Checking for negatives in an unsigned variable

[samus250]

I wrote a program that uses an unsigned long long as storage for the user input. Even though my program tells the user NOT to input negative numbers, I am extremely sure that it would be the first thing he/she will try. Also, I can't accept floating point numbers (duh). So, I need to show an error message somehow if the user inputs a negative number, instead of using the undefined number that will appear. My question is, how can I check for negative input or floating point input if I want to store the whole value to an unsigned long long? I read the user input like this:

scanf("%I64u",&var);

And after that, well, I can't check if the number is negative or floating point because what the variable contains is a positive piece of junk. I thought I could also use atoi() or atof(), but that is only for integers or floats and I need full unsigned long long!

Thanks

[tabstop]

If you need to do input verification, you can't read into the variable itself. Get a string (with fgets or scanf or whatever) and verify.

[samus250]

And after that, how do I pass the number to an unsigned long long?

[tabstop]

You would do that as part of the verification process. Look at strtoull.

[samus250]

Thanks very much. I didn't understand the 2nd argument of strtoull immediately, but I think I got it. I have to declare a char *pointer and pass the address of the pointer like this: &pointer, right?

So something like this should work:

unsigned long long number;
char input[32];
char *endptr;

scanf("%s", input);
// here goes error checking

number = strtoull(input, &endptr, 0);

// do what I want with the number

Thanks.

[tabstop]

Thanks very much. I didn't understand the 2nd argument of strtoull immediately, but I think I got it. I have to declare a char *pointer and pass the address of the pointer like this: &pointer, right?

Thanks.

Right. And endptr will point at the "rest" of the number, so if endptr doesn't point to just "\n" or the like, that means there was extra stuff typed in at the end.

[samus250]

Ok thanks, I understand completely now.

So then I could check everything endptr has by making a loop that uses pointer notation like (*(enptr+x)) until NULL is met.

Thanks a lot, I solved my problem! A very handy function strtoull is!

[Elysia]

Have a look at
http://cpwiki.sourceforge.net/Buffer_overrun
You're reading strings with scanf the wrong way.

[samus250]

Thanks a lot Elysia. I read that wiki and all of it on your signature I'll change that now.

Which one would you prefer? Using scanf("%63s",&var); or fgets(var, 64, stdin);? P.S., I noticed that you do not use the & in front of the variable in your scanf (the one on the single post). Wouldn't that cause compiler error?

[Elysia]

Arrays are passed as pointers by default, so using & on them would, in effect, take the address of the pointer, thus giving wrong results. So no, it's correct.
I recommend the fgets method, though, seeing as you can just pass sizeof(buffer) instead of hard-coding numbers and it doesn't leave anything in the input buffer.
Remember that fgets reads the newline, so you may have to get rid of it yourself.