I'm trying to convert a string to a int

I hope I’m on the right path here… I have got a DS1307 hooked up with a uno and a 4 digit 7 seg display. I can display numbers that I enter into the following function displayNumber(1234);

I think I need to convert the time into a string to give me the format I wish to display (Hour, Minutes). I have created a string to include the hours and minutes which prints out fine via serial, but I cant send the string to the display displayNumber(string);

I have tired to convert the string into an int using atol - but no sucess. I get the following error

cannot convert ‘String’ to ‘const char*’ for argument ‘1’ to ‘long int atol(const char*)’

Could somebody please help me out? ( the segment of code I’m having trouble with is at the start of the loop() function)

/*
 6-13-2011
 Spark Fun Electronics 2011
 Nathan Seidle
 
 This code is public domain but you buy me a beer if you use this and we meet someday (Beerware license).
 
 4 digit 7 segment display:
 http://www.sparkfun.com/products/9483
 Datasheet: 
 http://www.sparkfun.com/datasheets/Components/LED/7-Segment/YSD-439AR6B-35.pdf
 
 This is an example of how to drive a 7 segment LED display from an ATmega without the use of current limiting resistors.
 This technique is very common but requires some knowledge of electronics - you do run the risk of dumping too 
 much current through the segments and burning out parts of the display. If you use the stock code you should be ok, but 
 be careful editing the brightness values.
 
 This code should work with all colors (red, blue, yellow, green) but the brightness will vary from one color to the next
 because the forward voltage drop of each color is different. This code was written and calibrated for the red color.
 
 This code will work with most Arduinos but you may want to re-route some of the pins.
 
 7 segments
 4 digits
 1 colon
 =
 12 pins required for full control 
 
 */
#include <Wire.h>
#include "RTClib.h"

RTC_DS1307 rtc;

#include <stdlib.h> // for the atol() function


int digit1 = 11; //PWM Display pin 1
int digit2 = 10; //PWM Display pin 2
int digit3 = 9; //PWM Display pin 6
int digit4 = 6; //PWM Display pin 8

//Pin mapping from Arduino to the ATmega DIP28 if you need it
//http://www.arduino.cc/en/Hacking/PinMapping
int segA = A1; //Display pin 14
int segB = 3; //Display pin 16
int segC = 4; //Display pin 13
int segD = 5; //Display pin 3
int segE = A0; //Display pin 5
int segF = 7; //Display pin 11
int segG = 8; //Display pin 15

int col = 13; 

void setup() {  
  Serial.begin(57600);
#ifdef AVR
  Wire.begin();
#else
  Wire1.begin(); // Shield I2C pins connect to alt I2C bus on Arduino Due
#endif
  rtc.begin();

  if (! rtc.isrunning()) {
    Serial.println("RTC is NOT running!");
    // following line sets the RTC to the date & time this sketch was compiled
    //rtc.adjust(DateTime(__DATE__, __TIME__));
  }

  pinMode(segA, OUTPUT);
  pinMode(segB, OUTPUT);
  pinMode(segC, OUTPUT);
  pinMode(segD, OUTPUT);
  pinMode(segE, OUTPUT);
  pinMode(segF, OUTPUT);
  pinMode(segG, OUTPUT);

  pinMode(digit1, OUTPUT);
  pinMode(digit2, OUTPUT);
  pinMode(digit3, OUTPUT);
  pinMode(digit4, OUTPUT);
  pinMode(col, OUTPUT);

  pinMode(col, OUTPUT);
  digitalWrite(col, LOW);
}

void loop() {

  // find the time  
  DateTime now = rtc.now();
  byte hour = now.hour ();
  byte minute = now.minute ();

  String stringHour =  String(hour);    // converting a constant string into a String object
  String stringTime =  String(stringHour + minute);// concatenating two strings

  //stringOne.toInt();

  unsigned long num;
  num = atol(stringTime);
  Serial.println(num);


  displayNumber(num);
}

//Given a number, we display 10:22
//After running through the 4 numbers, the display is left turned off

//Display brightness
//Each digit is on for a certain amount of microseconds
//Then it is off until we have reached a total of 20ms for the function call
//Let's assume each digit is on for 1000us
//If each digit is on for 1ms, there are 4 digits, so the display is off for 16ms.
//That's a ratio of 1ms to 16ms or 6.25% on time (PWM).
//Let's define a variable called brightness that varies from:
//5000 blindingly bright (15.7mA current draw per digit)
//2000 shockingly bright (11.4mA current draw per digit)
//1000 pretty bright (5.9mA)
//500 normal (3mA)
//200 dim but readable (1.4mA)
//50 dim but readable (0.56mA)
//5 dim but readable (0.31mA)
//1 dim but readable in dark (0.28mA)

void displayNumber(int toDisplay) {
#define DISPLAY_BRIGHTNESS  50

#define DIGIT_ON  HIGH
#define DIGIT_OFF  LOW

  long beginTime = millis();

  for(int digit = 4 ; digit > 0 ; digit--) {

    //Turn on a digit for a short amount of time
    switch(digit) {
    case 1:
      digitalWrite(digit1, DIGIT_ON);
      break;
    case 2:
      digitalWrite(digit2, DIGIT_ON);
      break;
    case 3:
      digitalWrite(digit3, DIGIT_ON);
      break;
    case 4:
      digitalWrite(digit4, DIGIT_ON);
      break;
    }

    //Turn on the right segments for this digit
    lightNumber(toDisplay % 10);
    toDisplay /= 10;
    //toDisplay = 9;
    digitalWrite(col, HIGH); //turn on colon
    delayMicroseconds(DISPLAY_BRIGHTNESS); //Display this digit for a fraction of a second (between 1us and 5000us, 500 is pretty good)
    digitalWrite(col, LOW); //turn off colon
    //Turn off all segments
    lightNumber(10); 

    //Turn off all digits
    digitalWrite(digit1, DIGIT_OFF);
    digitalWrite(digit2, DIGIT_OFF);
    digitalWrite(digit3, DIGIT_OFF);
    digitalWrite(digit4, DIGIT_OFF);
  }

  while( (millis() - beginTime) < 10) ; //Wait for 20ms to pass before we paint the display again
}

//Given a number, turns on those segments
//If number == 10, then turn off number
void lightNumber(int numberToDisplay) {

#define SEGMENT_ON  LOW
#define SEGMENT_OFF HIGH

  switch (numberToDisplay){

  case 0:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_ON);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_OFF);
    break;

  case 1:
    digitalWrite(segA, SEGMENT_OFF);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_OFF);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_OFF);
    digitalWrite(segG, SEGMENT_OFF);
    break;

  case 2:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_OFF);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_ON);
    digitalWrite(segF, SEGMENT_OFF);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 3:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_OFF);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 4:
    digitalWrite(segA, SEGMENT_OFF);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_OFF);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 5:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_OFF);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 6:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_OFF);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_ON);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 7:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_OFF);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_OFF);
    digitalWrite(segG, SEGMENT_OFF);
    break;

  case 8:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_ON);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 9:
    digitalWrite(segA, SEGMENT_ON);
    digitalWrite(segB, SEGMENT_ON);
    digitalWrite(segC, SEGMENT_ON);
    digitalWrite(segD, SEGMENT_ON);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_ON);
    digitalWrite(segG, SEGMENT_ON);
    break;

  case 10:
    digitalWrite(segA, SEGMENT_OFF);
    digitalWrite(segB, SEGMENT_OFF);
    digitalWrite(segC, SEGMENT_OFF);
    digitalWrite(segD, SEGMENT_OFF);
    digitalWrite(segE, SEGMENT_OFF);
    digitalWrite(segF, SEGMENT_OFF);
    digitalWrite(segG, SEGMENT_OFF);
    break;
  }
}

As you are using a String class, you can access the char* by doing:

char *c = &string[0];

It has its own c_str function but is very inefficient.

However, the String class has its own conversion function.
Simply type:

string.toInt();

The main problem is that atol accepts a null (0) terminated array of characters (also known as a C string), and you are passing a String object (not a C string). To convert a String object to an int, just call the toInt() method on the String. Here is some example code:

void setup()
{
  Serial.begin(9600);
  String myString = "23";
  Serial.println(myString.toInt());
}

void loop()
{
  
}

Edit: Referring to atol instead of atoi (however, both accept C strings)

You can also use the standard C functions, too:

   char numString[6];
  
  itoa(12345, numString, 10);
  Serial.println(numString);

where the first argument to itoa() is the number, then the buffer to receive the conversion, and finally the numbering base you wish to use. Using the String class is sort of an H-bomb-to-kill-an-ant approach, as a plain character array will work fine. BTW, if you're working with the DS1307, it expects its to be in Binary Coded Decimal format.