Multi threaded Programming: Review Questions

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 "B" 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 */
}