\Header{seek}{Functions to Reposition Connections}
\alias{isSeekable}{seek}
\methalias{seek.connection}{seek}
\alias{truncate}{seek}
\methalias{truncate.connection}{seek}
\keyword{file}{seek}
\keyword{connection}{seek}
\begin{Description}\relax
Functions to re-position connections.
\end{Description}
\begin{Usage}
\begin{verbatim}
seek(con, ...)
## S3 method for class 'connection':
seek(con, where = NA, origin = "start", rw = "", ...)

isSeekable(con)

truncate(con, ...)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{con}] a connection.
\item[\code{where}] integer.  A file position (relative to the origin
specified by \code{origin}), or \code{NA}.
\item[\code{rw}] character.  Empty or \code{"read"} or \code{"write"},
partial matches allowed.
\item[\code{origin}] character.  One of \code{"start"}, \code{"current"},
\code{"end"}.
\item[\code{...}] further arguments passed to or from other methods.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{seek} with \code{where = NA} returns the current byte offset
of a connection (from the beginning), and with a non-missing \code{where}
argument the connection is re-positioned (if possible) to the
specified position. \code{isSeekable} returns whether the connection
in principle supports \code{seek}: currently only (possibly
compressed) file connections do.

File connections can be open for both writing/appending, in which case
\R{} keeps separate positions for reading and writing. Which \code{seek}
refers to can be set by its \code{rw} argument: the default is the
last mode (reading or writing) which was used.  Most files are
only opened for reading or writing and so default to that state.
If a file is open for reading and writing but has not been used, the
default is to give the reading position (0).

The initial file position for reading is always at the beginning.
The initial position for writing is at the beginning of the file
for modes \code{"r+"} and \code{"r+b"}, otherwise at the end of the
file.  Some platforms only allow writing at the end of the file in
the append modes.

\code{truncate} truncates a file opened for writing at its current
position.  It works only for \code{file} connections, and is not
implemented on all platforms.
\end{Details}
\begin{Value}
\code{seek} returns the current position (before any move),
as a byte offset, if relevant, or \code{0} if not.

\code{truncate} returns \code{NULL}: it stops with an error if
it fails (or is not implemented).

\code{isSeekable} returns a logical value, whether the connection is
support \code{seek}.
\end{Value}
\begin{SeeAlso}\relax
\code{\Link{connections}}
\end{SeeAlso}

