[CERT-C mapping] Create guideline that governs the mixing of read and write calls on the same I/O object

[darkwisebear]

There is no clear rule that tells how reads and writes on the same object interact with regards to the read / write pointer. There should be a rule that clarifies this and gives hints how reads and writes shall be treated in case they interleave (e.g. call seek before each operation, or when changing from reading to writing and vice versa). This came up when discussing FIO39.

[riking]

Remember that the rules should only apply to objects with a seek state: File-like, but not connection-like.

[riking]

There are also Rust objects with custom Read + Write behavior, e.g. VecDeque<u8> acts as a userspace Pipe:

https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#impl-Read-for-VecDeque%3Cu8,+A%3E
https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#impl-Write-for-VecDeque%3Cu8,+A%3E

[darkwisebear]

Remember that the rules should only apply to objects with a seek state: File-like, but not connection-like.

True. This rule can only apply to objects that have the notion of a seekable position, so for things like TCP streams, the assumption is that reading from such an object does not have any side effects on the writing part. Another rule?