### 4.1 Classification of Characters

This section explains the library functions for classifying characters. For example, `isalpha` is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this:

         if (isalpha (c))
           printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with ‘`is`’. Each of them takes one argument, which is a character to test, and returns an `int` which is treated as a boolean value. The character argument is passed as an `int`, and it may be the constant value `EOF` instead of a real character.

The attributes of any given character can vary between locales. See Locales, for more information on locales.

These functions are declared in the header file `ctype.h`.

— Function: int islower (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

— Function: int isupper (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

— Function: int isalpha (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is an alphabetic character (a letter). If `islower` or `isupper` is true of a character, then `isalpha` is also true.

> In some locales, there may be additional characters for which `isalpha` is true—letters which are neither upper case nor lower case. But in the standard `"C"` locale, there are no such additional characters.

— Function: int isdigit (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a decimal digit (‘`0`’ through ‘`9`’).

— Function: int isalnum (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is an alphanumeric character (a letter or number); in other words, if either `isalpha` or `isdigit` is true of a character, then `isalnum` is also true.

— Function: int isxdigit (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits ‘`0`’ through ‘`9`’ and the letters ‘`A`’ through ‘`F`’ and ‘`a`’ through ‘`f`’.

— Function: int ispunct (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.

— Function: int isspace (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a whitespace character. In the standard `"C"` locale, `isspace` returns true for only the standard whitespace characters:

> `' '`
>   space
> `'\f'`
>   formfeed
> `'\n'`
>   newline
> `'\r'`
>   carriage return
> `'\t'`
>   horizontal tab
> `'\v'`
>   vertical tab

— Function: int isblank (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99.

— Function: int isgraph (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

— Function: int isprint (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space (‘`` ’) character.

— Function: int iscntrl (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a control character (that is, a character that is not a printing character).

— Function: int isascii (int c)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Returns true if c is a 7-bit `unsigned char` value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension.