noble (2) lseek.2.gz

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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.

VERSIONS

       On Linux, using lseek() on a terminal device fails with the error ESPIPE.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, 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().

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

SEE ALSO

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