Unexpected behavior of sprintf with i32/u32

Question

I'm trying to build a string of characters from several elements given by the TinyGPS++ library with the use of sprintf().

This is my code making the string and printing it in the serial monitor.

char posmsg[100], lat[14+1], lon[14+1];

void sendString(){
  //- convert lat/lon
  dtostrf(gps.location.lat(), 10,6, lat);
  dtostrf(gps.location.lng(), 10,6, lon); 

  //- sprintf the string
  sprintf(posmsg, "$%4d-%02d-%02d %02d:%02d:%02d,%s,%s,%i,%i,%i,%i,%i$",
          gps.date.year(),gps.date.month(),gps.date.day(),
          gps.time.hour(), gps.time.minute(), gps.time.second(),
          lat, lon,
          gps.speed.value(), gps.course.value(), gps.altitude.value(),
          gps.satellites.value(), gps.hdop.value()
        );

  Serial.println("--------------------");
  Serial.println(posmsg);
  Serial.println("--------------------");

  Serial.print("Speed:");
  Serial.println(gps.speed.value());
  Serial.print("Course:");Serial.println(gps.course.value());
  Serial.print("Height:");Serial.println(gps.altitude.value());
  Serial.print("Sats:");Serial.println(gps.satellites.value());
  Serial.print("HDOP:");Serial.println(gps.hdop.value());
  Serial.println("--------------------");
}

This is the result and what I expect... (spaces added for readability)

 Result:
 --------------------
 $2014-06-24 15:17:11,  51.xxxxxx,   5.xxxxxx, 2, 0, 4964, 0, 3660$
 --------------------
 Speed:2
 Course:4964
 Height:3660
 Sats:8
 HDOP:110
 --------------------

 Expected:
 --------------------
 $2014-06-24 15:17:11,  51.xxxxxx,   5.xxxxxx, 2, 4964, 3660, 8, 110$
 --------------------

So in the end of the string the numbers get displaced...

According to the documentation the type of the variables are all integers or unsigned integers, but the sprintf messes things up.

 gps.speed.value(); // Raw speed in 100ths of a knot (i32)
 gps.course.value(); // Raw course in 100ths of a degree (i32)
 gps.altitude.value(); // Raw altitude in centimeters (i32)
 gps.satellites.value(); // Number of satellites in use (u32)
 gps.hdop.value(); // Horizontal Dim. of Precision (100ths-i32)

What am I doing wrong?

Answer 1

Ints and unsigned ints are 16 bits on AVR ATmegas. So while the argument passed is a 32-bit value, sprintf() is expecting a 16 bit value. The stack is packed with twice as much data as needed.

From the "2,0,4964" bit, it looks like it taking part of the previous/next parameter during processing, which makes me think this even more.

You need to use long types, e.g.: %ld or %li and %lu