C character classification

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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]


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>.


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.[clarification needed]

Overview of functions[edit]

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.

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


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

External links[edit]

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