why file with hole has smaller disk block than file without hole?

#include <fcntl.h>
#include <unistd.h>

char buf1[] = "abcdefghij";
char buf2[] = "ABCDEFGHIJ";
char buf3[10];

int
main(void)
{
  int fd;

  if ((fd = creat("file.hole", FILE_MODE)) < 0) {
    err_sys("creat error");
  }

  if (write(fd, buf1, 10) != 10) {                // offset is now = 10
    err_sys("buf1 write error");
  }

  if (lseek(fd, 16380, SEEK_SET) == -1) {         // offset now = 16380
    err_sys("lseek error");
  }

  if (write(fd, buf2, 10) != 10) {                // offset now = 16390
    err_sys("buf2 write error");
  }
  close(fd);

  if ((fd = open("file.hole", O_RDWR)) == -1) {
    err_sys("failed to re-open file");
  }

  ssize_t n;
  ssize_t m;
  while ((n = read(fd, buf3, 10)) > 0) {
    if ((m = write(STDOUT_FILENO, buf3, 10)) != 10) {
      err_sys("stdout write error");
    }
  }

  if (n == -1) {
    err_sys("buf3 read error");
  }
  exit(0);
}

I’m newbie in unix system programming

There is code making file with hole.

Output result is:

$ls -ls file.hole file.nohole
8 -rw-r--r-- 1 sar 16394 time file.hole
20 -rw-r--r-- 1 sar 16394 time file.nohole

Why file with hole has fewer disk block than file without hole?

In my thinking, file without hole takes smaller disk blocks

Because file with hole is more spreaded than without hole..

From “Advanced Programming in the UNIX Environment 3rd-Stevens Rago, example 3.2”

Answer

Why do you think that a file without hole takes smaller space ? This exactly the contrary. If the file has holes, then it is not necessary to reserve disk blocks for that space. The number of disk blocks is not related to the spreading of the file, but directly related to the size of the data you wrote in the file.

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Answer