Using file descriptors directly in R

fdtest.cpp

#include <cerrno>
#include <fcntl.h>
#include <cstdio>
#include <unistd.h>
#include <string.h>
#include <array>
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::plugins(cpp11)]]

// https://cboard.cprogramming.com/showthread.php?t=93683
int fd_is_valid(int fd) {
  return fcntl(fd, F_GETFD) != -1 || errno != EBADF;
}

// [[Rcpp::export]]
int makeFdForWrite(std::string filename) {
  int fd = open(filename.c_str(), O_WRONLY | O_CREAT, 0644);
  if(!fd_is_valid(fd)) Rcpp::stop("somthing went wrong :(");
  return fd;
}

// [[Rcpp::export]]
void writeToFd(int fd) {
  if(!fd_is_valid(fd)) Rcpp::stop("somthing went wrong :(");
  std::array<unsigned char, 17> mydata = 
    {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16};
  int bytes_written = write(fd, mydata.data(), mydata.size());
  if(bytes_written != mydata.size()) Rcpp::stop("somthing went wrong :(");
  close(fd);
}

// [[Rcpp::export]]
int makeFdForRead(std::string filename) {
  int fd = open(filename.c_str(), O_RDONLY);
  if(!fd_is_valid(fd)) Rcpp::stop("somthing went wrong :(");
  return fd;
}

// [[Rcpp::export]]
std::vector<unsigned char> readFromFd(int fd) {
  if(!fd_is_valid(fd)) Rcpp::stop("somthing went wrong :(");
  std::vector<unsigned char> output;
  std::array<unsigned char, 4> temp = {0,0,0,0};
  int bytes_read;
  while( (bytes_read = read(fd, temp.data(), 4)) > 0 ) {
    output.insert(std::end(output), std::begin(temp), std::end(temp));
  }
  close(fd);
  return output;
}

/*** R
setwd("~")
write_fd <- makeFdForWrite("fdtest.bin")
writeToFd(write_fd)
readBin("fdtest.bin", what="raw", n=1e9)
read_fd <- makeFdForRead("fdtest.bin")
readFromFd(read_fd)
*/