/* file: memshare.c
   purpose: 

   Demonstrate the use of mmap().

*/

#define _POSIX_C_SOURCE 199506L
#define __EXTENSIONS__
#include <unistd.h>
#include <stdlib.h>
#include <limits.h>
#include <stdio.h>
#include <stdarg.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <signal.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>

pid_t mainpid;    /* process id of the main program */

#define CHECK(CALL)\
  if (((int)(CALL)) == -1) {\
    perror (#CALL);\
    kill (-mainpid, SIGKILL);\
    exit (-1);\
 }

#define QUIT(MSG) {\
 fprintf (stderr, "fatal error: %s\n", MSG);\
 kill (-mainpid, SIGKILL);\
 exit (-1);\
 }

int xprintf (FILE *stream, const char * format, ...) {
/* behaves like fprintf except that the output is
   atomic with respect to other calls to xprintf */
   va_list arg;
   int done;
   va_start(arg, format);
   done = vfprintf(stream, format, arg);
   va_end(arg);
   return done;
}

const char sharefilename[] = "/tmp/my_share";    /* name of file to be mapped */
int sharefd;                                     /* file desc for mapped file */

#define MYSIZE 4096
typedef struct {
  char data[MYSIZE];
} sharedregion_t;

sharedregion_t *shared;                          /* pointer to mapped memory */

int main() {

   int status;
   pid_t child;
   int result;
   sharedregion_t init;

   mainpid = getpid();

   /* create a file to represent the shared memory region */

   CHECK(sharefd = open (sharefilename, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR));

   /* initialize the file contents */

   memset (init.data, 0, sizeof (init.data));
   result = write(sharefd, &init, sizeof(init));
   if (result != sizeof(init)) QUIT ("write");

   /* map the file into the address space of this process */

   shared = (sharedregion_t *)
     mmap(0,    /* suggested address */
       sizeof (sharedregion_t),   /* size of region to be mapped */
       PROT_READ | PROT_WRITE,    /* allow read and write access */
       MAP_SHARED,                /* share changes */
       sharefd,                   /* file descriptor */
       0                          /* offset in file */);
   if ((shared == MAP_FAILED)) QUIT ("mmmap");

   printf ("mapped memory address = %09lx\n", (long) shared);

   /* create a child process to share this region */

   child = fork();
   if (child) { /* executed by the parent */
     CHECK(waitpid (child, &status, 0));
     strcat (shared->data, "parent.");
     write (1, shared->data, strlen (shared->data));
     CHECK(munmap(shared, sizeof(sharedregion_t)));
     CHECK(close (sharefd));
     CHECK(unlink (sharefilename));
   } else { /* executed by the child */
     strcat (shared->data, "child.");
     write (1, shared->data, strlen (shared->data));
   }
   return 0;
}