printf_to_serial.cpp

/* allows c code to printf() within arduino sketches, given arm newlib or avr-libc */
#ifdef __arm__
/* TODO: proper check for newlib */

#ifdef USE_TINYUSB
#include <Adafruit_TinyUSB.h>
#endif
#include <Arduino.h>

extern "C"
int _write(int file, char * buf, int bytes) {
    if (file != 1 && file != 2) return -1;
    (void)file;

    static char initted = 0;
    if (!initted) {
        Serial.begin(9600);
        while (!Serial) __WFI();
        initted = 1;
    }

    if (!Serial.dtr()) NVIC_SystemReset();
    Serial.write(buf, bytes);
    return bytes;
}

#elif defined(__AVR__)

#include <avr/io.h>
#include <stdio.h>

#ifndef BAUD
#define BAUD 9600
#endif
#include <util/setbaud.h>

static int uart_putchar(char c, FILE * stream __attribute((unused))) {
    static char initted = 0;
    if (!initted) {
        initted = 1;
        UBRR0H = UBRRH_VALUE;
        UBRR0L = UBRRL_VALUE;
        
        UCSR0A &= ~(1 << U2X0);
        
        UCSR0C = (1 << UCSZ01) | (1 << UCSZ00); /* 8-bit data */
        UCSR0B = (1 << TXEN0); /* enable tx only */
    }
        
    loop_until_bit_is_set(UCSR0A, UDRE0);
    UDR0 = c;
    return c;
}

/* inefficient but required due to incompatibilities between avr-libc and c++ */
__attribute((constructor)) static void init(void) {
    static FILE uart_output;
    stdout = &uart_output;
    fdev_setup_stream(stdout, uart_putchar, NULL, _FDEV_SETUP_WRITE);
}

#endif

printf_to_serial_example_sketch.c

/* this must be included before any other headers, or not at all */
#include <Arduino.h>

static const unsigned long interval_milliseconds = 1000;

void setup() {
  /* function-like macro, which calls a c++ wrapper with c linkage, which interacts with Serial.print() for us */
  printf("hello world from %s\n", __func__);
}

void loop() {
  static unsigned long prev;
  const unsigned long now = millis();

  /* wait until interval_milliseconds since the previous wakeup */
  if (now - prev < interval_milliseconds) return;

  /* note we do this instead of prev = now s.t. error doesn't accumulate over time */
  prev += interval_milliseconds;

  printf("now = %lu ms\n", now);
}

printf_to_serial_example_sketch.ino

/* this file can be empty, but will be compiled the same as a .cpp with #include <Arduino.h> at the top if not */