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 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>.

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 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) 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 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.