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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 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
       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 file system  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 file system 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).

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.

       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

       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).

NOTES

       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
       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) share the
       current file position pointer, 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 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.