, else set by caller
IP Address
Port #
◦ addrlen: length of address structure
INADDR_ANY
0
Kernel chooses IP addr& port #
 = sizeof (struct sockaddr_in)
INADDR_ANY
nonzero
Kernel chooses IP addr, process specifies
port #
Local IP Addr
0
Process specifies IP addr, kernel assigns port
#
Local IP Addr
nonzero
Process specify both
sendto() & recvfrom() Functions
int sendto (int sockfd, char* buf, size_t nbytes,
int flags, struct sockaddr* destaddr, int
addrlen);
◦ Send a datagram to another UDP socket.
 Returns number of bytes written or -1.
int recvfrom (int sockfd, char* buf, size_t
nbytes, int flags, struct sockaddr* srcaddr,
int* addrlen);
◦ Read a datagram from a UDP socket.
 Returns number of bytes read or -1.
close() Function
int close (int sockfd);

Close a socket.
◦ Returns 0 on success, -1 on failure.
◦ sockfd: socket file descriptor (returned from socket)

Closes communication on socket in both directions.
◦ All data sent before close are delivered to other side.

After close, sockfd is not valid for reading or
writing.