Strange C++ Arduino serial behavior

Question

I am writing some code for my arduino to print 4 button states and the X,Y coordinates from an analogue joystick and I am having strange behavior.

Here is the code:

  int x,y;
  x = analogRead(joy_x);
  y = analogRead(joy_y);
  char* buf = new char[4];
  sprintf(buf, "%4d", x);
  Serial.print("X:");
  Serial.print(buf);
  Serial.print(" Y:");
  sprintf(buf, "%4d", y);
  Serial.println(buf);

  int one,two,three,four;
  one = digitalRead(touchOne);
  two = digitalRead(touchTwo);
  three = digitalRead(touchThree);
  four = digitalRead(touchFour);

  // create an array of states set to the default values
  char states[4] = {'X', 'X', 'X', 'X'};
  // change each value if it is not in the default state
  if(one   == 1) states[0] = 'O';
  if(two   == 1) states[1] = 'O';
  if(three == 1) states[2] = 'O';
  if(four  == 1) states[3] = 'O';
  // output the states to the OLED display
  //display.print(states);
  Serial.print(states);

When it is run the serial output looks like this:

XXXX3X: 515 Y: 520
XXXX3X: 516 Y: 520
XXXX3X: 516 Y: 519
XXXX3X: 516 Y: 520
XXXX3X: 515 Y: 520
XXXX3X: 516 Y: 519
XXXX3X: 516 Y: 520
XXXX3X: 515 Y: 519
XXXX3X: 516 Y: 520

Even though the X, Y should be before the XXXX and the number three appears from nowhere. I hope this mystery can be solved, thanks.

Answer 1

You have several buffer overruns in the code.

  char* buf = new char[4];
  sprintf(buf, "%4d", x);
  ...
  sprintf(buf, "%4d", y);

This needs room for the null terminator which sprintf adds.

  char* buf = new char[5];

Also easier: there's no need to use the free store here.

  char buf[5];

Same kind of thing here:

  char states[4] = {'X', 'X', 'X', 'X'};
  ...
  Serial.print(states);

We need to add a null terminator to make it a valid string.

  char states[5] = {'X', 'X', 'X', 'X', '\0'};

That should take care of the immediate memory issues.