Sockets and Other Necessities for Assignment 2

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Sockets and Other Necessities for Assignment 2

• Stephen Curial
• curial_at_cs.ualberta.ca
• http//www.cs.ualberta.ca/curial

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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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What is a Socket?
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What is a Socket?

• Inter Process Communication (IPC)
• Allows 2 processes to communicate
• Dont have to be on the same computer
• Give the programmer a file descriptor (fd) as a
  handle to the socket

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How do sockets work?
Created using socket() sys. call
Created using socket() sys. call
Server
Client
Port a
Port
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How do sockets work?
connect()
Server
Client
Port a
Port
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How do sockets work?
Server
Client
Port a
Port
Port b
Created with accept()
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How do sockets work?
Server
Client
Port a
Port
Port b
This is the connection the 2 processes use for
reading and writing.
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What functions are used?
Server
Client
socket()
socket()
bind()
connect()
listen()
accept()
/ block until connection from client /
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socket()

• Create a fd for use in network communication
• int socket(int domain, int type, int protocol)
• domain AF_UNIX, AF_INET, PF_INET,
• type SOCK_STREAM, SOCK_DGRAM, SOCK_RAW
• protocol 0 IPPROTO_TCP

IP
TCP
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bind()

• Bind a local IP address protocol to a socket
• int bind(int sockfd,
• struct sockaddr my_addr,
• socklen_t addrlen)

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listen()

• Place socket in passive mode and set the number
  of incoming TCP connections the system will
  en-queue.
• int listen(int s, int backlog)

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connect()

• Connect to the server
• int connect(int sockfd, const struct
  sockaddr serv_addr, socklen_t addrlen)

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accept()

• Accept the next incoming connection
• int accept(int s,
• struct sockaddr addr,
• socklen_t addrlen)

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close()

• Terminate communication and de-allocate file
  descriptor.
• int close(int fd)

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read()

• Read count bytes from fd into buf
• count bytes must have been written to the socket
  or the read will block
• ssize_t read(int fd, void buf,
• size_t count)

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write()

• Write count bytes from fd into buf
• ssize_t write(int fd, const void buf,
• size_t count)

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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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Endianness

• Assume we have the word 0xAABBCCDD at address
  0x100
• This can cause portability issues.

Big Endian
Little Endian
AA
BB
CC
DD
DD
CC
BB
AA
100
101
102
103
100
101
102
103
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Endianness Solution

• Send everything over the network in a common
  format
• Network Byte Order (Big Endian)
• uint16_t htons(uint16_t hostshort) / host to
  network short /
• uint32_t htonl(uint32_t hostlong) / host to
  network long /
• uint16_t ntohs(uint16_t netshort) / network to
  host short /
• uint32_t ntohl(uint32_t netlong) / network
  to host long /

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Port Numbers
Pier 10
400
BIN 707
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Port Numbers

• 0 - 1024 for well-known services
• Eg. http, ssh, ftp, etc.
• Free to use 1025 - 65535

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Header Payload Messages

• struct header
• int msg_type
• int msg_size

msg_type
msg_size

anything(int,char,)
int
int
header
payload
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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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What is I/O Multiplexing?
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What is I/O Multiplexing?

• Its how you (a single thread) talk to multiple
  sockets without blocking.

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I/O Multiplexing using select()

• fd_set (is simply a bit-vector)
• Think of it as an array of Booleans
• FD_CLR(int fd, fd_set set)
• FD_ISSET(int fd, fd_set set)
• / returns true/false (1/0) /
• FD_SET(int fd, fd_set set)
• FD_ZERO(int fd, fd_set set)

1023
0 1 2
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select()

• int select(int n, fd_set readfds,
• fd_set writefds, fd_set
  exceptfds, struct timeval
  timeout)
• n - the largest fd
• readfds - select will modify this bit-vector so
  only fds with something to read have a value of
  1.

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Typical usage of select() - Initialization (this
is in the server)

• fd_set allfds / contains a 1 at the location of
  the fd for every connection (also has a 1 in the
  location of the fd for the port we listen for
  connections on) /
• fd_set readfds
• int maxfd / The largest fd connected /
• int server_fd / returned from socket() /
• int i
• FD_ZERO(allfds)
• FD_SET(server_fd, allfds)
• maxfd server_fd

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Typical usage of select() - Main Server Loop

• while(server_open)
• memcpy(readfds, allfds, sizeof(fd_set) ) /
  memcpy(void dest, const void src, size_t n) /
• select(maxfd1,readfds,NULL,NULL,NULL)
• for(i 0 i lt maxfd i)
• if(FD_ISSET(i,readfds))
• if(i server_fd)
• / accept client
• add new fd to allfds
• adjust max /
• else
• / Service client with fd i/
• / end for /
• / end while /

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Yet Another Way to Multiplex poll()

• int poll(struct pollfd fdarray,
• unsigned int num_fds,
• int timeout)
• struct pollfd
• int fd
• short events / events of interest /
• short revents / events that occurred /

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poll()
revents
fd
events
fdarray0
revents
fd
events
fdarray1
revents
fd
events
fdarray2
revents
fd
events
fdarray3

revents
fd
events
fdarrayn
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poll()Example (Not Typical) Usage

• struct pollfd fdarray10
• for(i 0 i lt 10 i)
• fdarrayi.fd sock_fd
• fdarrayi.events POLLIN POLLPRI
• while(1)
• poll(fdarray, 10, INFTIM)
• for i 0 i lt 10 i)
• if(fdarray.revents (POLLIN POLLPRI))
• / Service event /

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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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Processes Threads

• How do you create a new process?
• Build one from scratch
• Load code and data into memory
• Create a dynamic memory workspace (heap)
• Create a process control block (pcb)
• Clone an existing one
• Stop current process save its state
• Make a copy of code, data, heap, pcb
• Use copy on write for efficiency

Based on Paul Lus 379 slide
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fork()

• Unix uses fork() to create a copy of the calling
  process
• pid_t fork()

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Typical usage of fork()
if( fork() ) / Parent / else /Child /
Based on Paul Lus 379 slide
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Process vs. Thread

• Creating a thread is cheap(er)
• Switching between threads is cheaper
• Threads within a process can share resources
• Same address space
• No memory protection

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Memory Layout of a Single Process
kernel
0x00000000
PCB
Stack
Heap
Data
Text
0xFFFFFFFF
Based on Paul Lus 379 slide
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Process vs. ThreadMemory Layout
Traditional Process
Multi-threaded Process
Text
Text
Data
Data
Stack
Stack
Stack
Stack
Based on Paul Lus 379 slide
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POSIX Threads - pthreads

• include ltpthread.hgt
• int pthread_create(pthread_t thread, const
  pthread_attr_t attr, void
  (start_routine)(void ), void arg)
• int pthread_join(pthread_t thread, void
  value_ptr)

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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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Time

• http//www.cplusplus.com/ref/ctime/

Functions clock Return number of clock
ticks since process start ctime Convert
time_t value to string difftime Return
difference between two times gmtime Convert
time_t value to tm structure as UTC
time localtime Convert time_t value to tm
structure as local time mktime Convert tm
structure to time_t value time Get
current time
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Timers - Using the Alarm Interrupt

• include ltstdio.hgt
• include ltsignal.hgt
• include ltunistd.hgt
• define KEEP_ALIVE_TIME 5
• void send_keep_alive()
• alarm(KEEP_ALIVE_TIME)
• printf(\n)
• int main()
• signal(SIGALRM,send_keep_alive)
• alarm(KEEP_ALIVE_TIME)
• while(1)
• return 0

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Outline

• Sockets
• Network Byte Order
• I/O Multiplexing
• Processes
• Timers
• General Comments

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Where to find more information

• If I havent presented something that you think
  that you need look for it.
• google
• man pages
• Stevens Advanced Programming In the Unix
  Environment
• Newsgroup - ask your classmates

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How do I get a good mark?

• Test your assignments before submission!
• 90 of the marks lost on AS1 could have been
  caught through some simple testing
• Once all of your files are tared create a new
  directory and un-tar the files, compile, re-test.
• Read the assignment spec. very carefully!

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Have a good weekend!