How is the blocksize currently calculated in Bitcoin Core? I'm specifically asking about how it's done for the purposes of enforcing the 1MB limit. If it's done by calculating the serialization length, what serialization format is used?
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The block size is the combination of the block header and the list of transactions. Specifically, the block header has these fields:
- version - 4 bytes
- previous block header hash - 32 bytes
- merkle root hash - 32 bytes
- time - 4 bytes
- nBits (encoded POW target) - 4 bytes
- nonce - 4 bytes
So the total bytes for a block header is 4 + 32 + 32 + 4 + 4 + 4 = 80 bytes
After the block header is the list of transactions which is
- transaction count - 1-9 bytes
- transactions - varies
Transactions can vary greatly in terms of the number of bytes, but the general serialization of a single transaction is:
- version - 4 bytes
- tx in size - 1-9 bytes
- tx ins - see below
- tx out size - 1-9 bytes
- tx outs - see below
- locktime - 4 bytes
Each tx in has these fields:
- previous tx out hash - 32 bytes
- previous tx out position - 4 bytes
- pubkey script - varies depending on script, max 10,000 bytes
- signature script - varies
- sequence - 4 bytes
Each tx out has these fields:
- value - 8 bytes
- pubkey script length - 1-9 bytes
- pubkey script - varies
The sum total of all these things in a block determine the size, which currently has to be less than 1 MB.
Jimmy Song
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block size = total block size - 8 bytes. The 8 being magic number and block size fields right? – Bitcoingraffiti Apr 22 '21 at 14:13
https://github.com/bitcoin/bitcoin/blob/master/src/consensus/consensus.h#L10
https://github.com/bitcoin/bitcoin/blob/master/src/main.cpp#L2640
https://github.com/bitcoin/bitcoin/blob/master/src/serialize.h
– Josh Cincinnati Nov 03 '15 at 01:50