SUMMARY: core in root that takes no space

From: Morten K. Barfoed (
Date: Wed Dec 07 1994 - 08:34:00 CST

Dear Sunmanagers:

I posted the following problem:

----- Begin Included Message -----
Dear sun-managers.

Here I am again with another puzzling thing:
I find an 8 Mbyte core in the /-directory.

When I do a df, it shows, that there's all
in all 15 Mbytes in the root-area, 4 of which
are still available. So I remove the core,
and do a 'df' again. Now the amount of free
space has risen with app. 400 Kbytes, and that
could be due to other activity on the partition.

As far as I can see, there is no running
process that could hold the core-file open,
so it should be removed really (at least I
think so, I used the program lsof to check).

When counting the files that are actually
on disk using du, the core-file does not
seem to count.

Unfortunately I removed the core file before
giving myselg proper time to look into it,
at least to see what program generated it.

Could anyone give me a hint on this one ?
 ----- End Included Message -----

Thanks to a lot of people, too many to list.

The solution was quite simple. The core in question
is a socalled sparse file (or a 'holy' file as one
calls it). The file's inode contains an indication
of the 8 Mbyte lenght, but the tree-structure that
administers the file is almost empty; few blocks
have actually been allocated on disk.

A very educative example was given to me by

        int fd;

        fd = creat("testfile", 0644);
        lseek(fd, 1000000, 0);
        write(fd, "a", 1);

(or dd if=/dev/zero of=testfile seek=1000000 count=1)

When running the program and listing it with 'ls -ls',
it give the following:

16 -rw-r--r-- 1 morten 1000001 Dec 7 10:16 testfile

...indicating that the file takes up 16 Kbytes, and that
the size of the file is 1 Mbyte (!). So, in effect only
16 Kbytes have been allocated on disk, but the inode
says 1 Mbyte. points out that it is normal for a core-
file to contain a large hole that corresponds to the gap
between the proces's memory space between the data and the

So, no mystery.

Best regards:

Morten Krabbe Barfoed

Danish Space Research Institute phone: +45 42 88 22 77 (switch-board)
Gl. Lundtoftevej 7 phone: +45 45 87 40 77 - 161 (direct)
DK 2800 Lyngby FAX: +45 45 93 02 83
Denmark TELEX: 37 198


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