String to unsigned long conversion

Question

I'm trying to use strtoul() to convert an Arduino String to an unsigned long (like the return of millis();) using the following code snippet:

unsigned long foo (String time)
{
 unsigned long mili;
 char Tim[9]="";
 uint16_t timsize = time.length()+1;
 char TIM[timsize];
 time.toCharArray(TIM,timsize);
 mili=strtoul(TIM,timsize,10);
 return mili;
}

The code seems to work. However, I'm getting the following warning:

invalid conversion from 'uint16_t {aka unsigned int}' to 'char**' [-fpermissive] mili=strtoul(TIM,timsize,10);

---

My question is twofold:

1. Is there a better way to convert a String to unsigned long without first converting to a C string?

2. If not, how can I eliminate this error?

• 001: unsigned long strtoul(const char *str, char **str_end, int base ); The 2nd parameter is char **. Can be nullptr.
• Code Gorilla: I believe that char TIM[timsize] is bad code. The size of arrays need to be know at compile time and since the value of time is not constant its length can't be known. I think you'd need to do a new, if you were still doing it which you aren't because you are using .c_str()
• ATE-ENGE: It may be poor code. I do know that it works though (however for a 2D or greater array, only the last value can be a var)
• KIIV: @CodeGorilla char TIM[timsize] usually works on newer compilers: Declaring an array with a non-constant size variable
• 001: VLAs are not part of the C++ standard. They work here because of a compiler extension.

Answer 1

Arduino String class provides method c_str(). So you don't have to convert it to C string, as it's already stored as a C string internally.

And as mentioned in comments, the second parameter of strtoul is:

endptr
Reference to an object of type char*, whose value is set by the function to the next character in str after the numerical value.
This parameter can also be a null pointer, in which case it is not used.

Basically you can reduce whole foo to the: return strtoul(time.c_str(), NULL, 10);

• ATE-ENGE: Can you tell me more about how to access the c_str() method of a String? just using sec=strtoul(tim,endptr,10); did not work.
• KIIV: @ATE-ENGE Added to the answer.
• Code Gorilla: Also be aware that strtol() can return 0 or ULONG_MAX if there is a problem, so the calling function should check for these because if you do a delay(ULONG_MAX); you'll think it has locked up :)
• ATE-ENGE: @CodeGorilla That would be a problem! Hypothetically, is there a good way to differentiate if a problem occurs? You know, in case a user actually does want to halt the program for 50 days :)
• KIIV: @ATE-ENGE You can use that endptr to check if whole string was parsed. However it returns 0 if there is nothing to parse and ULONG_MAX if the value is out of range so it must be even bigger than 32bit number.