When compiling the code with the MFRC522 library, I encountered an error in MFRC522Extended.cpp. The error involves an ordered comparison of a pointer with an integer.

[vidura2]

OS version: Windows 11
Arduino IDE version: 2.3.3
MFRC522 Library version: 1.4.11
Arduino device: ESP32
MFRC522 device: RC522 RFID Module

Affected file(s) or example(s):

MFRC522Extended.cpp

Steps to reproduce:

Add the MFRC522 library to the Arduino project.
Compile a program using the ESP32 and RC522 RFID Module.
Encounter the compilation error.

ERROR:
e:\Arduino IDE\libraries\MFRC522\src\MFRC522Extended.cpp: In member function 'MFRC522::StatusCode MFRC522Extended::TCL_Transceive(TagInfo*, byte*, byte, byte*, byte*)':
e:\Arduino IDE\libraries\MFRC522\src\MFRC522Extended.cpp:824:34: error: ordered comparison of pointer with integer zero ('byte*' {aka 'unsigned char*'} and 'int')
824 | if (backData && (backLen > 0)) {
| ~~~~~~~~^~~
e:\Arduino IDE\libraries\MFRC522\src\MFRC522Extended.cpp:847:42: error: ordered comparison of pointer with integer zero ('byte*' {aka 'unsigned char*'} and 'int')
847 | if (backData && (backLen > 0)) {
| ~~~~~~~~^~~

The code :

#include <WiFi.h>
#include <SPI.h>
#include <MFRC522.h>
#include <time.h>
#include <AESLib.h> // Library for AES encryption

// WiFi credentials (if needed, currently commented out)
// const char* ssid = "ESP32_Access_Point";
// const char* password = "123456789";

// RFID setup
#define SS_PIN 5 // CS/SDA pin for RFID
#define RST_PIN 22 // RST pin for RFID
#define RELAY_PIN 25 // Relay control pin

MFRC522 rfid(SS_PIN, RST_PIN);

// Authorized RFID UIDs (replace these with your own values)
byte managerKeyUID[4] = {0x3A, 0xC9, 0x6A, 0xCB};
byte secretaryKeyUID[4] = {0x30, 0x01, 0x8B, 0x15};

// Rolling code setup
byte rollingCode[4] = {0x00, 0x00, 0x00, 0x01}; // Initial rolling code
byte validRollingCode[16]; // Encrypted rolling code

// AES encryption key and initialization vector (IV)
byte aesKey[16] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F};
byte aesIV[16] = {0x00};

// WebServer for access log
#include <WebServer.h>
WebServer server(80);

// Log for RFID access attempts
String loggedEntries = "";
String systemStatus = "";

// Time synchronization
void setupTime() {
configTime(0, 0, "pool.ntp.org");
setenv("TZ", "UTC-5", 1); // Adjust timezone to your location
tzset();

struct tm timeinfo;
int retryCount = 0;
while (!getLocalTime(&timeinfo) && retryCount < 5) {
    Serial.println("Failed to sync time, retrying...");
    delay(1000);
    retryCount++;
}

if (retryCount == 5) {
    systemStatus = "Failed to sync time, using default time.";
} else {
    systemStatus = "Time synchronized.";
}

}

// Get the current time as a string
String getTime() {
struct tm timeinfo;
if (!getLocalTime(&timeinfo)) {
return "Failed to obtain time";
}
char timeStr[50];
strftime(timeStr, sizeof(timeStr), "%Y-%m-%d %H:%M:%S", &timeinfo);
return String(timeStr);
}

// Serve login page
void handleLoginPage() {
String html = "<style>";
html += "body { font-family: Arial; background-color: #f0f0f0; margin: 0; padding: 20px; }";
html += "h1 { color: #333; }";
html += "table { width: 100%; border-collapse: collapse; margin-top: 20px; }";
html += "th, td { border: 1px solid #ddd; padding: 8px; text-align: left; }";
html += "th { background-color: #4CAF50; color: white; }";
html += "tr:nth-child(even) { background-color: #f2f2f2; }";
html += "</style>";
html += "

RFID Door Lock System

";
html += "

System Status: " + systemStatus + "

";
html += "";
html += "";
html += loggedEntries;
html += "

UID	Date	Time

";

server.send(200, "text/html", html);

}

// Log RFID entry with UID and timestamp
void logEntry(byte uid[]) {
String uidStr = "";
for (int i = 0; i < 4; i++) {
uidStr += String(uid[i], HEX);
if (i < 3) uidStr += ":";
}

String dateTime = getTime();
String date = dateTime.substring(0, 10); // Extract date (YYYY-MM-DD)
String time = dateTime.substring(11);    // Extract time (HH:MM:SS)

String log = "<tr><td>" + uidStr + "</td><td>" + date + "</td><td>" + time + "</td></tr>";
loggedEntries += log;

}

// Check if RFID UID is authorized
bool isAuthorizedUID(byte uid[], byte authorizedUID[]) {
for (int i = 0; i < 4; i++) {
if (uid[i] != authorizedUID[i]) {
return false;
}
}
return true;
}

// Encrypt rolling code using AES
void encryptRollingCode(byte code[], byte encryptedCode[]) {
AESLib aes;
aes.encrypt(code, 16, encryptedCode, aesKey, 128, aesIV);
}

// Decrypt rolling code using AES
void decryptRollingCode(byte encryptedCode[], byte decryptedCode[]) {
AESLib aes;
aes.decrypt(encryptedCode, 16, decryptedCode, aesKey, 128, aesIV);
}

// Generate new rolling code
void generateNewRollingCode() {
for (int i = 0; i < 4; i++) {
rollingCode[i] = (rollingCode[i] + random(1, 255)) % 255;
}
encryptRollingCode(rollingCode, validRollingCode);
}

// Check if rolling code is valid
bool isValidRollingCode(byte currentCode[]) {
byte decryptedCode[16];
decryptRollingCode(validRollingCode, decryptedCode);

for (int i = 0; i < 4; i++) {
    if (currentCode[i] != decryptedCode[i]) {
        return false;
    }
}
return true;

}

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

// Start WiFi access point (optional)
// WiFi.softAP(ssid, password);
// systemStatus = "Access Point started. IP Address: " + WiFi.softAPIP().toString();

// Initialize RFID
SPI.begin();
rfid.PCD_Init();
pinMode(RELAY_PIN, OUTPUT);
digitalWrite(RELAY_PIN, HIGH); // Lock the door by default

// Start web server for log display
server.on("/", handleLoginPage);
server.begin();

// Setup time synchronization
setupTime();

// Initialize rolling code encryption
encryptRollingCode(rollingCode, validRollingCode);

}

void loop() {
server.handleClient(); // Handle web server requests

// Check for RFID card/tag
if (rfid.PICC_IsNewCardPresent() && rfid.PICC_ReadCardSerial()) {
    if (isAuthorizedUID(rfid.uid.uidByte, managerKeyUID) || isAuthorizedUID(rfid.uid.uidByte, secretaryKeyUID)) {
        
        // Validate the rolling code
        if (isValidRollingCode(rfid.uid.uidByte)) {
            logEntry(rfid.uid.uidByte); // Log the access
            digitalWrite(RELAY_PIN, LOW); // Unlock door
            delay(2000); // Keep door unlocked for 2 seconds
            digitalWrite(RELAY_PIN, HIGH); // Lock door

            // Generate new rolling code
            generateNewRollingCode();
            systemStatus = "Access granted and rolling code updated.";
        } else {
            systemStatus = "Invalid rolling code!";
        }
    } else {
        systemStatus = "Unauthorized access attempt.";
    }

    // Halt RFID reader
    rfid.PICC_HaltA();
    rfid.PCD_StopCrypto1();
}

}

[gcandrade10]

Same error here

[vidura2]

added a pull request

[syn-arch]

same

[frivera26]

El problema se resuelve de la siguiente manera:

if (backData && (*backLen > 0)) {

y ya puedo usar el RFID-RC522

[Jsuryaboi-08]

guys I have found the solution for this problem.

Open MFRC522Extended.cpp in a text editor. It should be located in your Arduino libraries folder:

c:\Users[username]\OneDrive\Documents\Arduino\libraries\MFRC522\src\MFRC522Extended.cpp

Locate lines 824 and 847 with the following code:

• if (backData && (backLen > 0)) {

Modify both lines to remove the invalid pointer comparison by replacing them with:

• if (backData != nullptr && backLen != nullptr && *backLen > 0) {

The original code incorrectly compares a pointer (backLen) with an integer (0). This modification uses != nullptr to check if backData and backLen pointers are valid and then dereferences backLen to compare its value with 0, fixing the error.