int isalpha(int c);

isalpha returns true if the character c is alphabetic. That is, any character for which isupper or islower returns true is considered alphabetic in addition to any of the locale-specific set of alphabetic characters for which none of iscntrl, isdigit, ispunct, or isspace is true.

In the ‘C’ locale, isalpha returns nonzero (true) if and only if isupper or islower return true for value of the argument c.