lib/libsys/lseek.2

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.Dd July 13, 2020
.Dt LSEEK 2
.Os
.Sh NAME
.Nm lseek
.Nd reposition read/write file offset
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft off_t
.Fn lseek "int fildes" "off_t offset" "int whence"
.Sh DESCRIPTION
The
.Fn lseek
system call repositions the offset of the file descriptor
.Fa fildes
to the
argument
.Fa offset
according to the directive
.Fa whence .
The argument
.Fa fildes
must be an open
file descriptor.
The
.Fn lseek
system call
repositions the file position pointer associated with the file
descriptor
.Fa fildes
as follows:
.Bl -item -offset indent
.It
If
.Fa whence
is
.Dv SEEK_SET ,
the offset is set to
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_CUR ,
the offset is set to its current location plus
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_END ,
the offset is set to the size of the
file plus
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_HOLE ,
the offset is set to the start of the next hole greater than or equal
to the supplied
.Fa offset .
The definition of a hole is provided below.
.It
If
.Fa whence
is
.Dv SEEK_DATA ,
the offset is set to the start of the next non-hole file region greater
than or equal to the supplied
.Fa offset .
.El
.Pp
The
.Fn lseek
system call allows the file offset to be set beyond the end
of the existing end-of-file of the file.
If data is later written
at this point, subsequent reads of the data in the gap return
bytes of zeros (until data is actually written into the gap).
However, the
.Fn lseek
system call does not, by itself, extend the size of a file.
.Pp
A
.Qq hole
is defined as a contiguous range of bytes in a file, all having the value of
zero, but not all zeros in a file are guaranteed to be represented as holes
returned with
.Dv SEEK_HOLE .
File systems are allowed to expose ranges of zeros with
.Dv SEEK_HOLE ,
but not required to.
Applications can use
.Dv SEEK_HOLE
to optimise their behavior for ranges of zeros, but must not depend on it to
find all such ranges in a file.
Each file is presented as having a zero-size virtual hole at the very
end of the file.
The existence of a hole at the end of every data region allows for easy
programming and also provides compatibility to the original implementation
in Solaris.
It also causes the current file size (i.e., end-of-file offset) to be returned
to indicate that there are no more holes past the supplied
.Fa offset .
Applications should use
.Fn fpathconf _PC_MIN_HOLE_SIZE
or
.Fn pathconf _PC_MIN_HOLE_SIZE
to determine if a file system supports
.Dv SEEK_HOLE .
See
.Xr pathconf 2 .
.Pp
For file systems that do not supply information about holes, the file will be
represented as one entire data region.
.Sh RETURN VALUES
Upon successful completion,
.Fn lseek
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise,
a value of -1 is returned and
.Va errno
is set to indicate
the error.
.Sh ERRORS
The
.Fn lseek
system call
will fail and the file position pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fildes
argument
is not an open file descriptor.
.It Bq Er EINVAL
The
.Fa whence
argument
is not a proper value
or the resulting file offset would
be negative for a non-character special file.
.It Bq Er ENXIO
For
.Dv SEEK_DATA ,
there are no more data regions past the supplied offset.
Due to existence of the hole at the end of the file, for
.Dv SEEK_HOLE
this error is only returned when the
.Fa offset
already points to the end-of-file position.
.It Bq Er EOVERFLOW
The resulting file offset would be a value which cannot be represented
correctly in an object of type
.Fa off_t .
.It Bq Er ESPIPE
The
.Fa fildes
argument
is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr open 2 ,
.Xr pathconf 2
.Sh STANDARDS
The
.Fn lseek
system call is expected to conform to
.St -p1003.1-2008 .
.Pp
The
.Dv SEEK_HOLE
and
.Dv SEEK_DATA
directives, along with the
.Er ENXIO
error, are extensions to that specification.
.Sh HISTORY
The
.Fn lseek
function appeared in
.At v7 .
.Sh BUGS
If the
.Fn lseek
system call is operating on a device which is incapable of seeking,
it will request the seek operation and return successfully,
even though no seek was performed.
Because the
.Ar offset
argument will be stored unconditionally in the file descriptor of that device,
there is no way to confirm if the seek operation succeeded or not
(e.g. using the
.Fn ftell
function).
Device types which are known to be incapable of seeking include
tape drives.
.Pp
The
.Fn lseek
system call will not detect whether media are present in changeable
media devices such as DVD or Blu-ray devices.
A requested seek operation will therefore return sucessfully when no
medium is present.
.Pp
This document's use of
.Fa whence
is incorrect English, but is maintained for historical reasons.