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:||
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
Early versions of Linux used a potentially faulty method similar to the first code sample:
#define isdigit >= '0' &&
This can cause problems if the variable x has a side effect. For example, if one calls isdigit or isdigit). 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.The functions that operate on wide characters are defined in wctype.h header file.
The classification is evaluated according to the effective locale.
| Byte character | Wide character | Description |
| | checks whether the operand is alphanumeric |
| | checks whether the operand is alphabetic |
| | checks whether the operand is lowercase |
| | checks whether the operand is an uppercase |
| | checks whether the operand is a digit |
| | checks whether the operand is hexadecimal |
| | checks whether the operand is a control character |
| | checks whether the operand is a graphical character |
| | checks whether the operand is space |
| | checks whether the operand is a blank space character |
| | checks whether the operand is a printable character |
| | checks whether the operand is punctuation |
| | converts the operand to lowercase |
| | converts the operand to uppercase |
| checks whether the operand falls into specific class | |
| converts the operand using a specific mapping | |
| returns a wide character class to be used with iswctype | |
| returns a transformation mapping to be used with towctrans |