\name{packing}
\alias{.pack}
\alias{packing}
\alias{.unpack}
\title{Handling of foreign C fundamental data types}
\description{Functions to unpack/pack (read/write) foreign C data types from/to R atomic vectors and C data objects such as arrays and pointers to structures.}
\usage{
.pack(x, offset, sigchar, value)
.unpack(x, offset, sigchar)
}
\arguments{
\item{x}{atomic vector (logical, raw, integer or double) or external pointer.}
\item{offset}{integer specifying \emph{byte offset} starting at 0.} 
\item{sigchar}{character string specifying the C data type by a \link{type signature}.}
\item{value}{R object value to be coerced and packed to a foreign C data type.} 
}
\details{
The function \code{.pack} converts an R \code{value} into a C data type specified by the \link{signature} \code{sigchar} 
and it writes the raw C foreign data value at byte position \code{offset} into the object \code{x}.
The function \code{.unpack} extracts a C data type according to the \link{signature} \code{sigchar} 
at byte position \code{offset} from the object \code{x} and converts the C value to an R value and returns it.

Byte \code{offset} calculations start at 0 relative to the first byte in an atomic vectors data area.

If \code{x} is an atomic vector, a bound check is carried out before read/write access.
Otherwise, if \code{x} is an external pointer, there is only a C NULL pointer check.
}
\value{
\code{.unpack} returns a read C data type coerced to an R value.
}
\seealso{
\code{\link{.dyncall}} for details on type signatures.
}
\examples{
# transfer double to array of floats and back, compare precision:
n <- 6
input <- rnorm(n)
buf <- raw(n*4)
for (i in 1:n) {
  .pack(buf, 4*(i-1), "f", input[i])
}
output <- numeric(n)
for (i in 1:n) {
  output[i] <- .unpack(buf, 4*(i-1), "f")
}
# difference between double and float
difference <- output-input
print( cbind(input,output,difference) )
}