CTYPE

Ctype refers to a set of character handling facilities provided by the C standard library, exposed through the header ctype.h. The functions in this module classify characters or convert their case. They are widely used for parsing, input validation, and text processing.

The typical functions include isalpha, isdigit, isalnum, isspace, iscntrl, isgraph, isprint, ispunct, isupper, islower, and isxdigit, as well as the case conversion functions tolower and toupper. Each function takes an int argument and returns a nonzero value if the character matches the tested property, or zero otherwise. The argument must be representable as an unsigned char or be the special value EOF. Passing a plain signed char with a negative value yields undefined behavior; the common pattern is to cast to unsigned char before passing to the function.

The tests are locale-sensitive to the current C locale, which can be changed with setlocale (locale.h). This means character classification can vary with locale settings, especially for functions like isalpha or isspace.

Implementation details vary by compiler, but most implementations use a lookup table of character properties. The functions are designed to be efficient and portable across platforms that support the C standard library.

Other notes: They do not perform encoding conversions; they work on the byte values of the input as defined by the locale. The header to include is ctype.h.