sparse.c

/**
   @file      sparse.c
   @author    Mitch Richling <http://www.mitchr.me/>
   @Copyright Copyright 1995 by Mitch Richling.  All rights reserved.
   @brief     UNIX sparse files@EOL
   @Keywords  UFS filesystem sparse hole file
   @Std       ISOC POSIX

              This C program is intended to illustrate how one can
              create a file with "holes" in it.  A holy file in UNIX
              is a file that is larger, logically speaking, than the
              space it takes up.  This can happen if a file is open,
              and one seeks past the end of file, and writes to the
              file.  Many people think of this as a UNIX thing, but in
              reality it has more to do with the UFS filesystem many
              UNIX variants use.  For example, do NOT try to run this
              program on a MacOS X system with an HFS+ filesystem.
              While MacOS X is BSD based, the filesystem it most often
              uses is HFS+.  HFS+ requires that the "empty space" be
              filled in with zeros that are actually written to the
              disk.  So if you try this program in such an
              environment, you will end up with a full disk --- unless
              you have 10Tb of free disk space.

   @Build     Solaris: cc -xarch=v9  (64-bit) 

   @Tested    
              - Solaris 2.8
              - BROKEN: Linux (RH 7.3)
              - BROKEN: MacOS X.2
*/

#include <sys/types.h>          /* UNIX types      POSIX */
#include <sys/stat.h>           /* UNIX stat       POSIX */
#include <fcntl.h>              /* UNIX file ctrl  UNIX  */
#include <stdio.h>              /* I/O lib         ISOC  */
#include <unistd.h>             /* UNIX std stf    POSIX */
#include <stdlib.h>             /* Standard Lib    ISOC  */

int main(int argc, char *argv[]);

int main(int argc, char *argv[]) {
  int fd;

  fd = creat("s", O_WRONLY);
  /* First we write a byte at the start of the file.  This is not 
     required to make a sparse file. */
  write(fd, "x", 5);
  /* Now we seek way past the end of the file. Note that we use an
     unsigned integer constant to avoid overflow. */
  lseek(fd, 1024 * 1024UL * 1024 * 1024, SEEK_SET);
  /* Now we write another byte to the end of the file.  We have
     now created a 10Tb file. */
  write(fd, "x", 5);
  close(fd);

  return 0;
} /* end func main */