isxdigit(*src) has another problem. The value *src is of type char, which can be negative. The function has undefined behavior if given any value that is not i

isxdigit(*src) has another problem. The value *src is of type char, which can be negative. The function has undefined behavior if given any value that is not in the range 0 to UCHAR_MAX, or the value EOF. Some implementations are robust in the face of the values -128 to -1 but ISO C doesn't require that.