Bitcoin Forum

Hi,

A quick question. Assume that a block body contains several transactions (tx), and none of these transactions are of equal length. On bit level, transactions are represented by a set of 0 and 1. A set of tx will therefore result in a set of 0 and 1. Now, if a block contains multiple tx, how can the receiver of a block distinguish each transaction in the block body?

Thanks.

Transactions have a known calculable size when it is parsed. So you will know that when a transaction ends and the next begins.

Specifically...

• Each transaction starts with 32 bits (set of thirty-two 0 and 1 digits) that store a version number.
• Following that is a "set of 0 and 1" that tell how many bits (actually how many bytes, but you can multiply by 8 to get the quantity of 0 and 1) in the transaction inputs.*
• Following that is a "set of 0 and 1" representing all the inputs. This set is exactly the amount indicated in the previous "how many bits" value.
• Following that is a "set of 0 and 1" that tell how many bits (actually how many bytes, but you can multiply by 8 to get the quantity of 0 and 1) in the transaction outputs.*
• Following that is a "set of 0 and 1" representing all the outputs. This set is exactly the amount indicated in the previous "how many bits" value.
• Following that is 32 bits that represent the locktime.

* note, there is a special pattern to the first eight 0 and 1 values of the section that "tells how many bits". This pattern allows the program to know when it has read enough 0 and 1 to determine the appropriate value. If this section starts with 11111111, then the next sixty-four 0 and 1 are used to indicate the size of the inputs or outputs. If it starts with 11111110, then the next thirty-two 0 and 1 are used to indicate the size of the inputs or outputs,  If it starts with 11111101, then the next sixteen 0 and 1 are used to indicate the size of the inputs or outputs. If it starts with anything else, then these 8 values are themselves the indication of the size of the inputs or outputs, and there is no need to use any additional digits to determine that size.

Thanks, that's the details I was looking for

Funny you want to have bit-scale details.  Have a look at the easy to understand and technically detailed doc : https://bitcoin.org/en/developer-reference#raw-transaction-format

Kex