1. UNIX Socket Programming CS 6378
Project Reference Book: Unix Network programming: Networking APIs: Sockets and XTI (2nd edition), Prentice Hall
>> UNIX System
>> Sockets
>> System Calls
>> TCP Client Server
>> UDP Client Server
>> Threads

2. User and System Programs
Kernel Support
Hardware

3. Socket API
Socket API originated with the 4.2 BSD system released in 1983
Sockets – A way to speak to other programs using UNIX file descriptors.
A file descriptor is an integer associated with an open file.This can be a network connection
Kinds of Sockets-DARPA Internet addresses(Internet Sockets) , Unix Sockets, X.25 Sockets etc
Types of Internet Sockets
SOCK_STREAM uses TCP (Transmission Control Protocol) Connection oriented and Reliable
SOCK_DGRAM uses UDP (User Datagram Protocol)
Connectionless and Unreliable

4. Structs and Data Handling
A socket descriptor is of type int
Byte ordering
Most significant byte first – Network byte order (Big Endian)
Least significant byte first – Host Byte order ( Little ..)
Socket Structures in Network byte order
struct sockaddr { unsigned short sa_family; // address family, AF_xxx char sa_data[14]; // 14 bytes of protocol address };
struct sockaddr_in { short int sin_family; // Address family
unsigned short int sin_port; // Port number
struct in_addr sin_addr; // Internet address
unsigned char sin_zero[8]; // Same size as struct sockaddr };

5. Convert the Natives
struct in_addr { unsigned long s_addr; // 32-bit long, or 4 bytes };
If ina is of type struct sockaddr_in
ina.sin_addr.s_addr references the 4-byte IP address (in Network Byte Order
htons() – Host to Network Short
htonl() -- "Host to Network Long"
ntohs() -- "Network to Host Short"
ntohl() -- "Network to Host Long"

6. IP Addresses socket01.utdallas.edu 129.110.43.11
sol2.utdallas.edu etc
Other UTD machines for use socket02 – socket06 , sol1 , jupiter
Please do not use apache for Network programming
inet_addr() converts an IP address in numbers-and-dots notation into unsigned long
ina.sin_addr.s_addr = inet_addr(“ ”) // Network byte order
Also can use inet_aton() “ascii to network”
int inet_aton(const char *cp,struct in_addr *inp);
inet_ntoa returns a string from a struct of type in_addr
inet_ntoa(ina.sin_addr) ;

7. Useful UNIX Commands
netstat –i prints information about the interfaces
netstat –ni prints this information using numeric addresses
loop back interface is called lo and the ethernet interface is called eth0 or le0 depending on the machine
netstat –r prints the routing table
netstat | grep PORT_NO shows the state of the client socket
ifconfig eth0 – Given the interface name ifconfig gives the details for each interface Ethernet Addr , inet_addr , Bcast , Mask , MTU
ping IP_addr -- Sends a packet to the host specified by IP_addr and prints out the roundtrip time ( Uses ICMP messages)
traceroute IP_addr -- Shows the path from this host to the destination printing out the roundtrip time for a packet to each hop in between
Tcpdump communicates directly with Data Link layer UDP Packet fail

8. System Calls socket() – returns a socket descriptor
int socket(int domain, int type, int protocol);
bind() – What port I am on / what port to attach to
int bind(int sockfd, struct sockaddr *my_addr, int addrlen);
connect() – Connect to a remote host
int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);
listen() – Waiting for someone to connect to my port
int listen(int sockfd, int backlog);
accept() – Get a file descriptor for a incomming connection
int accept(int sockfd, void *addr, int *addrlen);
send() and recv() – Send and receive data over a connection
int send(int sockfd, const void *msg, int len, int flags);
int recv(int sockfd, void *buf, int len, unsigned int flags);

9. sendto() and recvfrom() – Send and receive data without connection
int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen);
int recvfrom(int sockfd, void *buf, int len, unsigned int flags, struct sockaddr *from, int *fromlen);
close() and shutdown() – Close a connection Two way / One way
getpeername() – Obtain the peer name given the socket file descriptor
gethostname() – My computer name
int sock_get_port(const struct sockaddr *sockaddr,socklen_t addrlen);
Useful to get the port number given a struct of type sockaddr
Readn() writen() readline() Read / Write a particular number of bytes
Fork() – To start a new process with parents addr space
Exec() Load a new program on callers addr space

10. Client-Server Programming
TCP Client Server Example
Status of client-server before call to accept daytimetcpsrv1.c
Server Client
● connect()
listenfd ●
Status of clientserver after return from accept
● connect()
listenfd ●
connfd ●

11. Status of Client-Server after fork returns
Server( Parent)
listenfd ●
connfd ●
Client
● connect()
Server ( Child)
listenfd●
connfd ●

12. Status of Client Server after parent and Child close appropriate sockets
Server ( Parent)
listenfd ●
Client
● connect()
Server(Client)
connfd ●

13. Socket functions of UDP Client Server
Bind()
Recvfrom()
Socket()
Sendto()
Blocks until datagram received from a client
recvfrom
Sendto()
Close()
UDP Client UDP Server

14. Threads
Threads are lightweight process which share the process instructions , global variables , open files , signal handlers and signal dispositions , current working directory and user and group Ids
Each thread has its own thread ID , set of Registers, PC and Stack pointer, stack , errno ,signal mask , priority
Basic Thread Functions : Creation and Termination
Int pthread_create(pthread_t *tid,const pthread_attr_t *attr,void *(*func)(void*), void *arg);
Int pthread_join(pthread_t tid,void **status); // Wait for a thread to terminate
pthread_t pthread_self(void); //Returns thread ID of calling thread
Int pthread_detach(pthread_t pid);//We cannot wait for it to terminate, On its termination all resources are released
Void pthread_exit(void *status);
Threads/tcpserv01.c

15. Thread Synchronization
Shared data can be protected using mutex locks provided by the pthread library
int pthread_mutex_lock(pthread_mutex_t *mptr);
int pthread_mutex_unlock(pthread_mutex_t *mptr);
Threads/example02.c
Can use condition variables for signalling mechanism
int pthread_cond_wait(pthread_cond_t *cptr,pthread_mutex_t *mptr);
int pthread_cond_signal(pthread_cond_t *cptr);
Can also use System semaphores from <semaphores.h> See manual
~syrotiuk/pthreads.cc

16. References
UNIX Network Programming , Networking APIs: Sockets and XTI : W Richard Stevens
Beej’s Guide to Network Programming
Dr Syrotiuk’s web page with sample networking programs