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

/*
 * mmbackwards.c -  print a file backwards using mmap rather than lseek
 *
 * idea: map the entire file into a region of memory
 *	 then traverse the region of memory from end to start using *p--
 *	 as though it is an array.
 *
 * system calls used:
 *	open - to get a fd for a file
 *	stat - to get size of file
 *	mmap - to map file to memory
 */

#define oops(m,x) { perror(m); exit(x); }

main(int ac, char *av[])
{
	int	fd;
	char	*region;
	char	*p;
	off_t	filesize;
	struct	stat	info;
	int	rv;
	off_t	start_at = 0;

	if ( ac != 2 ){
		fprintf(stderr, "usage: mmbackwards filename\n");
		exit(1);
	}

	if ( stat(av[1], &info) == -1 )
		oops(av[1], 2);
	filesize = info.st_size;

	if ( (fd = open(av[1],O_RDONLY)) == -1 )
		oops(av[1], 3);

	region = (char*)mmap(NULL,filesize,PROT_READ,MAP_SHARED,fd,start_at);
	if ( region == (char *)MAP_FAILED )
		oops("mmap",4);

	for( p = region + filesize - 1; p >= region ; p-- )
		putchar(*p);

	rv = munmap(region, filesize);
	/* printf("\nmunmap returned %d\n", rv); */
}