cannot convert 'String' to 'const char*

Projects Programming
1

hi im kinda new to arduino but i was task todo this project where i have to interface arduino una + sd card +rfid + rtc . now before doing big code im taking it step by step coding each module individually but coming across interfacing 2 module im a bit having a problem with my code on the rfid and sd card . i give myself challenges weekly but im stuck with this almost 3 weeks no. my challenge was just simply putting 1 rfid id in a text file where when i tap my rfid it would just print access granted or denied. here is my code. to be honest im starting on doing random things just to make it work now . can you guys help me . i need to store my rfid scan digits to a variable then try looking for that var in my given text file.

sdsdsdcardfinalTRY.ino (2.06 KB)

2

What problem you would like help with?

If you are receiving the error "cannot convert 'String' to 'const char*" then you are trying to pass a String data type into a function that is expecting a character array. You can get two birds with one stone here though. Don't use String, anywhere anyway. When you change from String to character array (char[]) the error should go away (plus other problems you have yet to run into will never arise).

1 Like
3

OPs code.

#include <SD.h>
#include <MFRC522.h>
File myFile;
char buf[10];
#define SS_PIN 3 //rfid pin
#define RST_PIN 2 //rfid pin
MFRC522 mfrc522(SS_PIN, RST_PIN);

void setup()
{
    // Open serial communications and wait for port to open:
    Serial.begin(9600);
    
    while (!Serial) 
    {
        ; // wait for serial port to connect. Needed for Leonardo only
    }

    Serial.print("Initializing SD card...");
    pinMode(4, OUTPUT);//sd pin
    
    if (!SD.begin(10)) 
    {
        Serial.println("initialization failed!");
        return;
    }

    Serial.println("initialization done.");  
    // re-open the file for reading:
    SPI.begin();      // Initiate  SPI bus
    mfrc522.PCD_Init();   // Initiate MFRC522
    Serial.println("Approximate your card to the reader...");
    Serial.println();
    myFile = SD.open("test.txt");
    
    if (myFile) 
    {
        Serial.println("test.txt:");
        // read from the file until there's nothing else in it:
        while (myFile.available()) 
        {
            // Look for new cards
            if ( ! mfrc522.PICC_IsNewCardPresent()) 
            {
                return;
            }
            // Select one of the cards
            if ( ! mfrc522.PICC_ReadCardSerial()) 
            {
                return;
            }
            Serial.print("UID tag :");
            String content= "";
            char store= stringTochar(content);
            for (byte i = 0; i < mfrc522.uid.size; i++) 
            {
                Serial.print(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " ");
                Serial.print(mfrc522.uid.uidByte[i], HEX);
                content.concat(String(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " "));
                content.concat(String(mfrc522.uid.uidByte[i], HEX));
            }
            myFile.read(buf,10);       
            if(strncmp(buf,content, 8) == 0)
            {
                Serial.println("Match!");
                break;     
            }
            else if (strncmp(buf,content, 8) == 1)
            {
                Serial.println("No Match!");
                break;     
            };
        }
        // close the file:
        myFile.close();
    } 
    else 
    {
        // if the file didn't open, print an error:
        Serial.println("error opening test.txt");
    }
}

void loop()
{
    // nothing happens after setup
}
//
char stringTochar(String s)
{
    char arr[12];
    s.toCharArray(arr, sizeof(arr));
    return atol(arr);
}
4

Personally, I'd like to know what you 'think' you're doing here?

char stringTochar(String s)
{
char arr[12];
s.toCharArray(arr, sizeof(arr));
return atol(arr);
}