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

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

       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:

              The offset is set to offset bytes.

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

              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

              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.

              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 file system is not obliged to report holes, so these  opera-
       the value (off_t) -1 is returned and  errno  is  set  to  indicate  the

       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.

              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.

       SVr4, 4.3BSD, POSIX.1-2001.

       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).

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

       On Linux, using lseek() on a tty 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
       L_SET    SEEK_SET
       L_INCR   SEEK_CUR
       L_XTND   SEEK_END

       Note  that file descriptors created by dup(2) or fork(2) share the cur-
       rent file position pointer, so seeking on such files may be subject  to
       race conditions.

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

       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at
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