I'm using an Arduino MKR WiFi 1010, a SAMD ARM Cortex M3 board. I rely on the standard library a lot, using things like std::vector<> and std::string. I also want to use std::cout. I've managed to do this on an UNO R3 using the ArduinoSTL library, but that library neither works nor is necessary on the MKR WiFi 1010, because it has all the standard library stuff built in to the platform.

However, on MKR WiFi 1010, I cannot get std::cout to produce any output. I'm guessing its not really hooked up to anything.

I want std::cout to write to Serial (which is aliased to SerialUSB which is of type Serial_).

Is there a way of making std::cout write to the Serial stream?


The ARM gcc libraries offer a simple way to redirect standard outputs. It is enough to implement function _write(int fd, char *ptr, int len) and it will replace the default implementation used in library to direct the standard outputs to debugger semihosting. The function must be compiled as C to match.

#include <Arduino.h>
#undef max
#undef min
#include <stdio.h>
#include <iostream>

using namespace std;

extern "C" {
int _write(int fd, char *ptr, int len) {
  (void) fd;
  return Serial.write(ptr, len);

void setup() {

  while (!Serial);


  printf("HERE WE ARE\r\n");

  cout << "TEST COUT\r\n";
  cerr << "TEST ERR\r\n";

void loop() {


the #undefs undefine the Arduino macros, which are in conflict with includes used in <iostream>

  • it took me many hours to make it work and in the end it is so simple – Juraj Oct 27 '19 at 11:03
  • @TimLong, did the answer help? – Juraj Jan 30 '20 at 15:19

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