Thread API

Xiaohua Lu

Office : CS 3310

Tel. : 2621721

Email : lxh@cs.wisc.edu

Office hours: 11-12,3-5 T,TR

Pthread API

• thread creation & termination

• thread synchronization– mutex– condition variable– semaphore

Thread creation

status = pthread_create(&thread,&attr,start_function,arg);

int status;

pthread_t thread;

pthread_attr_t attr;

void *start_function(void *arg);

status: 0 if succeed and thread would contain the new thread id, otherwise EAGAIN or EINVAL.

An example of thread creationint status;

pthread_t thread_id;

pthread_attr_t thread_attr;

void *request_process(void *arg){

…

}

if ((status=pthread_attr_init(&thread_attr) ) != 0){

...

}

if ((status = pthread_create(&thread_id,&thread_attr,request_process,NULL)) != 0){

...

}

Thread join & exit

void pthread_exit(returnvalue);

status = pthread_join(thread_id, returnvalue_ptr);

pthread_t thread_id;

int status;

void *returnvalue;

void **returnvalue_ptr;

Mutexpthread_mutex_t mutex; const pthread_mutexattr_t attr;int status;

status = pthread_mutex_init(&mutex,&attr);

status = pthread_mutex_destroy(&mutex);

status = pthread_mutex_unlock(&mutex);

status = pthread_mutex_lock(&mutex); - block

status = pthread_mutex_trylock(&mutex); - unblock

Thread i

……lock(mutex)……critical region……unlock(mutex)……

Condition variablesint status;

pthread_condition_t cond;

const pthread_condattr_t attr;

pthread_mutex mutex;

status = pthread_cond_init(&cond,&attr);

status = pthread_cond_destroy(&cond);

status = pthread_cond_signal(&cond);

status = pthread_cond_broadcast(&cond);

Condition variables

status = pthread_cond_wait(&cond,&mutex);

wait on a condition variable.

First, a thread needs to get the lock of the mutex.

Then it could check whether or not a condition is satisfied.

If so, it could continue its work and unlock the mutex later. Otherwise, it would wait until some thread use pthread_signal to notify it that the condition is satisfied.

It will release the mutex while waiting and automatically regain it when awoken.

An example of pthread_cond_wait()

If there are lots of sending threads and each needs first check whether or not the sending buffers are available. If so, they could send, otherwise they should wait for the next available buffer. // For exclusive accessing of the number of available buffers if ( pthread_mutex_lock( &m_mutexSend ) < 0 ){

SHOW_ERROR( "Locking mutex failed" );return RTP_ERROR;

}

// Check if Sending buffers are available while ( m_nOutWndBufs <= 0 ) { if ( pthread_cond_wait( &m_condSend, &m_mutexSend ) < 0 )

{ SHOW_ERROR( "Waiting on condition variable failed" ); pthread_mutex_unlock( &m_mutexSend ); return RTP_ERROR;}

Semaphoreint status,pshared;

sem_t sem;

unsigned int initial_value;

status = sem_init(&sem,pshared,initial_value);

status = sem_destroy(&sem);

status = sem_post(&sem);

status = sem_wait(&sem); - block

status = sem_trywait(&sem); - unblock

Clarification of assignment 1

• Timestamp = file’s last modification time?

• At most 32 concurrent client requests.

• client server_address filepath+filename server address should allow formats like localhost, nova23.cs.wisc.edu and 128.105.120.123