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

int main (int argc, char* argv []) 
{
    char buffer [BUFSIZ];
    char* pMap;

    int fd = open ("MAPPED.FILE", O_CREAT | O_RDWR | O_SYNC, 0666);
    if (fd == -1) {
	perror ("open");
	return -1;
    }

    if (lseek (fd, BUFSIZ - 1, SEEK_SET) == -1) {
	perror ("lseek");
	return -1;
    }
    write (fd, "\0", 1);
    if (lseek (fd, 0, SEEK_SET) == -1) {
	perror ("lseek");
	return -1;
    }

    pMap = (char*) mmap (NULL, BUFSIZ, PROT_READ | PROT_WRITE, 
			 MAP_SHARED, fd, 0);
    if ((size_t) pMap == -1) {
	perror ("mmap");
	return -1;
    }
    
    while (1) {
	fprintf (stderr, "<Press ENTER to read> OR <Enter string to write> $ ");
	fgets (buffer, sizeof (buffer) - 1, stdin);

#if 0
	if (munmap (pMap, BUFSIZ)) {
	    perror ("munmap");
	    return -1;
	}
	pMap = (char*) mmap (NULL, BUFSIZ, PROT_READ | PROT_WRITE, 
				   MAP_SHARED, fd, 0);
	if ((size_t) pMap == -1) {
	    perror ("mmap");
	    return -1;
	}
#endif
	struct flock lck = { 
	    F_WRLCK,  /* Type of lock: F_RDLCK, F_WRLCK, or F_UNLCK    */
	    SEEK_SET, /* Where `l_start' is relative to (like `lseek') */
	    0,        /* Offset where the lock begins                  */
	    0,        /* Size of the locked area; zero means until EOF */
	    getpid () /* Process holding the lock                      */
	};

	if (fcntl (fd, F_SETLK, &lck)) {
	    perror ("fcntl");
	    return -1;
	}

	if (buffer [0] == '\n') {
	    fprintf (stderr, "Mapped file contents: %s\n", pMap);
	} else {
	    strncpy (pMap, buffer, strlen (buffer) - 1);
	    fprintf (stderr, "Written to mapped file: %s\n", pMap);
	}

 	if (msync (pMap, BUFSIZ, MS_SYNC | MS_INVALIDATE)) {
 	    perror ("msync");
 	    return -1;
 	}

	lck.l_type = F_UNLCK;
	if (fcntl (fd, F_SETLK, &lck)) {
	    perror ("fcntl");
	    return -1;
	}
    }

    return 0;
}