Pad byte in python struct

Question

What is a pad byte (x) in the python struct type? Is it an unsigned char with value 0, or what exactly does it look like / why is it one of the types that are available in the struct object?

For example, what would be the difference between doing one of the following:

>>> struct.pack('BB', 0, ord('a'))
b'\x00a'
>>> struct.pack('xB', ord('a'))
b'\x00a'

• Mad Physicist: I don't know that it's always zero, but I can answer the rest

Answer 1

It's useful for matching required length of another system.

For example. In my work, there is a server that sends a fixed sized header and expects fixed sized messages. This guarantees that, lets say the first 20 bytes are a header, with bytes 0-8 being the message size.

It doesn't really matter what type the pad is. It's basically junk data. unsigned char 0 is a good choice though and the one that struct.pack uses.

• Mad Physicist: Looks like the bytes are actually unset in struct.pack, at least for CPython