getline EOF Return Value Behavior Across Awk Implementations

Goal

Determine the portable way to detect end-of-file conditions when using getline in awk scripts, accounting for implementation differences in return values.

Constraints and Uncertainty

The POSIX specification defines getline return values as 1 for success, -1 for error, but leaves EOF detection ambiguous. GNU awk (gawk 4.0+) returns 0 on EOF, while mawk returns 1, creating portability challenges. This divergence affects scripts that test return values numerically rather than checking the NF variable or using explicit EOF detection.

Questions

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• Should scripts rely on NF checks or explicit EOF markers instead of return value comparisons for getline?
• How can one write getline loops that function correctly across gawk, mawk, nawk, and busybox awk?