How to generate a coinbase transaction?

Question

I am creating a miner in C++ for solo mining, and am struggling to create the proper coinbase transaction.

Say I have a bc1 address, like bc1qfc7fameteuguetm0kfzypnvf2ju6wppdvj6tkc , what would be and example of a coinbase transaction with this, and how would I get to it?

https://developer.bitcoin.org/reference/transactions.html

01000000 .............................. Version

01 .................................... Number of inputs
| 00000000000000000000000000000000
| 00000000000000000000000000000000 ...  Previous outpoint TXID
| ffffffff ............................ Previous outpoint index
|
| 29 .................................. Bytes in coinbase // height+message below, byte length
| |
| | 03 ................................ Bytes in height
| | | 4e0105 .......................... Height: 328014
| |
| | 062f503253482f0472d35454085fffed
| | f2400000f90f54696d65202620486561
| | 6c74682021 ........................ Arbitrary data // custom data, include extranonce here
| 00000000 ............................ Sequence // i thought this was all Fs?

01 .................................... Output count
| 2c37449500000000 .................... // coinbase value
| ????????????????? ............. // here, how do i properly fill this part??
| 00000000 ............................ Locktime

These are the parts needed to create the data for the transaction.

Everything up to the output, i understand. but i dont know what to put between the coinbase value and locktime. how does the address convert to hex? what OP codes do i need? and is this transaction layout correct?

And then I would then hash it and input it as a new transaction into the block as like so,

tx = {
  "data" : "<hex string mentioned above>",
  "hash" : "<double sha256 of data above>",
}

is that the only data i need for the json?

Thanks in advance.

Answer 1

To create a coinbase transaction you need to send this RPC command to your Bitcoin-Qt

createrawtransaction '[{"txid":"0000000000000000000000000000000000000000000000000000000000000000","vout":2147483647}]{"Your Bitcoin Adress":Amount}'

Why 2147483647 ?

Because the vout must be numeric, and you can't put 0xffffffff

So this will give you 0x7fffffff and the easiest way to do that is to change manually the 7 to f

Now as required in BIP34 you must include the block height inside the scriptSig otherwise your coinbase transaction will not be valid

PS: Be aware of little and big endian

Answer 2

i dont know what to put between the coinbase value and locktime.

This is the output scriptPubkey, the locking mechanism placed on a transaction output. In the case of a coinbase transaction it should spend the newly minted coins and any transaction fees.

how does the address convert to hex?

See this answer

what OP codes do i need?

You could use P2PK:

<pub key> OP_CHECKSIG

or P2PKH:

OP_DUP OP_HASH160 <RIPEMD160(SHA256(publickey))> OP_EQUALVERIFY OP_CHECKSIG

and is this transaction layout correct?

You could check it against Greg Walker's learnmeabitcoin coinbase description, or its block explorer:

https://learnmeabitcoin.com/explorer/transaction/json.php?txid=8b50f51b49f27e7bb0efb0b3bf38d12ce4f7e6258b90a75802a394cb585c879d