Arduino IDE ESP32 device using SPIFFS to save

Question

I writing ESP32 program and using SPIFFS to save some data because I do not want to loose it after I power down the device.

I have 2 functions:

char* readFile(fs::FS &fs, const char *path)
{
    Serial.printf("Reading file: %s\n", path);

    File file = fs.open(path);
    if (!file || file.isDirectory())
    {
        Serial.println("Failed to open file for reading");
        return "FAIL";
    }

    Serial.print("Read from file: ");
    while (file.available())
    {
        Serial.write(file.read());
        delayMicroseconds(100);
        //TODO
        //append all characters and return it as a list of char arrays at the end of reading
    }
    file.close();
    
}


void writeFile(fs::FS &fs, const char *path, const char *message)
{
    Serial.printf("Writing file: %s\n", path);

    File file = fs.open(path, FILE_WRITE);
    if (!file)
    {
        Serial.println("Failed to open file for writing");
        return;
    }
    if (file.print(message))
    {
        Serial.println("File written");
    }
    else
    {
        Serial.println("Write failed");
    }

    file.close();
}

First of all, I run my program and call writeFile() and then followed by readFile():

    int n = 0;
    Wifi_setup();
    Spiffs_setup();
    n = scan_wifi_networks();

    
    Serial.print("list_strings outside scan function = ");
    for (int i = 0 ; i<=n ; i++){
      Serial.println(found_networks[i]);
    }
    //compare list_strings with preset wifi networks. If match, everything is fine, if not, problem!


    writeFile(SPIFFS, "/wifi.txt", "Telia-33F8A3-Greitas");
    char* known_networks = readFile(SPIFFS, "/wifi.txt");

 
    //send USD to the server, go back to sleep
    //initialize_deep_sleep();
}

And that part works fine. From the serial monitor, I can see that it have sucesfully written my text to wifi.txt.

After I run write/read functions

Next, I comment out write function and only leave read function.I run the code again and it is not able to read back my text:

Only readFile

Can someone help me understand why is that happening? I thought that if I write to spiffs once, I will be able to access it afterwards but that is not the case. I have previously used EEPROM and that seemed to work. I can write to EEPROM address and simply access the same address later and the value will still be there after the power off. Any help is appreciated. Thanks in advance.

UPDATE1

I have managed to read back data from my SPIFFS after I wrote to it. I had missed one crucial step: https://randomnerdtutorials.com/install-esp32-filesystem-uploader-arduino-ide/

However, now, I am encountering another issue:

In my project folder, I have created a folder "data" which is supposed to be accessed by SPIFFS. In there, I create 2 files :

1. wifi.txt
2. update.bin

update.bin for now is not relevant. Lets talk about wifi.txt

After I have written to Spiffs, I now comment out the writeFile function and only leave out readFile. The result from my serial monitor:

load:0x3fff001c,len:1216
ho 0 tail 12 room 4
load:0x40078000,len:10944
load:0x40080400,len:6388
entry 0x400806b4
Listing directory: /
  FILE: /update.bin  SIZE: 0
  FILE: /wifi.txt  SIZE: 20
Reading file: /wifi.txt
Read from file: Telia-33F8A3-Greitas

As you can see from the serial monitor above, my program recognises the text that I have written to my spifs /wifi.txt file. Howeverm when I go to my actual project directory and open data/wifi.txt it is emtpy:

How can this happen? My program recongises that theres data inside but it wont show up in the file.

UPDATE2

I have done some further testing of SPIFFS.

In my data folder, I have created another txt file test.txt. In there, I have manually put some text "this is text message".

In my program, I have called function:

char* returned_data = readFile(SPIFFS, "/test.txt");

And the serial monitor sucesfully printed the message that I have put. So that proves that the SPIFFS is able to read back from the files without any issues.

Then, I have modified my program:

    writeFile(SPIFFS, "/test.txt", "hello123");
    char* returned_data = readFile(SPIFFS, "/test.txt");

The code above should overwrite whatever I have written to my txt file with "hello123" and then my program should read the "hello123" back from Spiffs. The serial monitor response:

Listing directory: /
  FILE: /update.bin  SIZE: 0
  FILE: /wifi.txt  SIZE: 2
  FILE: /test.txt  SIZE: 8
Writing file: /test.txt
File written
Reading file: /wifi.txt
Read from file: 
Reading file: /test.txt
Read from file: hello123

As you can see, it is able to sucesfully read back "hello123".

HOWEVER, when I go to my project folder, open the data/test.txt, I can still see my initial text not replaced with "hello123".

I do not understand how is this happening..