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.

• ATE-ENGE: I was under the impression that cmd[9]={SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo} would make the cmd array equal to {SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo,[unallocated]} so that when I said cmd[8]=0; Cmd would be {SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo,0} Is that not the case?
• ATE-ENGE: To clarify, I also removed the cmd[8]=0 line as well (sorry for the confusion)
• James Waldby - jwpat7: Perhaps you meant [uninitializ​ed] where you wrote [unallocat‌​ed]. But no matter, both are wrong. cmd[9]={SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo} initializes cmd to {SvAdd,FuCode,AddHi,AddLo,DatHi,DatLo,CrcHi,CrcLo,0} because it uses the default initializer for unspecified elements, and the default initializer is 0. See eg an answer to Is there a better way to initialize an array? and also see C++ array initialization,

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
}

• ATE-ENGE: Ok, I'll do that
• Macit: @ATE-ENGE you should have not edited your question. New readers are going to get confused :)
• ATE-ENGE: Ok, I'll undo that XD