Can you sprintf in Arduino with SAMD

Development Platforms
1

I have some sprintf commands that were working in the Arduino IDE, but after moving my program over to PlatformIO, they don’t seem to put anything into the char array. I am using an Adafruit ItsyBitsy M4, which is a Atmel SAMD 54.

char charBuf[20];
float sprintf(charBuf, "Ping: %.2f (s)", seconds);
Serial.println(charBuf);

It’s supposed to go to a TFT, but this shows the same issue - I get “Ping: (s)”

I did some searching on this forum, and I see a couple of posts about STM32 and setting a build flag, but they didn’t work, and I couldn’t see any flags for SAMD boards. The posts said that the sprintf was disabled on that architecture because of memory constraints. Is it disabled for SAMD boards too?

2

Writing float in front of the sprintf function call like that should give you a compile error. Is this the real code?

3

i use this function:

.h file:

#include <stdarg.h>
String s_printf(const char* format, ...)  __attribute__((format(printf, 1, 2)));

.cpp file:

String s_printf(const char* format, ...)
{
    char loc_buf[64];
    char* temp = loc_buf;
    va_list arg;
    va_list copy;
    va_start(arg, format);
    va_copy(copy, arg);
    int len = vsnprintf(temp, sizeof(loc_buf), format, copy);
    va_end(copy);
    if (len < 0) {
        va_end(arg);
        return "";
    };
    if (len >= sizeof(loc_buf)) {
        temp = (char*)malloc(len + 1);
        if (temp == NULL) {
            va_end(arg);
            return "";
        }
        len = vsnprintf(temp, len + 1, format, arg);
    }

    va_end(arg);        

    String s = String(temp);
    

    if (temp != loc_buf) {
        free(temp);
    }
    return s;

}
4

@maxgerhardt oops. Should be float seconds = 10.1 on one line, the sprintf on the next…. I was reusing seconds a few times in my clode and botched my example.

5

@savejeff is there a reason sprintf doesn’t work on platformio and you need this method?

6

i found this method to be the one that works on all platforms meaning SAMD, STM, ESP, Pico. with the correct compiler flags it even works on AVR.
i like to get a String from my sprint func instead of creating a char buffer. its just easier and the code is cleaner. you can do
Serial.print(s_printf(...)); in on line even if the Serial does not offer sprintf itself.

7

I think you are refering to Arm mbed: Serial.printf() fails to emit floats when using platformio - #5 by valeros. These flags absolutely work. They are not STM32 specific, they are specific to arm-none-eabi-gcc’s C library, so it applies to all ARM targets.

If I do

[env:adafruit_feather_m0]
platform = atmelsam
board = adafruit_feather_m0
framework = arduino
build_flags = -Wl,-u_printf_float
monitor_speed = 115200
monitor_dtr = 1

with code

#include <Arduino.h>

void setup() { Serial.begin(115200); }
void loop() {
   char charBuf[20] = {};
   float seconds = 1.3f;
   sprintf(charBuf, "Ping: %.2f (s)", seconds);
   Serial.println(charBuf);
   delay(1000);
}

I get

--- Miniterm on COM6  115200,8,N,1 ---
--- Quit: Ctrl+C | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
Ping: 1.30 (s)
Ping: 1.30 (s)

So, no problems & no additional function needed. Just enable it in the C library.

If that still doesn’t work for you, post your complete platformio.ini.

8

Thanks @maxgerhardt I must have not done the build_flags correctly. I think I had a different one or two that I tried, but that worked. That was very frustrating. Very glad to have that solved.

1 Like