lseek(2)



LSEEK(2)                   Linux Programmer's Manual                  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
       lseek()  repositions the file offset of the open file description asso-
       ciated 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.

       lseek()  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 allo-
       cated 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  opera-
       tions  are not a guaranteed mechanism for mapping the storage space ac-
       tually allocated to a file.  (Furthermore, a sequence of zeros that ac-
       tually  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  loca-
       tion  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  er-
       ror.

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 So-
       laris, 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 descrip-
       tors, open file descriptions, and files.

       If the O_APPEND file status flag is set on the open  file  description,
       then  a  write(2)  always moves the file offset to the end of the file,
       regardless of the use of lseek().

       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 fails with the error ES-
       PIPE.

SEE ALSO
       dup(2),   fallocate(2),   fork(2),   open(2),   fseek(3),   lseek64(3),
       posix_fallocate(3)

COLOPHON
       This  page  is  part of release 4.16 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/.

Linux                             2017-09-15                          LSEEK(2)

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