C character classification

C character classification is an operation provided by a group of functions in the ANSI C Standard Library for the C programming language. These functions are used to test characters for membership in a particular class of characters, such as alphabetic characters, control characters, etc. Both single-byte, and wide characters are supported.

History

Early C-language programmers working on the Unix operating system developed programming idioms for classifying characters into different types. For example, for the ASCII character set, the following expression identifies a letter, when its value is true:

('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z')

As this may be expressed in multiple formulations, it became desirable to introduce short, standardized forms of such tests that were placed in the system-wide header file ctype.h.

Implementation

Unlike the above example, the character classification routines are not written as comparison tests. In most C libraries, they are written as static table lookups instead of macros or functions.

For example, an array of 256 eight-bit integers, arranged as bitfields, is created, where each bit corresponds to a particular property of the character, e.g., isdigit, isalpha. If the lowest-order bit of the integers corresponds to the isdigit property, the code could be written as

#define isdigit(x) (TABLE[x] & 1)

Early versions of Linux used a potentially faulty method similar to the first code sample:

#define isdigit(x) ((x) >= '0' && (x) <= '9')

This can cause problems if the variable x has a side effect. For example, if one calls isdigit(x++) or isdigit(run_some_program()). It is not immediately evident that the argument to isdigit is evaluated twice. For this reason, the table-based approach is generally used.

Overview of functions

The functions that operate on single-byte characters are defined in ctype.h header file (cctype in C++). The functions that operate on wide characters are defined in wctype.h header file (cwctype in C++).

The classification is evaluated according to the effective locale.

Byte character	Wide character	Description
isalnum	iswalnum	checks whether the operand is alphanumeric
isalpha	iswalpha	checks whether the operand is alphabetic
islower	iswlower	checks whether the operand is lowercase
isupper	iswupper	checks whether the operand is an uppercase
isdigit	iswdigit	checks whether the operand is a digit
isxdigit	iswxdigit	checks whether the operand is hexadecimal
iscntrl	iswcntrl	checks whether the operand is a control character
isgraph	iswgraph	checks whether the operand is a graphical character
isspace	iswspace	checks whether the operand is space
isblank	iswblank	checks whether the operand is a blank space character
isprint	iswprint	checks whether the operand is a printable character
ispunct	iswpunct	checks whether the operand is punctuation
tolower	towlower	converts the operand to lowercase
toupper	towupper	converts the operand to uppercase
—	iswctype	checks whether the operand falls into specific class
—	towctrans	converts the operand using a specific mapping
—	wctype	returns a wide character class to be used with iswctype
—	wctrans	returns a transformation mapping to be used with towctrans