The R Project SVN R

% File src/library/base/man/seek.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2007 R Core Development Team
% Distributed under GPL 2 or later

\name{seek}
\alias{seek}
\alias{seek.connection}
\alias{truncate}
\alias{truncate.connection}
\alias{isSeekable}
\title{Functions to Reposition Connections}
\description{
  Functions to re-position connections.
}
\usage{
seek(con, \dots)
\method{seek}{connection}(con, where = NA, origin = "start", rw = "", \dots)

isSeekable(con)

truncate(con, \dots)
}
\arguments{
  \item{con}{a connection.}
  \item{where}{numeric.  A file position (relative to the origin
    specified by \code{origin}), or \code{NA}.}
  \item{rw}{character.  Empty or \code{"read"} or \code{"write"},
    partial matches allowed.}
  \item{origin}{character.  One of \code{"start"}, \code{"current"},
    \code{"end"}: see \sQuote{Details}.}
  \item{\dots}{further arguments passed to or from other methods.}
}
\details{
  \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
  gz-compressed) file connections do.  \code{gzfile} connections do not
  support \code{origin = "end"}; the file position they use is that
  of the uncompressed file.

  \code{where} is stored as a real but should represent an integer:
  non-integer values are likely to be truncated.  Note that the possible
  values can exceed the largest representable number in an \R
  \code{integer} on 64-bit OSes, and on some 32-bit OSes.
  
#ifdef windows
  Clipboard connections can seek too.

  We have found so many errors in the Windows implementation of file
  positioning that users are advised to use it only at their own risk,
  and asked not to waste the \R developers' time with bug reports on Windows'
  deficiencies.
#endif
  
  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 both 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.  (The reported write position for a file opened in
  an append mode will typically be unreliable until the file has been
  written to.)

  If \code{seek} is called with a non-\code{NA} value of \code{where},
  any pushback on a text-mode connection is discarded.

  \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: on others (including Windows) it will
  not work for large (> 2Gb) files.
}
\value{
  \code{seek} returns the current position (before any move), as a
  (numeric) byte offset from the origin, if relevant, or \code{0} if
  not.  Note that the position can exceed the largest representable
  number in an \R \code{integer} on 64-bit OSes, and on some 32-bit
  OSes.

  \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
  supports \code{seek}.
}
\seealso{
  \code{\link{connections}}
}
\keyword{file}
\keyword{connection}