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

int write_buffer(int fd, const void *buf, int count);

int outfile;
int main(void)
{
    	char *mapped;
    	char *ptr;

    	if(outfile=open("test.dat", O_RDWR|O_CREAT|O_TRUNC, 0640)==-1)
    	{
		printf("COULDN'T OPEN THIS FILE!\n");
		exit(254);
    	}

    	lseek(outfile,1000,SEEK_SET);
    	if(write(outfile,"\0",1)==-1)
	{
		printf("ERROR, WRITE FAILED!\n");
		exit(254);
    	}
	mapped=mmap(NULL, 1000, PROT_READ|PROT_WRITE,MAP_SHARED, outfile, 0);
	if(!mapped)
    		printf("ERROR, MMAP FAILED!\n");

	ptr=mapped;
	printf("PLEASE ENTER A NUMBER:");
	fgets(mapped, 80, stdin);

	ptr=mapped;
	sprintf(ptr,"YOUR NUMBER TIMES TWO IS:%d.\n",atoi(mapped)*2);
	printf("YOUR NUMBER TIMES TWO IS:%d\n",atoi(mapped)*2);

	msync(mapped, 1000, MS_SYNC);
	munmap(mapped,1000);

	if(!close(outfile))
	{
		printf("POSSIBLY SERIOUS ERROR,CLOSE FILE FAILED");
		exit(254);
    	}
	return 0;
}


int write_buffer(int fd, const void *buf, int count)
{
    	const void *pts=buf;
    	int status=0, n;

    	if(count<0)
		return(-1);
    	while(status!=count)
    	{
        	if(n=write(outfile, "\0",1)==-1)
		{
			printf("ERROR, WRITE FAILED!\n");
			exit(254);
        	}
        	if(n<0)
            		return(n);
        	status+=n;
    	}
	return(status);
}