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simplec - demo package for calling C code with .C()

simplec is a small demo package showing how C code could be included in a package and called with .C()

In general: Consider using .Call() or Rcpp instead of .C()

This is one of a series of small demo packages for
calling other languages from R:

• {simplec} - calling C with .C()
• {simplecall} - calling C with .Call()
• {simplercpp} - calling C++ with {Rcpp}
• {simplefortran} - calling Fortran with .Fortran()

Rough comparison of .C(), .Call(), {Rcpp} (and .Fortran())

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Installation

You can install from GitHub with:

# install.package('remotes')
remotes::install_github('coolbutuseless/simplec)

What’s in the box?

Package contains 2 C functions, and 2 functions in R which call the C functions using .C().

What’s in the R functions?

• A call using .C()
• First argument is the C function name e.g. add_
• x and y arguments are passed through from the R function
• the third argument to the function is space in which to store the result i.e. numeric(1)
• .C() returns as many arguments as you gave it originally
• We only want to return the actual result, which is the third argument, hence [[3]]
• @useDynLib [packagename] [C function name] is used to generate the NAMESPACE entry useDynLib(simplec,add_)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#' Add two numbers
#'
#' @param x,y numbers to add
#'
#' @useDynLib simplec add_
#' @export
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
add_C <- function(x, y) {
  .C(add_, x, y, numeric(1))[[3]]
}

What’s in the C functions?

• C function returns void
• all arguments passed as pointers
• Actual result must be stored in one of the arguments

void add_(double* x, double* y, double* res) {
  res[0] = x[0] + y[0];
}

How do R variables map to C variables?

• See R manual on writing Extensions for more info

What does the C function look like in R?

• An object of class NativeSymbolInfo
• Holds an externalptr to the loaded function

simplec:::add_
#> $name
#> [1] "add_"
#> 
#> $address
#> <pointer: 0x7f9780cb1c30>
#> attr(,"class")
#> [1] "RegisteredNativeSymbol"
#> 
#> $dll
#> DLL name: simplec
#> Filename:
#>         /Library/Frameworks/R.framework/Versions/4.0/Resources/library/simplec/libs/simplec.so
#> Dynamic lookup: FALSE
#> 
#> $numParameters
#> [1] 3
#> 
#> attr(,"class")
#> [1] "CRoutine"         "NativeSymbolInfo"

Resources

• Peter Dalgaard’s Keynote from UseR 2004 discusses R’s language interfaces
• Using R — Calling C Code ‘Hello World’
• Hadley’s Advanced R book chapter - ‘Rs C interface’
• Rcpp

Acknowledgements

• R Core for developing and maintaining such a wonderful language.
• CRAN maintainers, for patiently shepherding packages onto CRAN and maintaining the repository