why is my char array empty

Question

following sketch will run totally fine and as expected :

#include <Wire.h>

#include "SparkFun_External_EEPROM.h"
ExternalEEPROM ExtEEPROM;
char username[33] = "";
char password[65] = "";

void setup()
{
  Serial.begin(115200);
  delay(1000);
  Wire.begin();
  #define EEPROM_ADDRESS 0b1011000 // 0x58
  ExtEEPROM.setMemorySize(256000/8); // 256kbit = 32kbyte
  ExtEEPROM.setPageSize(64); // 64 byte page size.
  ExtEEPROM.enablePollForWriteComplete();
  ExtEEPROM.setPageWriteTime(10); // max. ms for AT24C128
  if (ExtEEPROM.begin(EEPROM_ADDRESS, Wire) == false) 
  {
    Serial.println("No memory detected. Freezing.");
    while (1);
  }
  Serial.println("Memory detected!");

  String uuu = "myusername";
  String ppp = "mypassword";

  uuu.toCharArray(username, sizeof(username) - 1);
  ppp.toCharArray(password, sizeof(password) - 1);
  Serial.print("password=");Serial.println(password);
  ExtEEPROM.put(0, username);
  //delay(100);
  ExtEEPROM.put(0 + sizeof(username), "greatnewpassword");
}
  
void loop() {
  char xxx[33] = "";
  ExtEEPROM.get(0, xxx);
  Serial.print("xxx=");Serial.println(xxx); // prints the username

  char yyy[33] = "";
  ExtEEPROM.get(0 + sizeof(username), yyy);
  Serial.print("yyy=");Serial.println(yyy); // prints the password
  delay(1000);
}

However, if I replace ExtEEPROM.put(0 + sizeof(username), "greatnewpassword"); to ExtEEPROM.put(0 + sizeof(username), password);, then yyy prints empty in the serial monitor. Why?

Answer 1

As Majenko already mentions:

.get() and .put() should only be used for atomic, self-contained, types like int or float, not pointers like char *.

Not sure what atomic means here, but you may .put and .get any data type, especially a struct.

struct EPROMData {
  char username[33];
  char password[65];
}; 
EPROMData data {"myusername","mypassword"};

void setup() {
   ...
   ExtEEPROM.put(0,data);
}
void loop() {
   EPROMData d;
   ExtEEPROM.get(0,d);
   Serial.print("user="); Serial.println(d.username); 
   ...
}

Sure, if the data type is a pointer or a String object (containing a pointer only), you won't be happy with .put / .get

Answer 2

.get and .put are being passed a char pointer, not the contents of a char array. Thus you're not writing the content of the string but the address in memory where it's being stored.

With a string literal that is fixed and never changes, so what you write and what you read are fine. With a variable though the address could be anything, and storing the address of some RAM where a string is stored is pointless.

.get() and .put() should only be used for atomic, self-contained, types like int or float, not pointers like char *.

Instead you need to be using the .read(address, buffer, size) and .write(address, buffer, size) functions to read and write the contents of your char arrays.

Answer 3

problem seems to be the lib I am using. I think it can not handle data overlapping page size.

if I replace from this

char username[33] = "";
char password[65] = "";

to this:

char username[16] = "";
char password[16] = "";

everything works fine as it saves and reads from the EEPROM via I2C.