I'm attempting to send data from a serial connection to a GSM modem, but having problems. The string from rx looks like this:

1.0 2.0 3.0 4.0

Here's my code to replace the spaces and then I just want to pass the whole string on, regardless of what's inside (there could be negative values too).

                data_current[data_index++] = ',';
                char received = bike_port.read(); //bike
                char test[32];
                inData += received;     
                  if (received == '\n')
                      String nospace = inData;
                      debug_println("Data: ");
                      inData = ""; // Clear recieved buffer
                  sprintf(test,32,"%d,%d,%d,%d",(String) nospace);
                //strtok(received, tmp);
                for (int i=0; i<strlen(test); i++) {
                  data_current[data_index++] = test[i];

I have also tried

sprintf(test,32,"%ld,(String) nospace);

What I get now is this error

cannot pass objects of non-trivially-copyable type 'class String' through '...'

Without the sprintf line I can successfully echo the string to debug_println (Serial Monitor). What am I doing wrong?

  • Your sprintf() line doesn't seem to make sense. What are you trying to get it to do? Commented Jul 16, 2015 at 14:10
  • I basically just need 'nospace' to be added to data_current. Maybe I should try for (int i=0; i<strlen(nospace); i++) { data_current[data_index++] = nospace[i];
    – Fid
    Commented Jul 16, 2015 at 14:44

3 Answers 3


The basic problem here is that you're trying to pass a String object to sprintf(). That isn't possible because String is a C++ class, and sprintf() is a C variadic function. The only way to pass a string to it is as a char *.

The rest of your call to sprintf() doesn't seem to make sense either, although I suspect it's not what you actually want here. You'd normally use sprintf() if you want to format several values into a string.

As you've suggested in the comments, adding the contents of nospace to data_current character-at-a-time would be the sensible approach. You could do it something like this:

for (int i = 0; i < nospace.length(); ++i) {
    data_current[data_index++] = nospace[i];

Note that strlen() won't work on nospace because it's a String object not a char array.

This is all assuming that there's enough space in data_current though. Without seeing the rest of your code, I can't tell you if it will definitely work or not.

As an alternative, you could use memcpy() to do a block transfer:

memcpy(data_current + data_index, nospace.c_str(), nospace.length());
data_index += nospace.length();

I'm not sure there's much benefit to doing that though. It's hypothetically more efficient, but it probably won't make a noticeable difference unless your string is moderately long and it's being called very frequently. (It's trading-off the loop overhead against a function-call overhead.)

  • There should be enough space, if not, I know where to change it. But is there a way to add the whole content of nospace, not a character at a time? Because it then gets sent through GSM. I don't know what the exact character length will be, but I do know the maximum.
    – Fid
    Commented Jul 16, 2015 at 15:06
  • @Fid You could use memcpy() in theory, although I don't know how much difference it will really make. I've updated my answer with details anyway. Commented Jul 16, 2015 at 15:59
  • "But is there a way to add the whole content of nospace, not a character at a time?" Yes and no: Yes, you can express it that way in C++; adding strings translates into concatenation. No, because for most processors, and for AVR's in particular, it ultimately involves a character by character copy. In other words, write it however it reads best to you. It won't make much difference at run time.
    – JRobert
    Commented Jul 22, 2015 at 14:39

As a companion to Nick Gammon's excellent answer, here is a streaming version of that (assuming this is coming in one serial port and out another to the modem).

if (Serial.available()) {
  int inByte = Serial.read();
  if (inByte == ' ')

Let me get this straight. You want to change:

1.0 2.0 3.0 4.0




How about:

  char indata[32] = "1.0 2.0 3.0 4.0";

  int len = strlen (indata);
  for (int i = 0; i < len; i++)
    if (isspace (indata [i]))
      indata [i] = ',';

  Serial.println (indata);   // prints: 1.0,2.0,3.0,4.0

Only problem is that when I change "1.0 2.0 3.0 4.0" to the actual incoming read (bike_port.read()), than I get error ...

In that case, as Jake C said, modify the incoming byte like this:

 char received = bike_port.read(); //bike
 if (isspace (received))
   received = ',';
  • This works wonderfully. Only problem is that when I change "1.0 2.0 3.0 4.0" to the actual incoming read (bike_port.read()), than I get error "array must be initialized with a brace-enclosed initializer".
    – Fid
    Commented Jul 21, 2015 at 16:00

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