nclude <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

int main( int argc, char **argv )
{
    int fd;
    void *pageaddr;
    int *ptr;

    fd = open( "/dev/mmap", O_RDWR );
    if( fd<0 ) {
       perror( "/dev/mmap" );
       return -1;
    }
    pageaddr = mmap( NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0 );

    printf("pageaddr mmap: %p\n", pageaddr );
    if( pageaddr ) {
       ptr = (int *)pageaddr;
       printf("*pageaddr: %d\n", *ptr );
       *ptr = 55;
    }
    munmap( pageaddr, 4096 );

    return 0;
}