Magic Bytes and Block Size

The blk.dat files are not just a sequence of blocks; they are a sequence of Block Records. Each record consists of three parts: the Magic Bytes, the Block Size, and the Block Header/Data.

1. The Magic Bytes (Network Delimiter)

To help a node identify where one block ends and another begins, every block in the file is preceded by a 4-byte Magic Value. For Bitcoin Mainnet, this value is: 0xD9 0xB4 0xBE 0xF9

These bytes act as a "start of record" marker. If a file becomes corrupted, a node can scan for these bytes to find the next valid block.

2. The Block Size Prefix

Immediately following the magic bytes is a 4-byte integer (Little-Endian) representing the Block Size.

• This value tells the node exactly how many bytes of raw block data follow.

• The size includes the 80-byte header and all transaction data.

• It does not include the 8 bytes used for the Magic Value and the Size prefix itself.

3. The Block Data

Following the size prefix is the actual block.

1. Block Header: 80 bytes.

2. Transaction Count: A Variable Length Integer (VarInt).

3. Transactions: The raw serialized transactions.

4. Visual Layout of a Record

[ 4 Bytes ] -> Magic Value (0xD9B4BEF9)
[ 4 Bytes ] -> Block Size (N bytes)
[ N Bytes ] -> Raw Block Data

5. Byte Order (Little-Endian)

The Block Size prefix is stored in little-endian.

• If the block size is 1,000,000 bytes (0x0F4240 in hex).

• It will be stored in the file as: 40 42 0F 00.

In the next section, we will discuss why blocks are not stored in height order in these files.