如何在C中的char数组末尾添加char(零)?

我是C语言的新手(我用的是delphi/pascal),尝试获取一些温度传感器值,并使其大小相等/固定,然后发送到MCU(使用arduino-ide),所以我必须使用C语言。

数据长度(strlen())可以是3(如5.3、0.9、0.0等)、4(如-4.2、19.8等)或5(如-15.6),以温度传感器为基础,代码以下;

    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..
    }

我需要修正数据的大小(我试图在末尾加零),IAM试图在下面做;

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;
}

但是,当我执行这个应用程序的时候,我把奇怪的字符作为Arduinoid(反问号、正方形等)的输出。 有什么问题?我怎样才能解决这个问题?或者你能建议更好的方法吗?现在谢谢……

注:这个问题的起源(问题)更多的是关于嵌入式系统,我已经问了另一个关于电子堆叠交换的问题,作为这个问题的参考(如果你想要/需要 you can read here )

编辑:我已经更正了编码 从

switch (num) {
case 3:
  temp_char[4] = "0";
  temp_char[5] = "0";
  temp_char[6] = "\0";
  //0.0 would become 0.000
  //5.4 would become 5.400
break;
case 4:
  temp_char[5] = "0";
  temp_char[6] = "\0";
  //15.2 would become 15.20
  //-7.4 would become -7.40
break;
  // -15.3 is in right format already
  }                              

到

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
  }