why does const_cast from and to a const char * result in strcmp != 0

Question

I'm coding Bluetooth on an ESP32 using Arduino IDE. Having a little trouble with const_cast const char *. Reading the paired device value has a '\n' line feed that I want to remove. So I cast it an editable char * then convert it back to a const char *, but somehow it isn't the same value.

#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>

BLEServer           *pServer = NULL;
BLEService          *pService;
BLECharacteristic   *pTxCharacteristic;
BLECharacteristic   *pRxCharacteristic;

bool    BLE_Paired       = false;

#define SERVICE_UUID           "6E400001-B5A3-F393-E0A9-E50E24DCCA9E" // UART service UUID
#define CHARACTERISTIC_UUID_RX "6E400002-B5A3-F393-E0A9-E50E24DCCA9E"
#define CHARACTERISTIC_UUID_TX "6E400003-B5A3-F393-E0A9-E50E24DCCA9E"

class MyServerCallbacks: public BLEServerCallbacks {
    void onConnect( BLEServer* pServer ) {
      BLE_Paired = true;
    };
    void onDisconnect( BLEServer* pServer ) {
      BLE_Paired = false;
    }
};

class MyCallbacks: public BLECharacteristicCallbacks {
    void onWrite( BLECharacteristic *pCharacteristic ) {
      std::string  rxValue = pCharacteristic->getValue();
    }
};

char*         _name = "your nm";
const char*  C_name = "Rick";
bool        name_OK = false;

void setup() {
  Serial.begin( 115200 );

  BLEDevice::init( "bluetooth" );
  // Create the BLE Server
  pServer = BLEDevice::createServer();
  pServer->setCallbacks(new MyServerCallbacks());
  pService = pServer->createService(SERVICE_UUID);
  pTxCharacteristic = pService->createCharacteristic(
                                          CHARACTERISTIC_UUID_TX,
                                          BLECharacteristic::PROPERTY_NOTIFY );
  pTxCharacteristic->addDescriptor( new BLE2902() );
  pRxCharacteristic = pService->createCharacteristic(
                                            CHARACTERISTIC_UUID_RX,
                                            BLECharacteristic::PROPERTY_WRITE   );
  pRxCharacteristic->setCallbacks( new MyCallbacks() );
  pService->start();
  pServer->getAdvertising()->start();
}

void loop() {
  while( BLE_Paired ) {
     while( !name_OK ) {  
       //  Receive the response from the Paired Device
       pTxCharacteristic->setValue( "Name? " );
       pTxCharacteristic->notify();
       delay( 100 );          // bluetooth stack can get congested

       //  read it as const char *, but cast it as just a char *
       _name = const_cast<char *>(pRxCharacteristic->getValue().c_str());

       //  the Paired Device adds a '\n' when the name is entered; remove it
       for( int j=0; _name[j] != '\0'; j++ ) {
         if( _name[j] == '\n' ) {
           _name[j] = NULL;   //  NULL or '\0'
         }
       }
       //  cast name back to a const char *
       const char* C_nm = const_cast<const char *>(_name);
       Serial.print( "const _names: " );
       Serial.print( C_name );
       Serial.print( " =? " );
       Serial.print( C_nm );
       Serial.print( " : " );
       Serial.println( strcmp( C_name, C_nm ));  // result is -13, not 0
       if( strcmp( C_name, C_nm ) == 0 )
         name_OK = true;
    }
  }
}

So, I pair my BLE device, enter Rick, and the code replaces the '\n' with NULL, but the value isn't the same a const char * variable set to Rick. It's -13 instead of 0. Why? Many thanks.

Answer 1

I also removed '/r' with the '/n' and that shortened the string to match. Thanks to datafiddler.