Adding null character to char array

Question

I have this code written and functioning that will write out the chars in cmd to a serial port for an input of ASCII text representing byte values.

void SendCTRL (String &input) 
{
  //Adjust Data
  const char *hin = input.c_str();    // Get character array
  int clen = input.length()/2;
  unsigned char cmd[clen+1]; // Leave a byte for null terminator
  for (int i=0; i < 2*clen; i+=2)
  {
    cmd[i/2] = dehex(hin[i])<<4 | dehex(hin[i+1]);
  }

  //Send Data
  cmd[clen] = 0;                     // Null-byte terminator
  digitalWrite(SSerialTxControl, RS485Transmit);  // Enable RS485 Transmit   
  for (int i=0; i< clen; i++)
  {
    RS485Serial.write(cmd[i]);   // Send string someplace
  }
  digitalWrite(SSerialTxControl, RS485Receive);  // Disable RS485 Transmit    
}

As the majority of the byte values fed are constant, I'm trying to create similar output for the following code when I feed the function below only the byte values that change frequently:

void Create_Message(int Address, int Data)
{
  //Adjust Data
  Address--;
  byte SvAdd = 1;
  byte FuCode = 6;
  byte AddHi = Address>>8;
  byte AddLo = Address;
  byte DatHi = Data>>8;
  byte DatLo = Data;
  byte AdDa[] = {SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo};
  unsigned int crc=CRC(AdDa,sizeof(AdDa)/sizeof(AdDa[0]));
  byte CrcHi = crc & 0xFF;
  byte CrcLo = crc >> 8;
  /*U8 Slave Address, U8, Function Code, U16 Address, U16 Data, U16 CRC
  * to
  *[U8 Address Split High, U8 Address Split Low, U8 Data Split High, U8 Data Split Low, U8 Function Code]
  */


  //Send Data
  unsigned char cmd[9]={SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo};
  cmd[8]=0; //Null-byte terminator
  digitalWrite(SSerialTxControl, RS485Transmit);  // Enable RS485 Transmit
  lcd.setCursor(0, 1);   
  for (int i=0; i< sizeof(cmd)/sizeof(cmd[0]); i++)
  {
    RS485Serial.write(cmd[i]);   // Send string someplace
  }
  digitalWrite(SSerialTxControl, RS485Receive);  // Disable RS485 Transmit    
}

I'm reading the Values written by RS485.write() and, in both cases, the data is good. However, for the 2nd section of code, the Arduino seems to write out the null-byte while it does not write the byte out in the 1st block. If I remove the allocation for the null-byte in the second code, it does not get a response from my device.

I'm guessing that the issue is that I am not setting my null-byte terminator correctly or otherwise missing something (I'm pretty new here XD )

Update: it seems like using sizeof(cmd)/sizeof(cmd[0]) to determine the send length interfered with data transmission when I set this to a constant the code works just fine.

Answer 1

In the second code example, the value of sizeof(cmd)/sizeof(cmd[0]) is 9, due to the declaration unsigned char cmd[9] ...

Thus, the for loop will write out 9 bytes, with indices 0, 1, ... 8. Due to cmd[8]=0; earlier in the code, the last byte will be zero. To prevent that, one might instead use the expression sizeof(cmd)/sizeof(cmd[0])-1 in the for loop:

for (int i=0; i< sizeof(cmd)/sizeof(cmd[0])-1; i++)
   RS485Serial.write(cmd[i]);

Note, this doesn't explain why it should be so that “If I remove the allocation for the null-byte in the second code, it does not get a response from my device”, although practically anything can happen if you say cmd[8]=0; after allocating only 8 bytes of storage.

Answer 2

In addition to James's answer, to print (or send) a null terminated string you do not have to know the length of the string. Just print the characters till you find the null. So this code works better.

for (int i=0; cmd[i] != 0; i++)
{
    RS485Serial.write(cmd[i]);   // Send string someplace
}

//or

int i=0;
while(cmd[i] != 0)
{
    RS485Serial.write(cmd[i++]);   // Send string someplace
}