#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <fcntl.h>

#include <errno.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <sys/mman.h>

/*

* max_mmap_size(filename)

* A not-so-great attempt to determine at run time how much

* of a given file can be mapped in.  Usually, the amount will

* be the minimum of filesize or 0x7fffffff. The math is a

* wee bit off on the binary search.

*/

size_t

max_mmap_size(const char *file) {

       int fd;

       struct stat sbuf;

       size_t retval;

       off_t min = 0, max = (size_t)(~0);

       fd = open(file, O_RDONLY);

       if (fd == -1)

               return 0;

       fstat(fd, &sbuf);

       if ((sbuf.st_mode & S_IFMT) != S_IFREG) {

               close(fd);

               return 0;

       } else if ((off_t)max > sbuf.st_size)

               max = (sbuf.st_size);

       retval = (size_t)((max + min) / 2);

       while (min != max) {

               caddr_t t;

               errno = 0;

               t = mmap(0, retval, PROT_READ, MAP_SHARED, fd, 0);

               if (t == (caddr_t)-1 && errno != 0) {

                       if (errno != EINVAL)

                               return 0;

                       /* too large */

                       max = retval - 1;

               } else {

                       min = retval + 1;

               }

               munmap(t, retval);

               retval = (size_t)((min + max) / 2);

       }

       close(fd);

       return (size_t)min - 1;

}

/*

* sum(filename, starting_offset, size)

* Sum the bytes in the file specified by the <filename> parameter,

* starting at the specified offset and continuing for <size> bytes.

* This is very slow, but this function will eventually touch

* every byte of the file segment you have asked for.

*/

long long

sum(const char *file, off_t offset, size_t count) {

       long long retval = 0;

       struct stat sbuf;

       int fd;

       caddr_t t;

       char *cp;

       fd = open(file, O_RDONLY);

       if (fd == -1) {

               return -1LL;

       }

       fstat(fd, &sbuf);

       if (offset > sbuf.st_size)

               return -1;

       sbuf.st_size -= offset;

       if (sbuf.st_size < count)

               count = sbuf.st_size;

       if (count == 0) {

               retval = -1LL;

               goto out;

       }

       errno = 0;

       fprintf(stderr, "mmap(NULL, %u, PROT_READ, MAP_FILE|MAP_SHARED, %d, %qu)\n",

               count, fd, offset);

       t = mmap(NULL, count, PROT_READ, MAP_FILE|MAP_SHARED, fd, offset);

       if (t == (caddr_t)-1 && errno != 0) {

               fprintf(stderr, "cannot mmap %s: %s\n", file, strerror(errno));

               retval = -1LL;

               goto out;

       }

       for (cp = t; cp < &t[count]; cp++)

               retval += *cp;

       munmap(t, count);

out:

       close(fd);

       return retval;

}

main(int ac, char **av) {

       size_t max;

       long long s = 0;

       long long t, off = 0;

       if (ac != 2) {

               fprintf(stderr, "usage: %s <filename>\n", av[0]);

               exit(1);

       }

       max = max_mmap_size(av[1]);

       printf("max = %u\n", max);

       /*

        * Cycle through the given filename, in <max>-byte

        * segments.

        */

       while ((t = sum(av[1], off, max)) != -1) {

               s += t;

               off += max;

       }

       printf("sum = %qd\n", s);

       return 0;

}