LSEEK(2)                  (2020-08-13)                   LSEEK(2)

     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
          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 fol-
          lows:

          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 (aq\0aq)
          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).

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          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 applica-
          tions such as file backup tools, which can save space when
          creating backups and preserve holes, if they have a mecha-
          nism 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 guar-
          anteed 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.

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     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.

     CONFORMING TO
          POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

          SEEK_DATA and SEEK_HOLE are nonstandard extensions also pre-
          sent in Solaris, FreeBSD, and DragonFly BSD; they are pro-
          posed 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)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about

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          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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