isascii() is not defined by ISO C. (It is defined by POSIX, but POSIX says it may be removed in a future version.) I see that POSIX explicitly says that isasci

isascii() is not defined by ISO C. (It is defined by POSIX, but POSIX says it may be removed in a future version.) I see that POSIX explicitly says that isascii(x) is "defined on all integer values" (it should have said "all int values"). Personally I'd rather cast to unsigned char.