KITFORMA · EDITORIAL ANSWER
Why can isalpha fail for bytes above ASCII on a signed-char system?
KitForma editorial guide. Character classification expects EOF or a value representable as unsigned char. Passing a negative promoted char from an arbitrary byte buffer violates that input contract.
Step-by-step answer
Character classification expects EOF or a value representable as unsigned char. Passing a negative promoted char from an arbitrary byte buffer violates that input contract.
For bytes already read into char storage, call
isalpha((unsigned char)ch). When processing getchar results, retain the int long enough to check EOF before converting. Decide whether locale-dependent classification is appropriate; a byte-wise function is not a general Unicode letter detector.
Test bytes with the high bit set and run under the locales your program supports. The cast repairs the input domain, not text encoding. A UTF-8 multibyte sequence must be decoded before applying Unicode-level character rules.Sources and verification
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