On Mon, Mar 14, 2022 at 9:07 AM محمد بومنذر
It seems to date back to when the Reader interface was first written,
back when Go looked somewhat different than it does today:
https://go.googlesource.com/go/+/7c9e2c2b6c2e0aa3090dbd5183809e1b2f53359b%5E%21/#F1
I don't know for sure but I'm going to guess that "p" is short for
"pointer", since back then the argument was indeed a pointer. And
we've just kept the name "p" ever since.
Ian