Trouble setting value of const char* variable

Troubleshooting
1

Here’s stripped down code of what I’m working with.

const char* on = "false";
float setTemp = 0.00;

void setup(void) {
  Particle.function("setData", setData);
}

void loop(void) {
  char tmpData[64];
  
  sprintf(tmpData, "{ \"setTemp\": %2.2f, \"on\": %s }", setTemp, on);
  Particle.publish("getData", tmpData);
  
  delay(2000);
}

int setData(String command) {
  int loc1 = 0;
  loc1 = command.indexOf(",");
  
  setTemp = command.substring(0,loc1).toFloat();
  on = command.substring(loc1+1).c_str();

  return 1;
}

When I call setData() from the CLI with:

$ particle call PHOTONIDHERE setData "67.25,true"

And then listen to my getData event with:

$ particle subscribe getData PHOTONIDHERE

The value of ‘on’ gets set to ‘P_’ instead of true. But I’ve also seen it set to ‘U’ and ‘t_’:

{"name":"getData","data":"{ \"setTemp\": 67.25, \"on\": P_ }","ttl":"60","published_at":"2015-10-26T14:00:38.795Z","coreid":"..."}

Admittedly, I’m new to C++… Any help is much appreciated!

2

Hi @danof

The const keyword means constant, as in not changeable, and on Particle devices the storage for const arrays/strings is allocated in Flash memory that is not writeable except to load a new program.

You need to:

  • Remove the const keyword from the declaration of on like this char on[] = "false";
  • Use strcpy(on,command.substring(loc1+1).c_str()); to copy the contents of the command string into your global on char array.
2 Likes
4

I agree with bko, but with the following difference: declare the on array with a fixed, specific size and then use strncpy() to avoid a buffer overflow.

2 Likes
5

Hi @Muskie

You are right that I was assuming the only legal values would be “true” and “false” and since “false” is longer than true, it should be safe, but it is best to be explicit.

6

Thanks bko and Muskie. That worked great!

1 Like