/* A simple readers/writers program using a shared buffer and spinlocks  */

#include <sys/types.h>
#include <unistd.h>          
#include <stdio.h>
#include <sys/mman.h>

#define BUF_SIZE 5            /* logical size of buffer */
#define SHARED_MEM_SIZE (BUF_SIZE+2)*sizeof(int) /* size of shared memory */
#define run_length 10  /* number of iterations in test run */

int main (void)
{  pid_t pid;          /* variable to record process id of child */
   caddr_t shared_memory;   /* shared memory base address */

   int *in;         /* pointer to logical 'in' address for writer */
   int *out;        /* pointer to logical 'out' address for reader */
   int *buffer;       /* logical base address for buffer */
 
   int i_child, j_child;   /* index variables */
   int value;              /* value read by child */

   /* set up shared memory segment */
   shared_memory=mmap(0, SHARED_MEM_SIZE, PROT_READ | PROT_WRITE, 
                              MAP_ANONYMOUS | MAP_SHARED, -1, 0);
   if (shared_memory == (caddr_t) -1)
     { perror ("error in mmap while allocating shared memory\n");
       exit (1);
     }

   /* shared memory segment will be organized as follows:
       0                                                   n-1 n  n+1
      ---------------------------------------------------------------
      |                                                       |  |  |
      ---------------------------------------------------------------
       ^                                                       ^  ^
      buffer                                                   in out     
   */

   buffer = (int*) shared_memory; /* logical buffer starts at shared segment */
   in  = (int*) shared_memory + BUF_SIZE*sizeof(int);
   out = (int*) shared_memory + (BUF_SIZE+1)*sizeof(int);

   *in = *out = 0;          /* initial starting points */

   if (-1 == (pid = fork())) /* check for error in spawning child process */
     { perror ("error in fork");  
       exit (1);
     }

   if (0 == pid)             
     { /* processing for child == reader */
       printf ("The reader process begins.\n");

       for (i_child = 0; i_child < run_length; i_child++)
          {  while (*in == *out) ;       /* spinlock waiting for data */
             value = buffer[*out];
             *out = (*out + 1) % BUF_SIZE;
             printf ("Reader's report: item %2d == %2d\n", i_child, value);
         }
       printf ("Reader done.\n");
     } 
  else 
     { /* processing for parent == writer */
       printf ("The writer process begins.\n");
       
       for (j_child = 0; j_child < run_length; j_child++)
         {  while ((*in + 1) % BUF_SIZE == *out);/* spinlock waiting for space */
            buffer[*in] = j_child*j_child;    /* put data in buffer */
            *in = (*in + 1) % BUF_SIZE;
            printf ("Writer's report: item %2d put in buffer\n", j_child);
          }
       wait (pid);
       printf ("Writer done.\n");  
     }
   exit (0);
}