libc/sys/lseek.2

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.\"     @(#)lseek.2	8.3 (Berkeley) 4/19/94
.\"
.Dd April 3, 2010
.Dt LSEEK 2
.Os
.Sh NAME
.Nm lseek ,
.Nm seek
.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
function 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.
.Fn lseek
repositions the file pointer
.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.
.El
.Pp
The
.Fn lseek
function 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).
.Pp
Some devices are incapable of seeking.
The value of the pointer associated with such a device is undefined.
.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
.Fn lseek
will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa fildes
is not an open file descriptor.
.It Bq Er EINVAL
.Fa whence
is not a proper value, or the resulting file offset would be invalid.
.It Bq Er ESPIPE
.Fa fildes
is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr open 2
.Sh STANDARDS
The
.Fn lseek
function conforms to
.St -p1003.1-90 .
.Sh HISTORY
A
.Fn seek
function appeared in
.At v2 ,
later renamed into
.Fn lseek
for
.Dq long seek
due to a larger
.Fa offset
argument type.
.Sh BUGS
This document's use of
.Fa whence
is incorrect English, but is maintained for historical reasons.