The R Project SVN R

% File src/library/base/man/seek.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2015 R Core 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 \link{connection}.}
  \item{where}{numeric.  A file position (relative to the origin
    specified by \code{origin}), or \code{NA}.}
  \item{rw}{character string.  Empty or \code{"read"} or \code{"write"},
    partial matches allowed.}
  \item{origin}{character string.  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
  \abbr{gz}-compressed) file connections do.

  \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 builds, and on some 32-bit builds.

  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.)

  \code{gzfile} connections support \code{seek} with a number of
  limitations, using the file position of the uncompressed file.
  They do not support \code{origin = "end"}.  When writing, seeking is
  only possible forwards: when reading seeking backwards is supported by
  rewinding the file and re-reading from its start.

  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 (> 2\abbr{Gb}) files.

  None of these should be expected to work on text-mode connections with
  re-encoding selected.
}
\section{Warning}{
  Use of \code{seek} on Windows is discouraged.  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.
}
\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 builds, and on some 32-bit
  builds.

  \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}