Calling an R function from C involves:
SET_TAG()eval()Creating the special list for the call can be done using the helper
methods lang1() to lang6(), or for R 4.4.1 or
later allocLang() can provide more flexibility if
needed.
lang1() - Calling an R function with no argumentslang1() is for assembling a call to an R function
without specifying any arguments.
In this example getwd() is called from C using
lang1()
#include <R.h>
#include <Rinternals.h>
SEXP call_r_from_c_with_lang1(void) {
  SEXP my_call = PROTECT(lang1(
    PROTECT(Rf_install("getwd"))
  ));
  SEXP my_result = PROTECT(eval(my_call, R_GlobalEnv)); 
  UNPROTECT(3);
  return my_result;
}lang2() - Calling an R function with a single unnamed
argumentlang2() is for assembling a call to an R function and
specifying a single argument.
In this example abs(-1) is called from C using
lang2()
#include <R.h>
#include <Rinternals.h>
SEXP call_r_from_c_with_lang2(void) {
  SEXP val = PROTECT(ScalarReal(-1));
  SEXP my_call = PROTECT(lang2(
    PROTECT(Rf_install("abs")),
    val
  ));
  SEXP my_result = PROTECT(eval(my_call, R_GlobalEnv)); 
  UNPROTECT(4);
  return my_result;
}code = r"(
#include <R.h>
#include <Rinternals.h>
SEXP call_r_from_c_with_lang2(void) {
  SEXP val = PROTECT(ScalarReal(-1));
  SEXP my_call = PROTECT(lang2(
    PROTECT(Rf_install("abs")),
    val
  ));
  SEXP my_result = PROTECT(eval(my_call, R_GlobalEnv)); 
  UNPROTECT(4);
  return my_result;
}
)"
callme::compile(code)lang2() - Calling an R function with a single named
argumentIn this example tempfile(fileext = ".txt") is called
from C using lang2()
#include <R.h>
#include <Rinternals.h>
SEXP call_r_from_c_with_lang2_named(void) {
 
  // Assemble the function + argument with name
  SEXP val = PROTECT(mkString(".txt"));
  SEXP my_call = PROTECT(lang2(
    PROTECT(Rf_install("tempfile")),
    val
  ));
  // Set the argument name
  SEXP t = CDR(my_call);
  SET_TAG(t, Rf_install("fileext"));
  
  // Evaluate the call
  SEXP my_result = PROTECT(eval(my_call, R_GlobalEnv)); 
  UNPROTECT(4);
  return my_result;
}code = r"(
#include <R.h>
#include <Rinternals.h>
SEXP call_r_from_c_with_lang2_named(void) {
 
  // Assemble the function + argument with name
  SEXP val = PROTECT(mkString(".txt"));
  SEXP my_call = PROTECT(lang2(
    PROTECT(Rf_install("tempfile")),
    val
  ));
  // Set the argument name
  SEXP t = CDR(my_call);
  SET_TAG(t, Rf_install("fileext"));
  
  // Evaluate the call
  SEXP my_result = PROTECT(eval(my_call, R_GlobalEnv)); 
  UNPROTECT(4);
  return my_result;
}
)"
callme::compile(code)allocLang() (R >= 4.4.1 only)The C code is equivalent to this R code:
#include <R.h>
#include <Rinternals.h>
SEXP call_print_from_c(SEXP value, SEXP digits) {
  // Allocate a new call object
  SEXP my_call = PROTECT(allocLang(3));
  // Manipulate a pointer to this call object to 
  // fill in the arguments and set argument names 
  SEXP t = my_call;
  SETCAR(t, install("print")); t = CDR(t);
  SETCAR(t,  value)          ; t = CDR(t);
  SETCAR(t, digits);
  SET_TAG(t, install("digits"));
  eval(my_call, R_GlobalEnv);
  UNPROTECT(1);
  return R_NilValue;
}code = r"(
#include <R.h>
#include <Rinternals.h>
SEXP call_print_from_c(SEXP value, SEXP digits) {
  // Allocate a new call object
  SEXP my_call = PROTECT(allocLang(3));
  // Manipulate a pointer to this call object to 
  // fill in the arguments and set argument names 
  SEXP t = my_call;
  SETCAR(t, install("print")); t = CDR(t);
  SETCAR(t,  value)          ; t = CDR(t);
  SETCAR(t, digits);
  SET_TAG(t, install("digits"));
  eval(my_call, R_GlobalEnv);
  UNPROTECT(1);
  return R_NilValue;
}
)"
callme::compile(code)