Multi threaded Programming: Review Questions

Warning - question numbers subject to change

Q1

Is the following code thread-safe? Redesign the following code to be thread-safe. Hint: A mutex is unnecessary if the message memory is unique to each call.

static char message[20];
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZAER;

void format(int v) {
  pthread_mutex_lock(&mutex);
  sprintf(message,":%d:",v);
  pthread_mutex_unlock(&mutex);
  return message;
}

Q2

Which one of the following does not cause a process to exit?

• Returning from the pthread's starting function in the last running thread.
• The original thread returning from main.
• Any thread causing a segmentation fault.
• Any thread calling exit.
• Calling pthread_exit in the main thread with other threads still running.

Q3

Write a mathematical expression for the number of "W" characters that will be printed by the following program. Assume a,b,c,d are small positive integers. Your answer may use a 'min' function that returns its lowest valued argument.

unsigned int a=...,b=...,c=...,d=...;

void* func(void* ptr) {
  char m = * (char*)ptr;
  if(m == 'P') sem_post(s);
  if(m == 'W') sem_wait(s);
  putchar(m);
  return NULL;
}

int main(int argv, char**argc) {
  sem_init(s,0, a);
  while(b--) pthread_create(&tid,NULL,func,"W"); 
  while(c--) pthread_create(&tid,NULL,func,"P"); 
  while(d--) pthread_create(&tid,NULL,func,"W"); 
  pthread_exit(NULL); 
  /*Process will finish when all threads have exited */
}

Q4

Complete the following code. The following code is supposed to print alternating A and B. It represents two threads that take turns to execute. Add condition variable calls to func so that the waiting thread does not need to continually check the turn variable. Hint: Why is cond_broadcast necessary?

pthread_cond_t cv = PTHREAD_COND_INITIALIZER;
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;

void* turn;

void* func(void*mesg) {
  while(1) {
// Add mutex lock and condition variable calls ...

    while(turn == mesg) { /* poll again ... Change me - This busy loop burns CPU time!*/ }

    /* Do stuff on this thread*/
    puts( (char*) mesg);
    turn = mesg;
    
  }
  return 0;
}

int main(int argc, char**argv){
  pthread_t tid1;
  pthread_create(&tid1,NULL, func, "A");
  func("B"); // no need to create another thread - just use the main thread
  return 0;
}

Q5

Identify the critical sections in the given code. Add mutex locking to make the code thread safe. Add condition variable calls so that total never becomes negative or above 1000. Instead the call should block until it is safe to proceed. Explain why pthread_cond_broadcast is necessary.

int total;
void add(int value) {
 if(value) <1) return;
 total += value;
}
void sub(int value) {
 if(value) <1) return;
 total -= value;
}

Q5

Identify the critical sections in the given code. Add mutex locking to make the code thread safe. Add condition variable calls so that total never becomes negative or above 1000. Instead the call should block until it is safe to proceed.

int total;
void add(int value) {
 if(value) <1) return;
 total += value;
}
void sub(int value) {
 if(value) <1) return;
 total -= value;
}

Q6

A non-threadsafe datastructure has size() enq and deq methods. Use condition variable and mutex lock to complete the thread-safe, blocking versions.

void enqueue(void* data) {
// should block if the size() > 256
enq(data);
}
void* dequeue() {
// should block if size() is 0
return deq();
}