Embellished sample of the ctype library interface

Most functions in this library classify ASCII characters, that is, decide whether a given byte represents a letter, or a digit, or a white-space, etc., in ASCII encoding.  The argument of each function is not a char but a positive integer or an EOF. Usually, the argument belongs to the interval −1..127.

// File ctype.h.
// Interface of the ctype library.
///////////////////////////////////////////////////////////

#ifndef _CTYPE_H
#define _CTYPE_H


///////////////////////////////////////////////////////////
// Section 1 -- Boolean functions
///////////////////////////////////////////////////////////
                                                           
// All the functions in this section return zero to say
// "no" and a nonnull integer to say "yes".


// The function isspace decides whether the argument
// represents a white-space character, that is, a character
// from the set  \  \t \n \v \f \r. Usage:
// if (isspace (c)) ...

int isspace (int c);


// The function isdigit decides whether the argument is a
// decimal digit (0 1 2 3 4 5 6 7 8 9). Usage: 
// if (isdigit (c)) .... 

int isdigit (int c);


// The function islower decides whether the argument
// represents a lowercase letter (a b c d e f g h i j k l
// m n o p q r s t u v w x y z). Usage:
// if (islower (c)) .... 

int islower (int c);


// The function isupper decides whether the argument
// represents an uppercase letter (A B C D E F G H I J K
// L M N O P Q R S T U V W X Y Z). Usage: 
// if (isupper (c)) .... 

int isupper (int c);


// The function isalpha decides whether the argument
// represents a letter (lowercase or uppercase). Usage:
// if (isalpha (c)) .... 

int isalpha (int c);


// The function isalnum decides whether the argument
// represents an alphanumeric character (i.e., letter or
// decimal digit). Usage: if (isalnum (c)) .... 

int isalnum (int c);


// The function ispunct decides whether the argument
// represents a punctuation character (! " # $ % & ' ( ) *
// + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~). Usage:
// if (ispunct (c)) .... 

int ispunct (int c);


///////////////////////////////////////////////////////////
// Section 1 -- Conversion functions
///////////////////////////////////////////////////////////
                                                           
// The function toupper receives a letter c and returns the
// corresponding uppercase letter. Usage: C = toupper (c); 

int toupper (int c);


// The function tolower receives a letter C and returns the
// corresponding lowercase letter. Usage: c = tolower (C); 

int tolower (int C);


#endif