Unable to convert uint8_t array to unsigned long in Arduino - c++

Question

I am trying to convert uint8_t readData[10] = "123456789" ; to unsigned long to do some math on this value, in Arduino. I'm using strtoul function. strtoul works fine if I define above array by my self and it converts this array to unsigned long successfully. But if I put some values in this array by reading DS1307 NVRAM, then strtoul fails to convert the array into unsigned long and gives 0 answer. I checked values in readData array, after reading NVRAM, using for loop, and found that values are same that i have saved in the NVRAM.
I am using NodeMCU board along with DS1307. My code and its Output is given below.
// Example of using the non-volatile RAM storage on the DS1307.
// You can write up to 56 bytes from address 0 to 55.
// Data will be persisted as long as the DS1307 has battery power.
#include "RTClib.h"
RTC_DS1307 rtc;
uint8_t readData[9] = "0"; //**this will store integer values from DS1307 NVRAM.
unsigned long convertedL1 = 0; //this will store converted value
uint8_t savedData[10] = "123456789"; //I have already filled this array for testing strtoul function.
unsigned long convertedL2 = 0; //this will store converted value of savedData array
void setup () {
Serial.begin(9600);
delay(3000);
#ifndef ESP8266
while (!Serial); // wait for serial port to connect. Needed for native USB
#endif
if (! rtc.begin()) {
Serial.println("Couldn't find RTC");
delay(3000);
while(1);
}
rtc.readnvram(readData,9,2); //Read NVRAM from address 2 to 11.
delay(20);
Serial.println("Prinitng values( using loop) stored in readData array, after reading NVRAM :");
for (int i = 0; i <9; i++) {
Serial.print(readData[i]);
}
Serial.println();
//Converting both arrays of same type using stroul
convertedL1 = (unsigned long)strtoul((char *)&readData[0],NULL,10);
convertedL2 = (unsigned long)strtoul((char *)&savedData[0],NULL,10);
Serial.print("converted value of readData array = ");
Serial.println(convertedL1);
Serial.println();
Serial.print("converted value of savedData array = ");
Serial.println(convertedL2);
}//setup end
void loop () {
// Do nothing in the loop.
}
the Output on Serial Monitor is:
Prinitng values( using loop) stored in readData array, after reading NVRAM :
123456789
converted value of readData array = 0
converted value of savedData array = 123456789
Why strtoul function works with one array and not with other.I searched many forums but couldn't find any solution.
Can anyone, please have a look at my code and kindly suggest me the solution. Any help will be highly appreciated.

Answer 1

Seems likely the reason for the difference is that your savedData array is null terminated but your readData array is not. strtoul requires that the array be null terminated.
Change your code like this
uint8_t readData[10] = "0"; // one extra byte for null terminator
...
rtc.readnvram(readData,9,2); //Read NVRAM from address 2 to 11.
readData[9] = '\0'; // add the null terminator