[1] Problem description
----------------------------------
a call to lseek() fails with EINVAL under the following conditions:
- it is called on a disk device file
- required offset is larger than the target disk device size
[2] Kernel version
---------------------------
This problem seems to be limited to version 2.4.18
I tested also on kernel 2.4.2 but everythng works fine there.
[3] Reproducing the problem
-----------------------------------------
Compile the following program (lseektest.c):
#define _LARGEFILE64_SOURCE
#define _FILE_OFFSET_BITS 64
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
main(int argc, char *argv[])
{
int fd;
int i;
off_t ret;
printf("Attempting to seek through file %s \n", argv[1]);
fd = open(argv[1], O_RDONLY | O_NONBLOCK, 0);
printf("Executed file open. errno = %d\n", errno);
for (i=1; i<10; i++ )
{
errno = 0;
ret = lseek (fd, (off_t)1024 *(off_t)1024 *(off_t)1024
*(off_t)i, SEEK_SET);
printf("lseek(%i Gb ): errno = %i ret = %lli\n", i,
errno, ret );
};
close (fd);
}
Run it with 1 parameter being an existing disk device file. For
example:
./lseektest /dev/hda3
Sample output on a machine with kernel 2.4.2, displaying the CORRECT
behaviour
(/dev/hda3 is a device file repersenting disk image of size 7517151
kb):
Attempting to seek through file /dev/hda3
Executed file open. errno = 0
lseek(1 Gb ): errno = 0 ret = 1073741824
lseek(2 Gb ): errno = 0 ret = 2147483648
lseek(3 Gb ): errno = 0 ret = 3221225472
lseek(4 Gb ): errno = 0 ret = 4294967296
lseek(5 Gb ): errno = 0 ret = 5368709120
lseek(6 Gb ): errno = 0 ret = 6442450944
lseek(7 Gb ): errno = 0 ret = 7516192768
lseek(8 Gb ): errno = 0 ret = 8589934592
lseek(9 Gb ): errno = 0 ret = 9663676416
Sample output on the same machine, but booted with kernel 2.4.18:
Attempting to seek through file /dev/hda3
Executed file open. errno = 0
lseek(1 Gb ): errno = 0 ret = 1073741824
lseek(2 Gb ): errno = 0 ret = 2147483648
lseek(3 Gb ): errno = 0 ret = 3221225472
lseek(4 Gb ): errno = 0 ret = 4294967296
lseek(5 Gb ): errno = 0 ret = 5368709120
lseek(6 Gb ): errno = 0 ret = 6442450944
lseek(7 Gb ): errno = 0 ret = 7516192768
lseek(8 Gb ): errno = 22 ret = -1
lseek(9 Gb ): errno = 22 ret = -1
[5] Special NOTE
---------------------------
Improper behaviour was noticed on device files only. When "normal"
files
are in question, everything seems to be fine.
[6] Reason for reporting this problem
---------------------------------------------------
Our multi-platform backup product relies on proper behaviour of the
lseek()
command to calculate a rawdisk size.
[7] Submitter
----------------------
Tomaz Susnik, Hermes SoftLab, Slovenia
[email protected]
On Jun 14, 2002 15:07 +0200, Tomaz Susnik wrote:
> [1] Problem description
> ----------------------------------
>
> a call to lseek() fails with EINVAL under the following conditions:
> - it is called on a disk device file
> - required offset is larger than the target disk device size
Is this behaviour mandated in a standard, or is it just different from
previous behaviour? I'm not saying it _isn't_ a bug, but I don't see
how seeking past the end of a block device is very useful.
> Attempting to seek through file /dev/hda3
>
> lseek(6 Gb ): errno = 0 ret = 6442450944
> lseek(7 Gb ): errno = 0 ret = 7516192768
> lseek(8 Gb ): errno = 0 ret = 8589934592
> lseek(9 Gb ): errno = 0 ret = 9663676416
>
>
> Sample output on the same machine, but booted with kernel 2.4.18:
>
> Attempting to seek through file /dev/hda3
>
> lseek(6 Gb ): errno = 0 ret = 6442450944
> lseek(7 Gb ): errno = 0 ret = 7516192768
> lseek(8 Gb ): errno = 22 ret = -1
> lseek(9 Gb ): errno = 22 ret = -1
>
> [6] Reason for reporting this problem
> ---------------------------------------------------
> Our multi-platform backup product relies on proper behaviour of the
> lseek() command to calculate a rawdisk size.
Well, e2fsprogs has a similar test that it uses if the BLKGETSZ ioctl
fails, but I don't see how this new behaviour is a real problem. All you
have to do is check if _either_ lseek(offset) fails or read() from that
offset fails to know you are past the end of the block device. It hardly
changes the algorithm at all.
Cheers, Andreas
--
Andreas Dilger
http://www-mddsp.enel.ucalgary.ca/People/adilger/
http://sourceforge.net/projects/ext2resize/
> a call to lseek() fails with EINVAL under the following conditions:
> - it is called on a disk device file
> - required offset is larger than the target disk device size
Is this behaviour mandated in a standard, or is it just different from
previous behaviour? I'm not saying it _isn't_ a bug, but I don't see
how seeking past the end of a block device is very useful.
I know many programs that use this. They seek and do not expect
an error, because the standard says no error is to be expected,
and then try to read or write.
Let me quote POSIX 1003.1-2001.
...
The lseek() function shall allow the file offset to be set beyond the
end of the existing data in the file.
...
[EINVAL]
The whence argument is not a proper value, or the resulting
file offset would be negative for a regular file, block special
file, or directory.
Andries