Casting from string to unsigned char* leaves garbage

Question

This is C++ newbie question. I have an Arduino sketch with following code to transmit bluetooth low energy UART notification.

When command is 5 characters I get the 5 characters at the bluetooth receiver side. However, when I follow a 5 character command with a single character command, what is received is the single character followed by last 3 characters from the previous command .

Example,

command is -1,0t . What I receive is -1,0t . But next command is just r . What I receive is r,0t .

What's happening here? How do I get just "r"?

int BLEsend(String command){
    byte length = sizeof(command);    
    unsigned char* notification = (unsigned char*) command.c_str(); // cast from string to unsigned char*
    Serial.print(length);
    BLE.sendData(UART_SEND, notification, length);
    Serial.print("Notification Sent: "); Serial.println(command);
    delay(100);
    return 1;
}

Answer 1

sizeof( ) doesn't give you the length of the string.

You can use int length = command.length( ); instead.

Also command.size( ); should work as well.

Answer 2

you need also to copy the string command.c_str() to notification :

notification = new char[length + 1];
strcpy(notification , command.c_str());