pow10-rule-07-check-return-values

name	pow10-rule-07-check-return-values
description	NASA Power of 10 Rule 7 — Check every non-void return value; validate every parameter. Severity: blocker.

Rule 7 — Check Return Values & Validate Parameters

Severity: blocker

Statement

Check the return value of every non-void function. Validate every parameter at function entry, before any state mutation. Callers cannot be trusted, even within the same module.

Rationale

Ignoring a return value discards error information the callee computed. Unvalidated parameters let bad input propagate into corrupted state. Both checks isolate errors at their source.

Universal violation patterns

• Discarded non-void returns ((void)foo(), _, _ = foo(), bare foo();)
• Function entry that mutates state before validating parameters
• Empty catch / except blocks swallowing errors
• Optional.get() / !! without prior null check
• Errors crossing module boundaries unwrapped

Universal remediation pattern

At function entry, validate every parameter against domain constraints and reject (return error or raise) before any side effect. After every fallible call, check the return; on failure, propagate (with wrapping for traceability) and roll back any partial mutation.

Per-language guidance

• C: references/c.md
• Go: references/go.md
• Python: references/python.md
• Java: references/java.md
• Kotlin: references/kotlin.md

Documented (void) casts

When you genuinely don't care about a return value (best-effort cleanup), state why:

(void)fclose(fp); // best-effort; fp already drained

//nolint:errcheck // best-effort; lock already released
_ = file.Close()

Citations

• Holzmann 2006 — Rule 7
• CERT C — ERR33-C