Losing a character when converting from String toCharArray

Projects Programming
1

I am using this routine to send a String via UDP:

void sendWiegandIDtoServer(String id) {
  if (checkWiegandIDforErrors(id))
    return;
  logOutput("Access Code '" + id + "' has been entered.");
  ws.textAll("access_code");

  String key = "multicontroller, key=" + id;
  uint8_t bufferLengh = key.length();
  uint8_t buffer[bufferLengh];
  key.toCharArray((char *)buffer, bufferLengh);

  Serial.print("To be sent to server: ");
  Serial.println(key);

  if (!wiegand_udp_on || wiegand_state.ip_wiegand == "not set" ||
      wiegand_state.ip_wiegand.length() == 0) {
    logOutput("ERROR ! Invalid IP Address for Wiegand. Please enter a valid IP.");
    return;
  }

  // send packet to server
  wiegand_udp.beginPacket(wiegand_state.ip_wiegand.c_str(),
                          wiegand_state.port_wiegand.toInt());
  wiegand_udp.write(buffer, sizeof(buffer));
  delay(30);
  logOutput(wiegand_udp.endPacket()
                ? (String) "Wiegand ID: " + id + " was sent over UDP."
                : "WARNING: Wiegand ID not sent.");
  memset(buffer, 0, bufferLengh);
}

// for context:
void logOutput(String string1)
{
    circle.push(string1);
    Serial.println(string1);
}

I have String key, which I place in a buffer uint8_t buffer[bufferLengh]; using key.toCharArray((char *)buffer, bufferLengh); and then I send it with wiegand_udp.write(buffer, sizeof(buffer));.

The issue is that even if logOutput(wiegand_udp.endPacket() ? (String) "Wiegand ID: " + id + " was sent over UDP." : "WARNING: Wiegand ID not sent."); logs a correct id, which is a 6 digit code, wiegand_udp.write(buffer, sizeof(buffer)); only sends "multicontroller, key=" + id with 5 digit instead of 6.

Where could I be losing a digit ?

2

Why is the key a String rather than a string ?

3

You need to add one extra byte than the length to store the trailing null char

There is no need to duplicate - just use the underlying cString

Your function should pass Strings by reference - as it stands you duplicate Strings all over the place and increase the RAM pressure needlessly

4

So, basically uint8_t bufferLength = key.length() + 1; // Add 1 for null terminator ?

5

yes, if you want to keep abusing RAM :slight_smile:

when you do

  key.toCharArray((char *)buffer, bufferLengh);

the function copies bufferLengh-1 bytes from the key into the buffer and then add a trailing null. So that's why you lose one character.

but my recommendation would be to use the c_str() method directly on the id and not use the String class so much

instead of

void sendWiegandIDtoServer(String id) {
...
  String key = "multicontroller, key=" + id;
  uint8_t bufferLengh = key.length();
  uint8_t buffer[bufferLengh];
  key.toCharArray((char *)buffer, bufferLengh);
...
  // send packet to server
  wiegand_udp.beginPacket(wiegand_state.ip_wiegand.c_str(),
                          wiegand_state.port_wiegand.toInt());
  wiegand_udp.write(buffer, sizeof(buffer));

just do

void sendWiegandIDtoServer(String& id) { // <== add & to pass the parameter by reference instead of copy
...
  // send packet to server
  const char * header = "multicontroller, key=";
  wiegand_udp.beginPacket(wiegand_state.ip_wiegand.c_str(), wiegand_state.port_wiegand.toInt());
  wiegand_udp.write(header, strlen(header) ); 
  wiegand_udp.write(id.c_str(), id.length()); // or id.length() + 1 if you want to send the trailing null char

and if you could use a const char * for your id instead of a String, that would be even better for the memory

6 
 wiegand_udp.write(header, strlen(header) ); 
 wiegand_udp.write(id.c_str(), id.length());

Both write () functions return : no instance of overloaded function "WiFiUDP::write" matches the argument listC/C++(304)

As write is:


size_t WiFiUDP::write(const uint8_t *buffer, size_t size){
  size_t i;
  for(i=0;i<size;i++)
    write(buffer[i]);
  return i;
}
7 
write(buffer[i]);

Should there be an object name in front of the write() function ?

8

cast to const uint8_t * (otherwise it's a const char * and it seems the developers of the library did not plan for that)

  wiegand_udp.write((const uint8_t * ) header, strlen(header) ); 
  wiegand_udp.write((const uint8_t * ) id.c_str(), id.length()); // or id.length() + 1 if you want to send the trailing null char

that was an extract from the class, so write() was referring to the local function in the class

9

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