An example of calling R functions from C

mRunif.c

//myRunif.c 
#include <R.h>
#include <Rinternals.h>

SEXP myRunif( SEXP n, SEXP min, SEXP max ){

  SEXP statsPackage;
  PROTECT(
    statsPackage = eval( lang2( install("getNamespace"),
      ScalarString(mkChar("stats")) ),
      R_GlobalEnv
    )   
  );  

  SEXP RCallBack;
  PROTECT( RCallBack = allocVector(LANGSXP, 4 )); 
  SETCAR( RCallBack,
    findFun( install("runif"), statsPackage )
  );  

  SETCADR( RCallBack, n );
  SET_TAG( CDR( RCallBack ), install("n") );

  SETCADDR( RCallBack, min );
  SET_TAG( CDDR( RCallBack ), install("min") );

  SETCADDDR( RCallBack, max );
  SET_TAG( CDR(CDDR( RCallBack )), install("max"));

  SEXP randoms;
  PROTECT(
    randoms = eval( RCallBack, statsPackage )
  );  

  UNPROTECT(3);
  return( randoms );

}

myRunif.R

#myRunif.R
myRunif <-
function( n, min = 0, max = 1 ){

  unifRandom <- .Call( 'myRunif', n, min, max )

  return( unifRandom )

}

myRunifConcise.c

//Alternate myRunif.c 
#include <R.h>
#include <Rinternals.h>

SEXP myRunif( SEXP n, SEXP min, SEXP max ){

  SEXP statsPackage;
  PROTECT(
    statsPackage = eval( lang2( install("getNamespace"),
      ScalarString(mkChar("stats")) ),
      R_GlobalEnv
    )   
  );  

  SEXP randoms;
  PROTECT(
    randoms = eval( lang4( install("runif"),
     n,
     min,
     max),
     statsPackage
  ));

  UNPROTECT(2); 


  return( randoms );

}

zzz.R

#zzz.R
.First.lib <-
function( libname, pkgname ){

  library.dynam( pkgname, pkgname, libname )

}