#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>

int wshmid;
int w_mutex, block_read, block_write;

void P ( int s )
{
   struct sembuf b;

   b.sem_num = 0;
   b.sem_flg = 0;
   b.sem_op = -1;
   semop(s, &b, 1);
}

void V ( int s )
{
   struct sembuf b;

   b.sem_num = 0;
   b.sem_flg = 0;
   b.sem_op = 1;
   semop(s, &b, 1);
}

void writer ( int i )
{
   int *write_count;

   srand((unsigned int)getpid());
   printf("Writer %d appears\n", i);
   write_count = (int *)shmat(wshmid,NULL,0);

   P(w_mutex);
   ++(*write_count);
   if (*write_count == 1) P(block_read);
   V(w_mutex);

   P(block_write);
   printf("Writer %d going to write: write_count = %d\n", i, *write_count);
   usleep(500000 + rand() % 500001);
   V(block_write);

   P(w_mutex);
   --(*write_count);
   printf("Writer %d finished writing: write_count = %d\n", i, *write_count);
   if (*write_count == 0) V(block_read);
   V(w_mutex);

   shmdt(write_count);

   exit(0);
}

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

   srand((unsigned int)getpid());
   n = (argc > 1) ? atoi(argv[1]) : 16;

   wshmid = shmget(ftok("/home",2), sizeof(int), 0666);

   w_mutex = semget(ftok("/home",4), 1, 0666);
   block_read = semget(ftok("/home",5), 1, 0666);
   block_write = semget(ftok("/home",6), 1, 0666);

   for (i=0; i<n; ++i)
      if (!fork()) writer(i);
      else if ((i < n-1) && (i % 4 == 3)) usleep(5000000 + rand() % 5000001);

   for (i=0; i<n; ++i) wait(NULL);

   exit(0);
}