#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 rshmid;
int r_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 reader ( int i )
{
   int *read_count;

   srand((unsigned int)getpid());
   read_count = (int *)shmat(rshmid,NULL,0);

   P(block_read);
   P(r_mutex);
   ++(*read_count);
   if (*read_count == 1) P(block_write);
   V(r_mutex);
   V(block_read);

   printf("Reader %d going to read: read_count = %d\n", i, *read_count);
   usleep(1000000 + rand() % 1000001);

   P(r_mutex);
   --(*read_count);
   printf("Reader %d finished reading: read_count = %d\n", i, *read_count);
   if (*read_count == 0) V(block_write);
   V(r_mutex);

   shmdt(read_count);

   exit(0);
}

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

   srand((unsigned int)getpid());
   n = (argc > 1) ? atoi(argv[1]) : 64;

   rshmid = shmget(ftok("/home",1), sizeof(int), 0666);

   r_mutex = semget(ftok("/home",3), 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()) reader(i); else usleep(300000 + rand() % 300001);

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

   exit(0);
}