NAME

       lseek - reposition read/write file offset

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

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

DESCRIPTION

       lseek()  repositions the file offset of the open file description associated 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 Linux 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>.

       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)

       •  GFS2 (since Linux 4.15)

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.

       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and offset is beyond the  end  of  the  file,  or
              whence is SEEK_DATA and offset is within a hole at 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.

STANDARDS

       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  descriptors,  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 ESPIPE.

SEE ALSO

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