Lecture 12 Overview

Lecture 12 Overview

UDP Connected mode

• A UDP socket can be used in a call to connect()

• This simply tells the O.S. the address of the peer

• No handshake is made to establish that the peer exists

• No data of any kind is sent on the network as a result of calling connect() on a UDP socket

2CPE 401/601 Lecture 12 : Socket Programming Issues

Connected UDP

• Once a UDP socket is connected:– can use sendto() with a null dest address – can use write() and send()– can use read() and recv()

• only datagrams from the peer will be returned

– Asynchronous errors will be returned to the process

3

OS Specific, some won’t do this!

CPE 401/601 Lecture 12 : Socket Programming Issues

Asynchronous Errors

• What happens if a client sends data to a server that is not running?– ICMP “port unreachable” error is generated by

receiving host and sent to sending host– The ICMP error may reach the sending host after

sendto() has already returned!– The next call dealing with the socket could return

the error

4CPE 401/601 Lecture 12 : Socket Programming Issues

I/O Multiplexing

• We often need to be able to monitor multiple descriptors:

– a generic TCP client (like telnet)

– a server that handles both TCP and UDP

– Client that can make multiple concurrent requests • browser

5CPE 401/601 Lecture 12 : I/O Multiplexing

Example - generic TCP client

• Input from standard input should be sent to a TCP socket

• Input from a TCP socket should be sent to standard output

• How do we know when to check for input from each source?

6

STDIN

STDOUT

TC

P S

OC

KE

T

CPE 401/601 Lecture 12 : I/O Multiplexing

Options

• Use multiple processes/threads

• Use nonblocking I/O– use fcntl() to set O_NONBLOCK

• Use alarm and signal handler to interrupt slow system calls

• Use functions that support checking of multiple input sources at the same time


Non blocking I/O• Tell kernel not to block a process if I/O requests

can not be completed• use fcntl() to set O_NONBLOCK:int flags;

flags = fcntl(sock,F_GETFL,0);

fcntl(sock,F_SETFL,flags | O_NONBLOCK);

• Now calls to read() (and other system calls) will return an error and set errno to EWOULDBLOCK


Non blocking I/Owhile (! done) {

if ( (n=read(STDIN_FILENO,…)<0))

if (errno != EWOULDBLOCK)

/* ERROR */

else write(tcpsock,…)

if ( (n=read(tcpsock,…)<0))

if (errno != EWOULDBLOCK)

/* ERROR */

else write(STDOUT_FILENO,…)

}


The problem with nonblocking I/O

• Using blocking I/O allows the OS to put your process to sleep when nothing is happening– Once input arrives, the OS will wake up your

process and read() (or whatever) will return

• With nonblocking I/O, the process will chew up all available processor time!!!


Using alarmssignal(SIGALRM, sig_alrm);

alarm(MAX_TIME);

read(STDIN_FILENO,…);

...

signal(SIGALRM, sig_alrm);

alarm(MAX_TIME);

read(tcpsock,…);

...

11

A function you write

CPE 401/601 Lecture 12 : I/O Multiplexing

“Alarming” Issues

• What will happen to the response time ?

• What is the ‘right’ value for MAX_TIME?


Select()

• The select() system call allows us to use blocking I/O on a set of descriptors – file, socket, …

• We can ask select to notify us when data is available for reading on either STDIN or a socket


select()int select( int maxfd,

fd_set *readset,

fd_set *writeset,

fd_set *excepset,

const struct timeval *timeout);

• maxfd: highest number assigned to a descriptor• readset: set of descriptors we want to read from• writeset: set of descriptors we want to write to• excepset: set of descriptors to watch for exceptions• timeout: maximum time select should wait


Using select()

• Create fd_set• Clear the whole thing with FD_ZERO• Add each descriptor you want to watch using

FD_SET

• Call select• when select returns, use FD_ISSET to see if I/O

is possible on each descriptor


System Calls and Errors

• In general, systems calls return a negative number to indicate an error– We often want to find out what error– Servers generally add this information to a log– Clients generally provide some information to the

user

16CPE 401/601 Lecture 12 : Error Handling

What is fatal?

• How do you know what should be a fatal error?– Common sense– If the program can continue – it should

– if a server can't create a socket, or can't bind to it's port

• there is no sense continuing…


extern int errno;

• Whenever an error occurs, system calls set the value of the global variable errno– You can check errno for specific errors

• errno is valid only after a system call has returned an error– System calls don't clear errno on success– If you make another system call you may lose the

previous value of errno• printf makes a call to write!


General Strategies

• Include code to check for errors after every system call

• Develop "wrapper functions" that do the checking for you

• Develop layers of functions, each hides some of the error-handling details


Wrappers are great!

• Wrappers like those used in the text can make code much more readable

• There are always situations in which you cannot use the wrappers– Sometimes system calls are "interrupted" (EINTR)

• this is not always a fatal error !


Another approach

• Instead of simple wrapper functions, you might develop a layered system

• The idea is to "hide" the sockaddr and error handling details behind a few custom functions:– int tcp_client(char *server, int port);– int tcp_server(int port);


Layers and Code Re-use

• Developing general functions that might be re-used in other programs is obviously "a good thing"

• Layering is beneficial even if the code is not intended to be re-used:– hide error-handling from "high-level" code– hide other details– often makes debugging easier


The Best Approach to handling errors

• There is no best approach• Do what works for you

• Make sure you check all system calls for errors!– Not checking can lead to security problems!– Not checking can lead to bad grades on

assignments!


Lecture 13

Client/Server Programming

CPE 401 / 601

Computer Network Systems

slides are modified from Dave Hollingerslides are modified from Dave Hollinger

Issues in Client/Server Programming

• Identifying the Server

• Looking up an IP address

• Looking up a well known port name

• Specifying a local IP address

• UDP/TCP client designCPE 401/601 Lecture 13 : Client/Server Issues 25

Identifying the Server

• Options:– hard-coded into the client program– require that the user identify the server– read from a configuration file– use a separate protocol/network service to lookup

the identity of the server.

CPE 401/601 Lecture 13 : Client/Server Issues 26

Identifying a TCP/IP server

• Need an IP address, protocol and port– We often use host names instead of IP addresses– usually the protocol is not specified by the user

• UDP vs. TCP

– often the port is not specified by the user


Services and Ports• Many services are available via “well known”

addresses (names)• There is a mapping of service names to port

numbers:

struct *servent getservbyname(

char *service, char *protocol );

• servent->s_port is the port number in network byte order


Specifying a Local Address

• When a client creates and binds a socket, it must specify a local port and IP address

• Typically clients don’t care what port it is on:haddr->port = htons(0);


give me any available port !give me any available port !

Local IP address

• A client can also ask the operating system to take care of specifying the local IP address:

haddr->sin_addr.s_addr=

htonl(INADDR_ANY);


Give me the appropriate addressGive me the appropriate address

UDP Client Design

• Establish server address (IP and port)• Allocate a socket• Specify that any valid local port and IP

address can be used

• Communicate with server (send, recv)• Close the socket


Connected mode UDP

• A UDP client can call connect() to establish the address of the server

• The UDP client can then use read() and write() or send() and recv()

• A UDP client using a connected mode socket can only talk to one server– using the connected-mode socket


TCP Client Design

• Establish server address (IP and port)• Allocate a socket• Specify that any valid local port and IP

address can be used

• Call connect()

• Communicate with server (read, write)• Close the connection


Closing a TCP socket

• Many TCP based application protocols support – multiple requests and/or – variable length requests over a single TCP

connection

• How does the server known when the client is done ?– and it is OK to close the socket ?


Partial Close

• One solution is for the client to shut down only it’s writing end of the socket

• shutdown() system call provides this function

shutdown(int s, int direction);– direction can be 0 to close the reading end or 1 to

close the writing end– shutdown sends info to the other process!


TCP sockets programming

• Common problem areas:– null termination of strings

– reads don’t correspond to writes

– synchronization (including close())

– ambiguous protocol


TCP Reads

• Each call to read() on a TCP socket returns any available data– up to a maximum

• TCP buffers data at both ends of the connection

• You must be prepared to accept data 1 byte at a time from a TCP socket!


Server Design


Iterative

Small, fixed size requestsEasy to program

Iterative

Small, fixed size requestsEasy to program

Concurrent

Large or variable size requestsHarder to program

Typically uses more system resources

Concurrent

Large or variable size requestsHarder to program

Typically uses more system resources

Server Design


Connection-Oriented

EASY TO PROGRAMtransport protocol handles the tough stuff.

requires separate socket for each connection.

Connection-Oriented

EASY TO PROGRAMtransport protocol handles the tough stuff.

requires separate socket for each connection.

Connectionless

less overheadno limitation on number of clients

Connectionless

less overheadno limitation on number of clients

Server Design


IterativeConnectionless

IterativeConnectionless

IterativeConnection-Oriented

IterativeConnection-Oriented

ConcurrentConnection-Oriented

ConcurrentConnection-Oriented

ConcurrentConnectionless

ConcurrentConnectionless

Statelessness

• State: Information that a server maintains about the status of ongoing client interactions

• Connectionless servers that keep state information must be designed carefully!


Messages can be duplicated!Messages can be duplicated!

The Dangers of Statefullness

• Clients can go down at any time

• Client hosts can reboot many times

• The network can lose messages

• The network can duplicate messages


Concurrent Server Design Alternatives

• One child per client

• Spawn one thread per client

• Preforking multiple processes

• Prethreaded Server


One child per client

• Traditional Unix server:– TCP: after call to accept(), call fork()– UDP: after recvfrom(), call fork()– Each process needs only a few sockets– Small requests can be serviced in a small amount

of time

• Parent process needs to clean up after children!!!! – call wait()


One thread per client

• Almost like using fork– call pthread_create instead

• Using threads makes it easier to have sibling processes share information– less overhead

• Sharing information must be done carefully– use pthread_mutex


Prefork()’d Server

• Creating a new process for each client is expensive

• We can create a bunch of processes, each of which can take care of a client

• Each child process is an iterative server


Prefork()’d TCP Server

• Initial process creates socket and binds to well known address.

• Process now calls fork() a bunch of times

• All children call accept()

• The next incoming connection will be handed to one child


Preforking

• Having too many preforked children can be bad

• Using dynamic process allocation instead of a hard-coded number of children can avoid problems

• Parent process just manages the children– doesn’t worry about clients


Sockets library vs. system call

• A preforked TCP server won’t always work if sockets is not part of the kernel– calling accept() is a library call, not an atomic

operation

• We can get around this by making sure only one child calls accept() at a time using some locking scheme


Prethreaded Server

• Same benefits as preforking

• Can also have the main thread do all the calls to accept() – and hand off each client to an existing thread


What’s the best server design?

• Many factors:– expected number of simultaneous clients– Transaction size

• time to compute or lookup the answer

– Variability in transaction size– Available system resources

• perhaps what resources can be required in order to run the service


Server Design

• It is important to understand the issues and options

• Knowledge of queuing theory can be a big help

• You might need to test a few alternatives to determine the best design


Threaded Programming

Threads vs. Processes

• Creation of a new process using fork is expensive– Time – Memory

• A thread does not require lots of memory or startup time– called a lightweight process

Threads Programming 54

fork()

Process A

GlobalVariables

Code

Stack

Process B

GlobalVariables

Code

Stack

fork()

Process AThread 1

GlobalVariables

Code

Stack

Process AThread 2

Stack

pthread_create()

pthread_create()

Multiple Threads

• Each process can include many threads.

• All threads of a process share: – memory

• program code and global data

– open file/socket descriptors– signal handlers and signal dispositions– working environment

• current directory, user ID, etc.


Thread-Specific Resources

• Each thread has it’s own:– Thread ID (integer)– Stack, Registers, Program Counter– errno

• Threads within the same process can communicate using shared memory.– Must be done carefully!


Posix Threads

• We will focus on Posix Threads– most widely supported threads programming

API.

• Solaris– you need to link with “-lpthread”

• On many systems, this also forces the compiler to link in re-entrant libraries – instead of plain C libraries



Thread Creation

pthread_create(

pthread_t *tid,

const pthread_attr_t *attr,

void *(*func)(void *),

void *arg );

func is the function to be called.When func() returns the thread is terminated


pthread_create()

• The return value is 0 for OK.– positive error number on error

• Does not set errno !!!

• Thread ID is returned in tid

pthread_t *tid

• The book says you can specify NULL for tid (thread ID), – this doesn't always work!

• Thread attributes can be set using attr,– including detached state and scheduling policy. – You can specify NULL and get the system

defaults.


Thread IDs

• Each thread has a unique ID, a thread can find out it's ID by calling pthread_self().

• Thread IDs are of type pthread_t which is usually an unsigned int. When debugging, it's often useful to do something like this:

• printf("Thread %u:\n",pthread_self());


Thread Arguments

• When func() is called the value arg specified in the call to pthread_create() is passed as a parameter– func can have only 1 parameter, and it can't be

larger than the size of a void *

• Complex parameters can be passed by using structures– The structure can't be a local variable of the function

calling pthread_create !!• threads have different stacks!


Thread args example

struct { int x,y } 2ints;

void *blah( void *arg) {

struct 2ints *foo = (struct 2ints *) arg;

printf("%u sum of %d and %d is %d\n",

pthread_self(), foo->x, foo->y,

foo->x+foo->y);

return(NULL);

}


Thread Lifespan

• Once a thread is created, – it starts executing function func() specified in

the call to pthread_create()

• If func() returns, thread is terminated

• A thread can also be terminated by calling pthread_exit()

• If main() returns or any thread calls exit()– all threads are terminated.


Detached State

• Each thread can be either joinable or detached.

• Detached: on termination all thread resources are released by the OS.

• A detached thread cannot be joined.

• No way to get at the return value of the thread.– a pointer to something: void *


Joinable Thread

• Joinable: on thread termination thread ID and exit status are saved by the OS

• One thread can "join" another by calling pthread_join – which waits (blocks) until a specified thread

exits

int pthread_join( pthread_t tid, void **status);


Shared Global Variables

int counter=0;

void *pancake(void *arg) {

counter++;

printf("Thread %u is number %d\n",

pthread_self(), counter);

}

main() {

int i; pthread_t tid;

for (i=0;i<10;i++)

pthread_create(&tid,NULL,pancake,NULL);

}Threads Programming 69

DANGER! DANGER! DANGER!

• Sharing global variables is dangerous– two threads may attempt to modify the same

variable at the same time.

• Just because you don't see a problem when running your code doesn't mean it can't and won't happen!!!!


Avoiding Problems

• pthreads includes support for Mutual Exclusion primitives that can be used to protect against this problem.

• The general idea is to lock something before accessing global variables and to unlock as soon as you are done.

• Shared socket descriptors should be treated as global variables!!!

Netprog: Threads Programming 71

pthread_mutex

• A global variable of type pthread_mutex_t is required:

pthread_mutex_t counter_mtx=PTHREAD_MUTEX_INITIALIZER;

• Initialization to PTHREAD_MUTEX_INITIALIZER

is required for a static variable!Threads Programming 72


Locking and Unlocking

• To lock use:pthread_mutex_lock(pthread_mutex_t &);

• To unlock use:pthread_mutex_unlock(pthread_mutex_t &);

• Both functions are blocking!

Example Problem

• A server creates a thread for each client.

• No more than n threads (and therefore n clients) can be active at once.

• How can we have the main thread know when a child thread has terminated and it can now service a new client?


pthread_join() doesn’t help

• pthread_join requires that we specify a thread id. – is sort of like wait()

• We can wait for a specific thread,– but we can't wait for "the next thread to exit"


Use a global variable• When each thread starts up:

– acquires a lock on the variable (using a mutex)– increments the variable– releases the lock.

• When each thread shuts down:– acquires a lock on the variable (using a mutex)– decrements the variable– releases the lock.


What about the main loop?

active_threads=0;

// start up n threads on first n clients

// make sure they are all running

while (1) {

// have to lock/relase active_threads

if (active_threads < n)

// start up thread for next client

busy_ waiting(is_bad);

}Threads Programming 77

Condition Variables

• pthreads support condition variables, – which allow one thread to wait (sleep) for an

event generated by any other thread.

• This allows us to avoid the busy waiting problem.

pthread_cond_t foo = PTHREAD_COND_INITIALIZER;


Condition Variables (cont.)

• A condition variable is always used with mutex.

pthread_cond_wait(pthread_cond_t *cptr,

pthread_mutex_t *mptr);

pthread_cond_signal(pthread_cond_t *cptr);


don’t let the word signal confuse you -this has nothing to do with Unix signals

Revised strategy

• Each thread decrements active_threads when terminating and calls pthread_cond_signal to wake up the main loop

• The main thread increments active_threads when each thread is started and waits for changes by calling pthread_cond_wait

• All changes to active_threads must be inside the lock and release of a mutex

• If two threads are ready to exit at (nearly) the same time– the second must wait until the main loop recognizes the first

• We don’t lose any of the condition signals


Global Variables

// global variable the number of active

// threads (clients)

int active_threads=0;

// mutex used to lock active_threads

pthread_mutex_t at_mutex = PTHREAD_MUTEX_INITIALIZER;

// condition var. used to signal changes

pthread_cond_t at_cond = PTHREAD_COND_INITIALIZER;


Child Thread Code

void *cld_func(void *arg) {

. . .

// handle the client

. . .

pthread_mutex_lock(&at_mutex);

active_threads--;

pthread_cond_signal(&at_cond);

pthread_mutex_unlock(&at_mutex);

return();

}


Main thread

// no need to lock yet

active_threads=0;

while (1) {

pthread_mutex_lock(&at_mutex);

while (active_threads < n ) {

active_threads++;

pthread_start(…)

}

pthread_cond_wait( &at_cond, &at_mutex);

pthread_mutex_unlock(&at_mutex);

}

IMPORTANT!Must happen while the mutex lock is

held.


Other pthread functions

• Sometimes a function needs to have thread specific data– eg, a function that uses a static local

• Functions that support thread specific data:pthread_key_create()

pthread_once()

pthread_getspecific()

pthread_setspecific()Threads Programming 84

Thread Safe library functions

• You have to be careful with libraries.

• If a function uses any static variables (or global memory) it’s not safe to use with threads!

• The book has a list of the Posix thread-safe functions…


Thread Summary

• Threads are awesome, but dangerous.

• You have to pay attention to details or it's easy to end up with code that is incorrect – doesn't always work, or hangs in deadlock

• Posix threads provides support for – mutual exclusion, – condition variables and – thread-specific data.


Documents

Lecture 12 Overview