How to add a char (zero) at the end of char array in C?

Question

I am newbie in C (I'm using Delphi/pascal instead) and trying to get some temperature sensor values and make them equal/fixed size to send to the MCU (with Arduino IDE- so I have to work with C).

Length of data (strlen()) can be 3(like 5.3, 0.9, 0.0 etc), 4(like -4.2, 19.8 etc) or 5(like -15.6) based on temp sensor and below code;

    char value[5]; // must be char in order to send to MCU
    if ((temp_data>=temp_max){
      fix_size(value,true); //Error part. writes: "EEEEE" to reach fix size: 5
    } else {
      dtostrf(temp_data, 0, 1, value);
     fix_size(value,false); //I'll use this to send data to screen later..
    }

I need to fixed the size of data (to do that I am trying to add zeros at the end) and Iam trying to do with below;

char fix_size(char temp_char[5],bool err=false){
  if(err){
    temp_char= "EEEEE";
    Serial.println(temp_char);
    return temp_char;
  }
  int num = strlen(temp_char);
  // If strlen is 5 then it is OK and strlen cannot be 2 and 1 because of my temp sensor data processing (dtostrf(temp_data, 0, 1, value)) so I only need to take care 3 and 4
  switch (num) {
    case 3:
      temp_char[3] = "0";
      temp_char[4] = "\0";
      //0.0 would become 0.000
      //5.4 would become 5.400
    break;
    case 4:
      temp_char[4] = "\0";
      //15.2 would become 15.20
      //-7.4 would become -7.40
    break;
      // -15.3 is in right format already
  }                              | E.g. I tried for 15.4 and get
  Serial.println(temp_char[0]);  | 1
  Serial.println(temp_char[1]);  | 5
  Serial.println(temp_char[2]);  | .
  Serial.println(temp_char[3]);  | 4
  Serial.println(temp_char[4]);  | ؟
  return temp_char;
}

But, when I execute the app I am taking strange characters as output in ArduinoIDE (reversed question marks, squares etc..). What can be the problem?How can I solve this problem? or can you suggest better way? thanks right now..

NOTE: Origin of this question (problem) is more about Embedded Systems and I've asked another question on Electronics StackExchage as a reference to this question (If you want/need you can read here )

Answer 1

At least three problems. First, temp_char[] is declared at size 5, but you're trying to put 6 characters into it with "EEEEE" (which has a trailing zero), and by using temp_char[5] (only values 0..4 are legal).

Second, the assignment temp_char = "EEEEE" just changes the pointer, it doesn't actually copy anything into temp_char . You need strcpy() or something similar for that.

Third, you're confusing types:

temp_char[4] = "0";

temp_char[4] is of type char . "0" is of type char * , that is, it's a pointer to a character, not a character, so you're just getting the lower 8 bits of some random memory address. You probably mean:

temp_char[4] = '0';

because '0' is of type int , representing an ASCII value, that will be properly truncated to 8 bits on assignment.