\HeaderA{readChar}{Transfer Character Strings To and From Connections}{readChar}
\aliasA{writeChar}{readChar}{writeChar}
\keyword{file}{readChar}
\keyword{connection}{readChar}
\begin{Description}\relax
Transfer character strings to and from connections, without assuming
they are null-terminated on the connection.
\end{Description}
\begin{Usage}
\begin{verbatim}
readChar(con, nchars, useBytes = FALSE)

writeChar(object, con,
          nchars = nchar(object, type="chars"), eos = "")
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{con}] A connection object, or a character string naming a file,
or a raw vector.
\item[\code{nchars}] integer, giving the lengths in characters of (unterminated)
character strings to be read or written.  Must be >= 0 and not
missing.
\item[\code{useBytes}] logical: should \code{nchars} be regarded as a number
of bytes not characters in a multi-byte locale?
\item[\code{object}] A character vector to be written to the connection, at
least as long as \code{nchars}.
\item[\code{eos}] \sQuote{end of string}: character string .  The terminator
to be written after each string, followed by an ASCII \code{nul};
use \code{NULL} for no terminator at all.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
These functions complement \code{\LinkA{readBin}{readBin}} and
\code{\LinkA{writeBin}{writeBin}} which read and write C-style zero-terminated
character strings.  They are for strings of known length, and
can optionally write an end-of-string mark.  They are intended only
for character strings valid in the current locale.

If \code{con} is a character string, the functions call
\code{\LinkA{file}{file}} to obtain an file connection which is opened for
the duration of the function call.

If the connection is open it is read/written from its current
position.  If it is not open, it is opened for the duration of the
call in an appropriate mode (binary read or write) and then closed
again.  An open connection must be in binary mode.

If \code{readChar} is called with \code{con} a raw vector, the data in
the vector is used as input.  If \code{writeChar} is called with
\code{con} a raw vector, it is just an indication that a raw vector
should be returned.  

Character strings containing ASCII \code{nul}(s) will be read
correctly by \code{readChar} but truncated at the first
\code{nul} with a warning.

If the character length requested for \code{readChar} is longer than
the data available on the connection, what is available is
returned.  For \code{writeChar} if too many characters are requested
the output is zero-padded, with a warning.

Missing strings are written as \code{NA}.
\end{Details}
\begin{Value}
For \code{readChar}, a character vector of length the number of
items read (which might be less than \code{length(nchars)}).

For \code{writeChar}, a raw vector (if \code{con} is a raw vector) or
invisibly \code{NULL}.
\end{Value}
\begin{Note}\relax
Earlier versions of \R{} allowed embedded nul bytes within character
strings, but not \R{} >= 2.8.0.  \code{readChar} was commonly used to
read fixed-size zero-padded byte fields for which \code{readBin} was
unsuitable.  \code{readChar} can still be used for such fields if
there are no embedded nuls: otherwise \code{readBin(what="raw")}
provides an alternative.

\code{nchars} will be interpreted in bytes not characters in a
non-UTF-8 multi-byte locale, with a warning.

There is little validity checking of UTF-8 reads.
\end{Note}
\begin{SeeAlso}\relax
The \emph{R Data Import/Export} manual.

\code{\LinkA{connections}{connections}}, \code{\LinkA{readLines}{readLines}},
\code{\LinkA{writeLines}{writeLines}}, \code{\LinkA{readBin}{readBin}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
## test fixed-length strings
zz <- file("testchar", "wb")
x <- c("a", "this will be truncated", "abc")
nc <- c(3, 10, 3)
writeChar(x, zz, nc, eos=NULL)
writeChar(x, zz, eos="\r\n")
close(zz)

zz <- file("testchar", "rb")
readChar(zz, nc)
readChar(zz, nchar(x)+3) # need to read the terminator explicitly
close(zz)
unlink("testchar")
\end{ExampleCode}
\end{Examples}

