Basic printf functionality for the Arduino serial ports

SerialPrintf.txt

/*
 * Simple printf for writing to an Arduino serial port.  Allows specifying Serial..Serial3.
 * 
 * const HardwareSerial&, the serial port to use (Serial..Serial3)
 * const char* fmt, the formatting string followed by the data to be formatted
 * 
 * int d = 65;
 * float f = 123.4567;
 * char* str = "Hello";
 * serial_printf(Serial, "<fmt>", d);
 * 
 * Example:
 *   serial_printf(Serial, "Sensor %d is %o and reads %1f\n", d, d, f) will
 *   output "Sensor 65 is on and reads 123.5" to the serial port.
 * 
 * Formatting strings <fmt>
 * %B    - binary (d = 0b1000001)
 * %b    - binary (d = 1000001)  
 * %c    - character (s = H)
 * %d/%i - integer (d = 65)\
 * %f    - float (f = 123.45)
 * %3f   - float (f = 123.346) three decimal places specified by %3.
 * %o    - boolean on/off (d = On)
 * %s    - char* string (s = Hello)
 * %X    - hexidecimal (d = 0x41)
 * %x    - hexidecimal (d = 41)
 * %%    - escaped percent ("%")
 * Thanks goes to @alw1746 for his %.4f precision enhancement 
 */
void serial_printf(HardwareSerial& serial, const char* fmt, ...) { 
    va_list argv;
    va_start(argv, fmt);

    for (int i = 0; fmt[i] != '\0'; i++) {
        if (fmt[i] == '%') {
            // Look for specification of number of decimal places
            int places = 2;
            if (fmt[i+1] == '.') i++;  // alw1746: Allows %.4f precision like in stdio printf (%4f will still work).
            if (fmt[i+1] >= '0' && fmt[i+1] <= '9') {
                places = fmt[i+1] - '0';
                i++;
            }
            
            switch (fmt[++i]) {
                case 'B':
                    serial.print("0b"); // Fall through intended
                case 'b':
                    serial.print(va_arg(argv, int), BIN);
                    break;
                case 'c': 
                    serial.print((char) va_arg(argv, int));
                    break;
                case 'd': 
                case 'i':
                    serial.print(va_arg(argv, int), DEC);
                    break;
                case 'f': 
                    serial.print(va_arg(argv, double), places);
                    break;
                case 'l': 
                    serial.print(va_arg(argv, long), DEC);
                    break;
                case 'o':
                    serial.print(va_arg(argv, int) == 0 ? "off" : "on");
                    break;
                case 's': 
                    serial.print(va_arg(argv, const char*));
                    break;
                case 'X':
                    serial.print("0x"); // Fall through intended
                case 'x':
                    serial.print(va_arg(argv, int), HEX);
                    break;
                case '%': 
                    serial.print(fmt[i]);
                    break;
                default:
                    serial.print("?");
                    break;
            }
        } else {
            serial.print(fmt[i]);
        }
    }
    va_end(argv);
}

For this to work on PlatformIO with an Arduino MKR1300, I had to make a few changes. Testing them in the Arduino IDE made it fail to compile, so I think these should not be added to the script, but I placed them here anyway for the few people using this in PlatformIO with the MKR1300.

• First I had to include cstdarg.h for va_start to be defined: #include <cstdarg>.
• For passing the serial interface to the function, I had to change HardwareSerial in the signature to Serial_. void serial_printf(Serial_& serial, const char* fmt, ...). (Probably because it uses SerialUSB)

Thanks for making this gist, I will be using it in my project and updating this comment if I find anything new.

I use templates to support various serial interfaces in my forked snippet below. Tested in Arduino IDE for Nano, ESP, STM32 mcus.

Serial_printf.h