C character classification

C character classification is a group of operations in the C standard library that test a character for membership in a particular class of characters; such as alphabetic, control, etc. Both single-byte, and wide characters are supported.<ref>Template:Cite book</ref>

History

Early C programmers working on the Unix operating system developed programming idioms for classifying characters. For example, the following code evaluates as true for an ASCII letter character c:

<syntaxhighlight lang="c"> ('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z') </syntaxhighlight>

Eventually, the interface to common character classification functionality was codified in the C standard library file ctype.h.

Implementation

For performance, the standard character classification functions are usually implemented as macros instead of functions. But, due to limitations of macro evaluation, they are generally not implemented today as they were in early versions of Linux like:

<syntaxhighlight lang="c">

1. define isdigit(c) ((c) >= '0' && (c) <= '9')

</syntaxhighlight>

This can lead to an error when the macro parameter x is expanded to an expression with a side effect; for example: isdigit(x++). If the implementation was a function, then x would be incremented only once. But for this macro definition it is incremented twice.

To eliminate this problem, a common implementation is for the macro to use table lookup. For example, the standard library provides an array of 256 integers Template:Endash one for each character value Template:Endash that each contain a bit-field for each supported classification. A macro references an integer by character value index and accesses the associated bit-field. For example, if the low bit indicates whether the character is a digit, then the isdigit macro could be written as:

<syntaxhighlight lang="c">

1. define isdigit(c) (TABLE[c] & 1)

</syntaxhighlight>

The macro argument, c, is referenced only once, so is evaluated only once.

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
Template:Anchorisalnum	Template:Anchoriswalnum	checks whether the operand is alphanumeric
Template:Anchorisalpha	Template:Anchoriswalpha	checks whether the operand is alphabetic
Template:Anchorislower	Template:Anchoriswlower	checks whether the operand is lowercase
Template:Anchorisupper	Template:Anchoriswupper	checks whether the operand is an uppercase
Template:Anchorisdigit	Template:Anchoriswdigit	checks whether the operand is a digit
Template:Anchorisxdigit	Template:Anchoriswxdigit	checks whether the operand is hexadecimal
Template:Anchoriscntrl	Template:Anchoriswcntrl	checks whether the operand is a control character
Template:Anchorisgraph	Template:Anchoriswgraph	checks whether the operand is a graphical character
Template:Anchorisspace	Template:Anchoriswspace	checks whether the operand is space
Template:Anchorisblank	Template:Anchoriswblank	checks whether the operand is a blank space character
Template:Anchorisprint	Template:Anchoriswprint	checks whether the operand is a printable character
Template:Anchorispunct	Template:Anchoriswpunct	checks whether the operand is punctuation
Template:Anchortolower	Template:Anchortowlower	converts the operand to lowercase
Template:Anchortoupper	Template:Anchortowupper	converts the operand to uppercase
Template:N/a	Template:Anchoriswctype	checks whether the operand falls into specific class
Template:N/a	Template:Anchortowctrans	converts the operand using a specific mapping
Template:N/a	Template:Anchorwctype	returns a wide character class to be used with iswctype
Template:N/a	Template:Anchorwctrans	returns a transformation mapping to be used with towctrans