So, I'm working on a mod which needs to check an NPC's equipment slots, using OBSE. Right now, I have a loop that checks each of the bitmasks to see what's equipped in each spot.

I've narrowed it down to this function:

let curEquip := (curNPC.getEquipmentSlotMask itemMask)

itemMask will always be a power of 2, meaning I will only check one slot at a time. The problem is, this always returns 0. No matter what.

Basically, I want to be able to, if I search for something equipped on the hand, I will get the item on the hand, whether or not that item takes up other slots or not.

I'm not sure why it's always returning 0, but that method won't find multi-slotted items that cover the hand slot.
For example, an item that covers the Upper Torso and Hand, like the Grey Tunic from SI, will have an itemMask value of 20. When you check for an itemMask of 4 or 16, the function will return a 0.

Alright, I'll have to think up something else to do, then.

As an aside, though, what does the value parameter do? It says it'll be equal to the slotmask if it is omitted, but there is no mention of what it *does*

GetEquippedObject will do what you want. The usual problem is the opposite of yours, i.e. not knowing what other slots the item occupies.

The value operand allows you to check for an item that occupies one slot and not another. The mask indicates which slots to check and the value has the same bits on, if the item should be there, and off if its should not. e.g GetEquipmentSlotMask 12 4 would check upper body slot occupied, and lower body slot not occupied by the same item. So a Cuirass would be returned, but a Robe wouldn't. GetEquipmemtSlotMask 12 12 would find a Robe, but not a Cuirass.

Hopefully GetEquippedObject won't suffer the same issues that other slotID functions suffer... how strange slot combination (say... hand and tail) confuse the functions and thus return 0... which is the head slot. Which would cause problems, because I'm also checking the head.

But if checking the hand gives me what's only on the hand, all will be good, and I hopefully won't run into this problem. Plus it seems like I can convert single slot masks to slot ids by taking the log base 2 of the mask.

...as long as I don't test ranged weapons, I guess o.o

UPDATE:
With a little fiddling, everything is working as expected. Thanks much.

I don't know why you were having trouble with GetEquipmentSlotMask to return items occupying the slot you're checking, even if they occupy other slots as well. It did always work that way for me, and if it'd have changed, I guess the users of my mods would have complained already.
GetEquippedObject was of no use for my purposes, because the slots occupied had to be an "exact match", while at the same time "not" enabling any arbitrary combination of slots, which of course can very well always occur with user-made content. It's of no use for scripts meant to work reliably under all circumstances, but GetEquipmentSlotMask always was.

Maybe it's the infamous "let"-bug again... I don't know. I only know I wouldn't use "let" explicitly when there's no function in use which requires it.