Threads

Single and Multithreaded Processes

Benefits: Responsiveness & Resource Sharing

Multithreaded Server Architecture

Pthreads

ü A POSIX standard API for thread creation and synchronization

ü Common in UNIX operating systems (Solaris, Linux, Mac OS X)

EXAMPLE:

âthrd2-posix.c:

int sum; /* this data is shared by the thread(s) */
int sum2; /* this data is shared by the thread(s) */

void *runner(void *param); /* the thread */
void *runner2(void *param); /* the thread */

int main(int argc, char *argv[])
{
pthread_t tid; /* the thread identifier */
pthread_t tid2; /* the thread identifier */
pthread_attr_t attr; /* set of attributes for the thread */

/* get the default attributes */
pthread_attr_init(&attr);

/* create the thread */
pthread_create(&tid, &attr, runner, argv[1]);
/* create the 2nd thread */
pthread_create(&tid2, &attr, runner2, argv[1]);

/* now wait for the thread to exit */
pthread_join(tid, NULL);
pthread_join(tid2, NULL);

printf("sum = %d\n", sum);
printf("sum2 = %d\n", sum2);
}


void *runner(void *param)
{
int i, upper = atoi(param);
sum = 0;

if (upper >= 0) {
for (i = 0; i <= upper; i++) {
sum += i;
}
}
pthread_exit(0);
}

void *runner2(void *param)
{
int i, upper = atoi(param);
sum2 = 0;

if (upper >= 0) {
for (i = 0; i <= upper; i++) {
sum2 += pow(2, i);
}
}
pthread_exit(0);
}

To execute:

% thrd2-posix 1

sum = 1

sum2 = 3

% thrd2-posix 20

sum = 210

sum2 = 2097151

Java Threads

ü Java threads are managed by the JVM

ü Typically implemented using the threads model provided by underlying OS

EXAMPLE:

âSimpleThread.java:

public class SimpleThread extends Thread {

private int countDown = 50;

private static int threadCount = 0;

private int threadNumber = ++threadCount;

public SimpleThread() {

System.out.println("Making " + threadNumber);

}

public void run() {

while(true) {

System.out.println("Thread " +

threadNumber + "(" + countDown + ")");

if(--countDown == 0) return;

}

}

public static void main(String[] args) {

for(int i = 0; i < 5; i++)

new SimpleThread().start();

System.out.println("All Threads Started");

}

}

To execute:

% java SimpleThread

Threading Issues

· Semantics of fork() and exec()

Does fork() duplicate only the calling thread or all threads?

No, it does not duplicate any of the threads.

âExample: thread-fork.c

main(int argc, char *argv[])

{

pthread_create(&tid, &attr, runner1, 0);

if ( fork() == 0){

runner2();

exit(0);

}

else {

pthread_join(tid, NULL);

printf("thread done\n");

wait(NULL);

printf("child process done\n");

}

}

void *runner1()

{

int i;

for(i=0;i<=7;i++) {

printf("HI %d\n", i);

sleep(i);

}

}

void *runner2()

{

int i;

for(i=0;i<=7;i++) {

printf("HI %c\n", i+'a');

sleep(i);

}

}

To execute:

% thread-fork

HI 0

HI 1

HI a

HI b

HI 2

HI c

HI d

HI 3

HI e

HI 4

HI f

HI 5

Etc...

Signal Handling

Signals are used in UNIX systems to notify a process that a particular event has occurred

A signal handler is used to process signals

Signal is generated by particular event

Signal is delivered to a process

Options:

Deliver the signal to the thread to which the signal applies

Deliver the signal to every thread in the process

Deliver the signal to certain threads in the process

Assign a specific thread to receive all signals for the process

In UNIX:

· The signal is delivered to EVERY process in the program

· The parent process of all threads gets the signal and NONE of the threads gets it.

âExample 1: thread-fork-signal.c

int
main(int argc, char *argv[])
{

pthread_t tid;
pthread_attr_t attr;

pthread_attr_init(&attr);

signal(SIGINT, SIG_IGN);
pthread_create(&tid, &attr, ThreadRunner, 0);
if (fork() == 0) {
ChildRunner();
exit(0);
}
ParentRunner();
wait(NULL);
printf("child process done\n");

pthread_join(tid, NULL);
printf("thread done\n");
}

void *ParentRunner()
{
int i;

printf("\nParentRunner started, PID: %d\n", getpid());
for(i=0;i<=7;i++) {
printf("Parent: %c\n", i+'A');
sleep(i);
}
}

void *ThreadRunner()
{
int i;
signal(SIGINT, ThreadSIGhandler);

printf("\nThreadRunner started, PID: %d\n", getpid());
for(i=0;i<=7;i++) {
printf("Thread: %d\n", i);
sleep(i);
}
}

void *ChildRunner()
{
int i;
signal(SIGINT, ChildSIGhandler);

printf("\nChildRunner started, PID: %d\n", getpid());

for(i=0;i<=7;i++) {
printf("Child: %c\n", i+'a');
sleep(i);
}
}
void ThreadSIGhandler(int sig)
{
signal(sig, ThreadSIGhandler);
printf("\nPID: %d", getpid());
psignal(sig, "\nReceived ThreadSIGhandler signal");
printf("pauseing...\n\n");
pause();
}
void ChildSIGhandler(int sig)
{
signal(sig, ChildSIGhandler);
printf("\nPID: %d", getpid());
psignal(sig, "\nReceived ChildSIGhandler signal");
printf("pauseing...\n\n");
pause();
}

To execute:

% thread-fork-signal

Type: CTRL-C

· Thread Pools

Create a number of threads in a pool where they await work

Advantages:

Usually slightly faster to service a request with an existing thread than create a new thread

Allows the number of threads in the application(s) to be bound to the size of the pool

· Thread Specific Data

Allows each thread to have its own copy of data

Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

EXAMPLES of Threaded Servers:

âEchoServerThread.c:

Int main(void)
{
int len, listenfd, connfd;
struct sockaddr_in servaddr, cliaddr;
char buff[512];
int nread;
pthread_t tid;
pthread_attr_t attr;

if ( (listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1 ) {
printf ("error\n");
exit(-1);
}

bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(10123);

if ( bind(listenfd, (SA *) & servaddr, sizeof(servaddr)) < 0) {
printf("can not bind\n");
exit(-1);
}

listen(listenfd, 0);

pthread_attr_init(&attr);

for (;;) {
len = sizeof(cliaddr);
connfd = accept(listenfd, (SA *) & cliaddr, &len);

printf("connection from: %s:%d\n", inet_ntoa(cliaddr.sin_addr), ntohs(cliaddr.sin_port));

pthread_create(&tid, &attr, &EchoToClient, (void *) connfd);

}
}

void *EchoToClient(void *sockfd)
{
int nread;
char buffer[512];
for (;;) {
nread = read((int) sockfd, buffer, 512);
if (nread == 0)
pthread_exit(0);
// exit(0);
write((int) sockfd, buffer, nread);
}
}

To execute:

% EchoServerThread

ü In another window for the same host:

% EchoClientFork localhost 10123

ü In another window for the same host:

% EchoClientFork localhost 10123

ü Repeat for any number of other windows.

âEchoServerThreadJava:

class EchoToClient extends Thread {

private Socket socket;

private BufferedReader in;

private PrintWriter out;

public EchoToClient(Socket s) throws IOException {

socket = s;

in = new BufferedReader( new InputStreamReader( socket.getInputStream()));

out = new PrintWriter( new BufferedWriter(

new OutputStreamWriter(socket.getOutputStream())), true);

start();

}

public void run() {

try {

while (true) {

String str = in.readLine();

if (str.length() == 0 )

System.exit(0);

else

out.println(str);

}

} catch(IOException e) {

}

}

}

public class EchoServerThreadJava {

public static void main(String[] args)

throws IOException {

ServerSocket s = new ServerSocket(10123);

try {

while(true) {

Socket socket = s.accept();

try {

new EchoToClient (socket);

} catch(IOException e) {

}

}

} catch(IOException e) {

}

}

}

To execute:

% java EchoServerThreadJava

ü In another window for the same host:

% EchoClientFork localhost 10123

ü In another window for the same host:

% EchoClientFork localhost 10123

ü Repeat for any number of other windows.