lseek

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

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

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

       SEEK_SET
              The offset is set to offset bytes.

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

       SEEK_END
              The 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 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
       actually allocated to a file.  (Furthermore, a sequence of  zeros  that

       *  Btrfs (since Linux 3.1)

       *  OCFS (since Linux 3.2)

       *  XFS (since Linux 3.5)

       *  ext4 (since Linux 3.8)

       *  tmpfs (since Linux 3.8)

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
       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.

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

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

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

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 descrip-
       tors, open file descriptions, and files.

       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 follow-
       ing macros:

        old       new
       0        SEEK_SET
       1        SEEK_CUR
       2        SEEK_END
       This page is part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-08-08                          LSEEK(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019 Hurricane Electric. All Rights Reserved.