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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 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.
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.
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 |
| N/A | iswctype | checks whether the operand falls into specific class |
| N/A | towctrans | converts the operand 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 |
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