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From: jonb@specialix.co.uk (Jon Brawn)
Newsgroups: comp.unix.internals
Subject: Re: How do you find the symbolic links to files.
Message-ID: <1990Dec5.183223.28304@specialix.co.uk>
Date: 5 Dec 90 18:32:23 GMT
References: <25146@adm.brl.mil> <1990Dec5.052124.28435@erg.sri.com> <10960:Dec507:07:4190@kramden.acf.nyu.edu>
Organization: Specialix International, London
Lines: 117

brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:

>In article <1990Dec5.052124.28435@erg.sri.com> zwicky@erg.sri.com (Elizabeth Zwicky) writes:
>> Unfortunately, you
>> have to get pretty intimate with the disk to tell that the 20 meg of
>> nulls aren't there

>Hardly. You just look at the file size. Other than the file size, there
>is no way a portable program can tell the difference between a hole and
>an allocated block of zeros. If an archiver knows the block size and
>sees that a file has N holes, it can just squish the first N holes it
>finds, and write explicit zeros in the remaining zero-filled blocks.

Umm? really? I wrote this program:

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

/*
** This program was written on SCO Unix System V release 3.2.something.
*/
char	buffer[1024];

main()
{
	int fd;			/* file desctiptor */
	int block;		/* a block number  */
	long offset;		/* offset at which to write above block */
	struct stat statb;	/* stat structure used to read the file size */

	/*
	** set the buffer to a known data pattern:
	*/
	memset(buffer,42,sizeof(buffer));

	/*
	** create a new file
	*/
	if ( (fd = creat("hole_file",0666))==-1 )
	{
		perror("cant creat hole_file");
		exit(1);
	}

	/*
	** write ten (sparse) blocks to it
	*/
	for ( block=0; block<10; block++ )
	{
		/*
		** blocks are at 10K intervals in the file
		*/
		offset = block * 10240;

		/*
		** seek...
		*/
		if ( lseek(fd,offset,0) != offset )
		{
			perror("cant seek into hole_file");
			exit(1);
		}
		/*
		** ...write
		*/
		if ( write(fd,buffer,sizeof(buffer)) != sizeof(buffer) )
		{
			perror("cant write hole_file");
			exit(1);
		}
	}

	/*
	** close the file
	*/
	if ( close(fd) == -1 )
	{
		perror("trouble closeing hole_file");
	}

	/*
	** ask the OS how big the file is
	*/
	if ( stat("hole_file",&statb)==-1 )
	{
		perror("cant stat hole_file");
		exit(1);
	}
	printf("stat information for hole_file:\n");

	printf("st_size %d\n",statb.st_size);

	system("ls -ils hole_file");
}

And ran it, producing this output:

stat information for hole_file:

st_size 93184

16314  184 -rw-rw----   1 jonb     soft       93184 Dec  5 18:23 hole_file
inode size  mode      num user     group       size date   time  name
    (blocks)        links name     name      (bytes)

The size of the file is indeed 9*10240+1024.

Now, please demonstrate to your audience where the holes can be detected?

>---Dan
Jonb

--
``Let the myth be expelled. I stand here before you. Can you not see me?
  Do you not hear my voice?''