#include <pthread.h>
#include <stdlib.h>

typedef struct {
  pthread_mutex_t lock;
  pthread_cond_t a_cond;
  pthread_cond_t b_cond;
  int b_blocked;
} Info;

Info *initialize_Info()
{
  Info *I;

  I = (Info *) malloc(sizeof(Info));
  pthread_mutex_init(&(I->lock), NULL);
  pthread_cond_init(&(I->a_cond), NULL);
  pthread_cond_init(&(I->b_cond), NULL);
  I->b_blocked = 0;
}

void safe_modify_data(struct Data *D, Info *I)
{
  pthread_mutex_lock(&(I->lock));
  while (!I->b_blocked) pthread_cond_wait(&(I->a_cond), &(I->lock));
  modify_data(D);
  I->b_blocked = 0;
  pthread_cond_signal(&(I->b_cond));
  pthread_mutex_unlock(&(I->lock));
}

void BM(Info *I)
{
  pthread_mutex_lock(&(I->lock));
  I->b_blocked = 1;
  pthread_cond_signal(&(I->a_cond));
  pthread_cond_wait(&(I->b_cond), &(I->lock));
  pthread_mutex_unlock(&(I->lock));
}