/* A simple readers/writers program using a one-word shared memory. */
/* Program by Henry M. Walker, October 7, 2000 */
/* File:  /home/walker/c/concurrency-linux/read-write-1.c */
#include <sys/types.h>
#include <unistd.h>          
#include <stdio.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

#define SIZE sizeof(int)   /* size of [int] integer */
#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 i_parent, i_child;   /* index variables */
   int value;               /* value read by child */
   int fd;                  /* device designator for shared memory */

   /* set up shared memory segment */
   /* Linux requires the use of a file in the binding of shared memory,
      so "temp.file" serves that purpose */
   fd = open("temp.file", O_RDWR);
   if (fd  < 0) {
     perror ("open error");  
     exit (1); 
   }
   shared_memory=mmap(NULL, SIZE, PROT_READ | PROT_WRITE, 
                              MAP_SHARED, fd, 0);
   close (fd);

   if (shared_memory == (caddr_t) -1)
     { perror ("error in mmap while allocating shared memory");
       exit (1);
     }

   if ((pid = fork()) < 0)  /* apply fork and check for error */
     { perror ("error in fork");  
       exit (1);
     }

   if (0 == pid)             
     { /* processing for child */
       printf ("The child process begins.\n");
       for (i_child = 0; i_child < run_length; i_child++)
         { sleep(1);  /* wait for memory to be updated */
           value = *shared_memory;
           printf ("Child's report:  current value = %2d\n", value);
         }
       printf ("The child is done\n");
     }
  else 
     { /* processing for parent */
       printf ("The parent process begins.\n");
       for (i_parent = 0; i_parent < run_length; i_parent++)
         { *shared_memory = i_parent * i_parent;/* square into shared memory */
           printf ("Parent's report: current index = %2d\n", i_parent ); 
           sleep(1);  /* wait for child to read value */
         }
       waitpid(pid, NULL, 0);
       printf ("The parent is done\n");
     }
   exit (0);
}