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?

• Nick Gammon: Try doing a serial print of password to see if the toCharArray call did what it was supposed to do.
• the busybee: OT: Don't write any_boolean == false, use !any_boolean. If you think think, you should make sure that anyone understands, then I'd recommend (any_boolean == false) == true. :-D
• the busybee: OT: sizeof is an operator, not a function, so use sizeof variable. However, its operand can be a type, and then it needs parentheses for syntactIcal reasons. This leads to this ubiquitous bad habit to write it as a function.
• Edgar Bonet: Are you sure you are putting the actual array password, rather than a pointer produced by its decay (like char *foo = password; ExtEEPROM.put(..., foo);)?
• Majenko: 256kbit is not 256000 bits. It's 256 * 1024 bits, or 262144.

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

• Majenko: By atomic I mean "it is a thing complete in itself". A struct is complete. An int is complete. A char * is not complete since it doesn't contain the data, it is only a pointer to it. So it's not "atomic". Same goes for a String object, since the object doesn't contain the string data, only a pointer to allocated memory that contains the string data.

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.

• Edgar Bonet: In the OP's code, get() and put() are being passed actual arrays, not pointers. As the arrays are being passed by reference, they do not decay to pointers.

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.

• Juraj: the conversation has been moved to chat.