How many bitmask values can I have

[jgp4]

Hi,

 

I've just started looking at using bitmasks for permissions in my work as they seem like a nice way to handle things, however I've just had a thought.

 

As the different permission values will be stored as a power of 2, will i only be able to have 32 of them because my server is 32bit? I.E. if I try and set a permission with a bitmask of 2^33 will the server say no?

 

I must admit I haven't tried this out much, as I'm so new to it, but if anyone can offer some insight I'd be very grateful.

 

Thanks.

[Daniel0]

An n bit integer can have 2ⁿ different values, so for an unsigned 32-bit integer, that means 0 through 4,294,967,295. If you assign a larger value then it'll overflow.

[Yesideez]

Here's how I've done this. First set some constants (this is from a class)

  const BFLG_ISADMIN  = 8;
  const BFLG_CANDELETE= 4;
  const BFLG_CANWRITE = 2;
  const BFLG_CANREAD  = 1;

 

Then I check to see if the user has delete access:

  public function canDelete($field) {
  	return (boolean) (((int)$this->{$field} & (int)db_staff_permissions::BFLG_CANDELETE) == (int)db_staff_permissions::BFLG_CANDELETE);
  }

 

$field that is passed is a value between 0 and 15:

8+4+2+1=15

 

No privileges is 0.

[Yesideez]

With that you can replace BFLG_CANDELETE with the constants and make new functions.

[Mchl]

And 2^33 is 8 589 934 592

In other words 32bitmask has 32bits and not a single one more.

You can however use multiple bitmasks... a bit more complicated but will work...

 

You can also think about more flexible and intuitive solution, like ACL (access control lists)

[jgp4]

Thanks guys, all great answers, I'll look into the multiple bitmasks and ACL like mchl suggests.

 

Just to clarify the situation, I'm working on a CMS/Ecommerce platform which has many modules each with lots of functions, so there will be more than 32 different things a user could potentially do  - at the very least add, approve and delete in each module - i may see how simple it would be to have a bitmask for each module.

 

Will update once I've found what seems to be the best solution.

 

Thanks again.

[Daniel0]

Okay, wait it seems like I misunderstood you. Yes, you'll only be able to have 32 permission flags with a 32-bit integer. I was actually talking about the different combinations you could make using these.

 

Anyway, an ACL would be far more superior.