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.^[1]

History

Early toolsmiths writing in C under Unix began developing idioms at a rapid rate to classify characters into different types. For example, in the ASCII character set, the following test identifies a letter:

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

However, this idiom does not necessarily work for other character sets such as EBCDIC.

Pretty soon, programs became thick with tests such as the one above, or worse, tests almost like the one above. A programmer can write the same idiom several different ways, which slows comprehension and increases the chance for errors.

Before long, the idioms were replaced by the functions in <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 thus:

#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 x has a side effect---for instance, if one calls isdigit(x++) or isdigit(run_some_program()). It would not be immediately evident that the argument to isdigit is being evaluated twice. For this reason, the table-based approach is generally used.

The difference between these two methods became a point of interest during the SCO v. IBM case.

Overview of functions

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

The classification is done according to the current locale.

Byte character	Wide character	Description
isalnum	iswalnum	checks if a byte/wchar_t is alphanumeric
isalpha	iswalpha	checks if a byte/wchar_t is alphabetic
islower	iswlower	checks if a byte/wchar_t is lowercase
isupper	iswupper	checks if a byte/wchar_t is an uppercase byte/wchar_t
isdigit	iswdigit	checks if a byte/wchar_t is a digit
isxdigit	iswxdigit	checks if a byte/wchar_t is a hexadecimal byte/wchar_t
iscntrl	iswcntrl	checks if a byte/wchar_t is a control byte/wchar_t
isgraph	iswgraph	checks if a byte/wchar_t is a graphical byte/wchar_t
isspace	iswspace	checks if a byte/wchar_t is a space byte/wchar_t
isblank	iswblank	checks if a byte/wchar_t is a blank byte/wchar_t (C99/C++11)
isprint	iswprint	checks if a byte/wchar_t is a printing byte/wchar_t
ispunct	iswpunct	checks if a byte/wchar_t is a punctuation byte/wchar_t
tolower	towlower	converts a byte/wchar_t to lowercase
toupper	towupper	converts a byte/wchar_t to uppercase
N/A	iswctype	checks if a wchar_t falls into specific class
N/A	towctrans	converts a wchar_t using a specific mapping
N/A	wctype	returns a wide character class to be used with iswctype
N/A	wctrans	returns a transformation mapping to be used with towctrans

References

1. ISO/IEC 9899:1999 specification (PDF). p. 193, § 7.4.

External links

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