Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

[Eric Blake]

On 04/21/2011 07:22 PM, Josef Bacik wrote:
>> You absolutely need to match Solaris semantics, which are documented as follows:
>>
>>         ï  If whence is SEEK_HOLE, the offset of the start of the  next  hole
>>            greater than or equal to the supplied offset is returned. The def-
>>            inition of a hole is provided near the end of the DESCRIPTION.
>>
>> Note that in that definition, SEEK_HOLE does _not_ reposition the file offset
>> (it returns the offset of the next hole, which might be at the end of the file
>> since all files have a virtual hole at the end, but leaves the position
>> unchanged).  I'd have to write a test program on Solaris to see whether that
>> definition is actually true, or if it is a bug in the Solaris man page.
> 
> lseek's purpose is to reposition the file position, so I'd imagine
> this is just a bug in the man page.

Confirmed that the Solaris man page is buggy and does not accurately
describe the semantics that are actually implemented.  My testing showed:

lseek(fd, off, SEEK_HOLE) has four cases:

off < end, in data: repositions to the next hole as if by lseek(fd,
next_hole, SEEK_CUR); the next hole is guaranteed to exist, even if at
end of file

off < end, in hole, and later in file has data: repositions as if by
lseek(fd, off, SEEK_CUR)

off < end, in hole, and no further data in file: repositions as if by
lseek(fd, 0, SEEK_END)

off == end: fails with ENXIO

lseek(fd, off, SEEK_DATA) has four cases:

off < end, in data: repositions as if by lseek(fd, off, SEEK_CUR)

off < end, in hole, and later in file has data: repositions to the next
data as if by lseek(fd, next_data, SEEK_CUR)

off < end, in hole, and remainder of file is hold: fails with ENXIO

off == end: fails with ENXIO


Here's how I tested:

$ uname -sr
SunOS 5.10
$ cat foo.c
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>

static int
make_sparse (const char *name)
{
  int fd = open (name, O_RDWR | O_CREAT | O_TRUNC, 0600);
  if (fd < 0)
    return fd;
  printf ("creating %s\n", name);
  if (write (fd, "a", 1) != 1)
    goto cleanup;
  if (lseek (fd, 1024*1024, SEEK_CUR) == -1)
    goto cleanup;
  if (write (fd, "b", 1) != 1)
    goto cleanup;
  if (lseek (fd, 0, SEEK_SET) != 0)
    goto cleanup;
  return fd;
 cleanup:
  close (fd);
  return -1;
}

int
main (int argc, char **argv)
{
  off_t off;
  off_t end;
  int fd = argc > 2 ? open (argv[1], O_RDONLY)
    : make_sparse (argc > 1 ? argv[1] : "file");

  perror(NULL);
  if (fd < 0)
    return 2;

  printf ("fpathconf gives %ld, ENXIO is %d\n",
          fpathconf (fd, _PC_MIN_HOLE_SIZE), ENXIO);

  puts ("\ntesting at start");
  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_DATA);
  printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  puts ("\ntesting at end");

  errno = 0;
  off = end = lseek (fd, 0, SEEK_END);
  printf ("end at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, end, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, end, SEEK_DATA);
  printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, end - 1, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, end - 1, SEEK_DATA);
  printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  puts ("\ntesting at offset 1");

  errno = 0;
  off = lseek (fd, 1, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 1, SEEK_DATA);
  printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  puts ("\ntesting at offset 200000");

  errno = 0;
  off = lseek (fd, 200000, SEEK_HOLE);
  printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 200000, SEEK_DATA);
  printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);

  errno = 0;
  off = lseek (fd, 0, SEEK_CUR);
  printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);

  if (close (fd))
    return 3;
  return 0;
}
$ ./foo
creating file
Error 0
fpathconf gives 512, ENXIO is 6

testing at start
CUR at offset 0, errno 0
HOLE gives offset 131072, errno 0
CUR at offset 131072, errno 0
DATA gives offset 0, errno 0
CUR at offset 0, errno 0
HOLE gives offset 131072, errno 0
CUR at offset 131072, errno 0

testing at end
end at offset 1048578, errno 0
HOLE gives offset -1, errno 6
CUR at offset 1048578, errno 0
DATA gives offset -1, errno 6
CUR at offset 1048578, errno 0
HOLE gives offset 1048578, errno 0
CUR at offset 1048578, errno 0
DATA gives offset 1048577, errno 0
CUR at offset 1048577, errno 0

testing at offset 1
HOLE gives offset 131072, errno 0
CUR at offset 131072, errno 0
DATA gives offset 1, errno 0
CUR at offset 1, errno 0

testing at offset 200000
HOLE gives offset 200000, errno 0
CUR at offset 200000, errno 0
DATA gives offset 1048576, errno 0
CUR at offset 1048576, errno 0
$ gtruncate -s 1M sparse
$ ./foo sparse 2
Error 0
fpathconf gives 512, ENXIO is 6

testing at start
CUR at offset 0, errno 0
HOLE gives offset 0, errno 0
CUR at offset 0, errno 0
DATA gives offset -1, errno 6
CUR at offset 0, errno 0
HOLE gives offset 0, errno 0
CUR at offset 0, errno 0

testing at end
end at offset 1048576, errno 0
HOLE gives offset -1, errno 6
CUR at offset 1048576, errno 0
DATA gives offset -1, errno 6
CUR at offset 1048576, errno 0
HOLE gives offset 1048576, errno 0
CUR at offset 1048576, errno 0
DATA gives offset -1, errno 6
CUR at offset 1048576, errno 0

testing at offset 1
HOLE gives offset 1, errno 0
CUR at offset 1, errno 0
DATA gives offset -1, errno 6
CUR at offset 1, errno 0

testing at offset 200000
HOLE gives offset 1048576, errno 0
CUR at offset 1048576, errno 0
DATA gives offset -1, errno 6
CUR at offset 1048576, errno 0
$

-- 
Eric Blake   eblake@xxxxxxxxxx    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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