% File src/library/base/man/showConnections.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2018 R Core Team
% Distributed under GPL 2 or later

\name{showConnections}
\alias{showConnections}
\alias{getConnection}
\alias{getAllConnections}
\alias{closeAllConnections}
\alias{stdin}
\alias{stdout}
\alias{stderr}
\alias{nullfile}
\alias{isatty}
\title{Display Connections}
\description{
  Display aspects of \link{connections}.
}
\usage{
showConnections(all = FALSE)
getConnection(what)
closeAllConnections()

stdin()
stdout()
stderr()
nullfile()

isatty(con)

getAllConnections()
}
\arguments{
  \item{all}{logical: if true all connections, including closed ones
    and the standard ones are displayed.  If false only open user-created
    connections are included.}

  \item{what}{integer: a row number of the table given by
    \code{showConnections}.}

  \item{con}{a connection.}
}
\details{
  \code{stdin()}, \code{stdout()} and \code{stderr()} are standard
  connections corresponding to input, output and error on the console
  respectively (and not necessarily to file streams).  They are text-mode
  connections of class \code{"terminal"} which cannot be opened or
  closed, and are read-only, write-only and write-only respectively.
  The \code{stdout()} and \code{stderr()} connections can be
  re-directed by \code{\link{sink}} (and in some circumstances the
  output from \code{stdout()} can be split: see the help page).

  The encoding for \code{\link{stdin}()} when redirected can
  be set by the command-line flag \option{--encoding}.

  \code{nullfile()} returns filename of the null device (\code{"/dev/null"}
  on Unix, \code{"nul:"} on Windows).

  \code{showConnections} returns a matrix of information.  If a
  connection object has been lost or forgotten, \code{getConnection}
  will take a row number from the table and return a connection object
  for that connection, which can be used to close the connection,
  for example.  However, if there is no \R level object referring to the
  connection it will be closed automatically at the next garbage
  collection (except for \code{\link{gzcon}} connections).

  \code{closeAllConnections} closes (and destroys) all user
  connections, restoring all \code{\link{sink}} diversions as it does
  so.

  \code{isatty} returns true if the connection is one of the class
  \code{"terminal"} connections and it is apparently connected to a
  terminal, otherwise false.  This may not be reliable in embedded
  applications, including GUI consoles.

  \code{getAllConnections} returns a sequence of integer connection
  descriptors for use with \code{getConnection}, corresponding to the
  row names of the table returned by \code{showConnections(all =
  TRUE)}. 
}
\note{
  \code{stdin()} refers to the \sQuote{console} and not to the C-level
  \file{stdin} of the process.  The distinction matters in GUI consoles
  (which may not have an active \file{stdin}, and if they do it may not
  be connected to console input), and also in embedded applications.
  If you want access to the C-level file stream \file{stdin}, use
  \code{\link{file}("stdin")}.

  When \R is reading a script from a file, the \emph{file} is the
  \sQuote{console}: this is traditional usage to allow in-line data (see
  \sQuote{An Introduction to R} for an example).
}
\value{
  \code{stdin()}, \code{stdout()} and \code{stderr()} return connection
  objects.

  \code{showConnections} returns a character matrix of information with
  a row for each connection, by default only for open non-standard connections.

  \code{getConnection} returns a connection object, or \code{NULL}.
}
\seealso{\code{\link{connections}}}

\examples{
showConnections(all = TRUE)
\dontrun{
textConnection(letters)
# oops, I forgot to record that one
showConnections()
#  class     description      mode text   isopen   can read can write
#3 "letters" "textConnection" "r"  "text" "opened" "yes"    "no"
mycon <- getConnection(3)
}

c(isatty(stdin()), isatty(stdout()), isatty(stderr()))
}
\keyword{connection}
