Why does IsCoinBase() check if there is exactly one input?

Question

In the Bitcoin Core source code there is a function to check if a given transaction is a coinbase or not:

    bool IsCoinBase() const
    {
        return (vin.size() == 1 && vin[0].prevout.IsNull());
    }

Does it really need to check if vin.size() is equal to 1? Isn't vin[0].prevout.IsNull() a sufficient condition for a transaction to be a coinbase? Is there any situation that the later condition is true but we have more than one input?

The only reason I can guess is to avoid run-time error when vin is empty (so there is no vin[0]).

Hi Amir, I edited the title of this topic because the previous felt a bit vague. I tried to capture the main idea of your question. Please feel free to revert or otherwise edit it if this doesn't align with what you wanted to express. — Murch, Jul 05 '22 at 10:55
@Murch Hi Murch, thanks for your edit. It's now really better. — Amir reza Riahi, Jul 05 '22 at 11:17

score 8 · Accepted Answer · answered Jul 05 '22 at 03:22

A coinbase transaction is not allowed to have any other inputs, hence this check. Furthermore, because this check is part of consensus code, it is the definition for a coinbase transaction, so this ends up being a bit circular - the function says that a coinbase transaction must only have one input, and it checks that.

score 0 · Answer 2 · answered Jul 05 '22 at 22:18

A coinbase transaction uses an input to store metadata. Some types of metadata are the block height and extra nonce. See:

Why does IsCoinBase() check if there is exactly one input?

2 Answers2