lseek(2)

NAME

   lseek - reposition read/write file offset

SYNOPSIS

   #include <sys/types.h>
   #include <unistd.h>

   off_t lseek(int fd, off_t offset, int whence);

DESCRIPTION

   The  lseek()  function  repositions  the  file  offset of the open file
   description associated with the file  descriptor  fd  to  the  argument
   offset according to the directive whence as follows:

   SEEK_SET
          The file offset is set to offset bytes.

   SEEK_CUR
          The  file  offset  is  set  to  its current location plus offset
          bytes.

   SEEK_END
          The file offset is set to the  size  of  the  file  plus  offset
          bytes.

   The lseek() function allows the file offset to be set beyond the end of
   the file (but this does not change the size of the file).  If  data  is
   later written at this point, subsequent reads of the data in the gap (a
   "hole") return null bytes ('\0') until data is  actually  written  into
   the gap.

   Seeking file data and holes
   Since  version  3.1, Linux supports the following additional values for
   whence:

   SEEK_DATA
          Adjust the file offset to the next location in the file  greater
          than  or  equal  to offset containing data.  If offset points to
          data, then the file offset is set to offset.

   SEEK_HOLE
          Adjust the file offset to the next hole in the file greater than
          or equal to offset.  If offset points into the middle of a hole,
          then the file offset is set to offset.  If there is no hole past
          offset,  then the file offset is adjusted to the end of the file
          (i.e., there is an implicit hole at the end of any file).

   In both of the above cases, lseek() fails if offset points past the end
   of the file.

   These  operations  allow  applications  to  map  holes  in  a  sparsely
   allocated file.  This can be  useful  for  applications  such  as  file
   backup  tools,  which can save space when creating backups and preserve
   holes, if they have a mechanism for discovering holes.

   For the purposes of these operations, a hole is  a  sequence  of  zeros
   that  (normally) has not been allocated in the underlying file storage.
   However, a  filesystem  is  not  obliged  to  report  holes,  so  these
   operations are not a guaranteed mechanism for mapping the storage space
   actually allocated to a file.  (Furthermore, a sequence of  zeros  that
   actually has been written to the underlying storage may not be reported
   as a hole.)  In the simplest implementation, a filesystem  can  support
   the  operations by making SEEK_HOLE always return the offset of the end
   of the file, and making SEEK_DATA always return offset (i.e.,  even  if
   the  location  referred to by offset is a hole, it can be considered to
   consist of data that is a sequence of zeros).

   The _GNU_SOURCE feature test macro must be defined in order  to  obtain
   the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.

   The  SEEK_HOLE and SEEK_DATA operations are supported for the following
   filesystems:

   *  Btrfs (since Linux 3.1)

   *  OCFS (since Linux 3.2)

   *  XFS (since Linux 3.5)

   *  ext4 (since Linux 3.8)

   *  tmpfs(5) (since Linux 3.8)

   *  NFS (since Linux 3.18)

   *  FUSE (since Linux 4.5)

RETURN VALUE

   Upon  successful  completion,  lseek()  returns  the  resulting  offset
   location  as  measured  in  bytes  from  the beginning of the file.  On
   error, the value (off_t) -1 is returned and errno is  set  to  indicate
   the error.

ERRORS

   EBADF  fd is not an open file descriptor.

   EINVAL whence  is  not  valid.   Or: the resulting file offset would be
          negative, or beyond the end of a seekable device.

   ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the file offset is  beyond
          the end of the file.

   EOVERFLOW
          The resulting file offset cannot be represented in an off_t.

   ESPIPE fd is associated with a pipe, socket, or FIFO.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

   SEEK_DATA  and  SEEK_HOLE  are  nonstandard  extensions also present in
   Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in
   the next POSIX revision (Issue 8).

NOTES

   See   open(2)  for  a  discussion  of  the  relationship  between  file
   descriptors, open file descriptions, and files.

   The off_t data type is a signed integer data type specified by POSIX.1.

   Some devices are incapable of seeking and POSIX does not specify  which
   devices must support lseek().

   On Linux, using lseek() on a terminal device returns ESPIPE.

   When  converting  old  code,  substitute  values  for  whence  with the
   following macros:

    old       new
   0        SEEK_SET
   1        SEEK_CUR
   2        SEEK_END
   L_SET    SEEK_SET
   L_INCR   SEEK_CUR
   L_XTND   SEEK_END

   Note that file descriptors created by dup(2) or fork(2)  refer  to  the
   same open file descriptions (and thus file offsets), so seeking on such
   files may be subject to race conditions.

SEE ALSO

   dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

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