magic-io-unit (C032)

What it does

Checks for literal integers as units in IO statements.

Why is this bad?

Hardcoding unit numbers makes programs more brittle as it becomes harder to verify units have been opened before reading/writing. Instead, units should be passed in to procedures as arguments, or the newunit= argument used for open statements. Having a named variable also makes it much clearer what a given IO statement is for, and allows tools like LSP and IDEs to find all references.

Bad:

open(10, file="example.txt", action="read")
read(10, fmt=*) int
close(10)

Good:

open(newunit=example_unit, file="example.txt", action="read")
read(example_unit, fmt=*) int
close(example_unit)