GoKawiilBlog post details idiomatic C pattern for safely iterating backwards with unsigned indices
Developer David Priver published a post explaining common mistakes programmers make when writing backward loops over unsigned size_t counters in C, including tautological comparisons and signed-integer workarounds that introduce portability or truncation issues. He presents an alternative idiom, 'for (size_t i = count; i--; )', which uses the postdecrement operator to keep the index in bounds and relies on well-defined unsigned wraparound behavior in C to terminate the loop safely.
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The post addresses a recurring low-level bug pattern that the author says has become more common as LLM-assisted coding tools reproduce flawed idioms without understanding C's unsigned integer semantics. Clearer guidance on this pattern could reduce subtle off-by-one or wraparound bugs in systems code where size_t indexing is standard, particularly in performance-sensitive or security-relevant C codebases.
- Using 'i >= 0' with an unsigned size_t index for backward loops is always true and never terminates correctly.
- Converting the loop index to a signed type like ssize_t avoids the wraparound issue but introduces portability and integer-promotion concerns.
- The idiom 'for (size_t i = count; i--; )' uses postdecrement semantics and well-defined unsigned wraparound to safely iterate backwards without signed types.
Source: davidpriver.com, 2026-10-26
Published there as: “Iterating Backwards in C”
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