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getcchar(3NCURSES) Library calls getcchar(3NCURSES)

NAME

getcchar, setcchar - convert between a wide-character string and a curses complex character

SYNOPSIS

#include <curses.h>
int gettchar(const cchar_t * wch, wchar_t * wc,


      attr_t * attrs, short * pair, void * opts);
int settchar(cchar_t * wch, const wchar_t * wc,


      const attr_t attrs, short pair, const void * opts);

DESCRIPTION

The curses complex character data type cchar_t is a structure type comprising a wide-character string, a set of attributes, and a color pair identifier. The cchar_t structure is opaque; do not attempt to access its members directly. The library provides functions to manipulate this type.

getcchar

getcchar destructures a cchar_t into its components.

If wc is not a null pointer, getcchar:

  • stores the wide-character string in the curses complex character wch into wc;
  • stores the attributes in attrs; and
  • stores the color pair identifier in pair.

If wc is a null pointer, getcchar counts the wchar_t wide characters in wch, returns that value, and leaves attrs and pair unchanged.

setcchar

setcchar constructs a curses complex character wch from the components wc, attrs, and pair. The wide-character string wch must be terminated with a null wide character L'\0' and must contain at most one spacing character, which, if present, must be the first wide character in the string.

Up to CCHARW_MAX - 1 non-spacing characters may follow (see curses_variables(3NCURSES)). ncurses ignores any additional non-spacing characters.

The string may contain a single control character instead. In that case, no non-spacing characters are allowed.

RETURN VALUE

If getcchar is passed a null pointer as its wc argument, it returns the number of wide characters for a given wch that it would store in wc, counting a trailing null wide character. If getcchar is not passed a null pointer as its wc argument, it returns OK on success and ERR on failure.

In ncurses, getcchar returns ERR if either attrs or pair is a null pointer and wc is not.

setcchar returns OK on success and ERR on failure.

In ncurses, setcchar returns ERR if

  • wch is a null pointer,
  • wc starts with a (wide) control character and contains any other wide characters, or
  • pair has a negative value.

NOTES

wch may be a value stored by setcchar or another curses function with a writable cchar_t argument. If wch is constructed by any other means, the library's behavior is unspecified.

EXTENSIONS

X/Open Curses documents the opts argument as reserved for future use, saying that it must be a null pointer. The ncurses 6 ABI uses it with functions that have a color pair parameter to support extended color pairs.

  • In functions that assign colors, such as setcchar, if opts is not a null pointer, ncurses treats it as a pointer to int, and interprets it instead of the short pair parameter as a color pair identifier.
  • In functions that retrieve colors, such as getcchar, if opts is not a null pointer, ncurses treats it as a pointer to int, and stores the retrieved color pair identifier there as well as in the short pair parameter (which may therefore undergo a narrowing conversion).

PORTABILITY

Applications employing ncurses extensions should condition their use on the visibility of the NCURSES_VERSION preprocessor macro.

These functions are described in X/Open Curses Issue 4. It specifies no error conditions for them.

X/Open Curses does not detail the layout of the cchar_t structure, describing only its minimal required contents:

  • a spacing wide character (wchar_t),
  • at least five non-spacing wide characters (wchar_t; see below),
  • attributes (at least 15 bits' worth, inferred from the count of specified WA_ constants),
  • a color pair identifier (at least 16 bits, inferred from the short type used to encode it).

Non-spacing characters are optional, in the sense that zero or more may be stored in a cchar_t. XOpen/Curses specifies a limit:

Implementations may limit the number of non-spacing characters that can be associated with a spacing character, provided any limit is at least 5.

Then-contemporary Unix implementations adhered to that limit.

  • AIX 4 and OSF/1 4 used the same declaration with a single spacing wide character c and an array of 5 non-spacing wide characters z.
  • HP-UX 10 used an opaque structure of 28 bytes, large enough for 6 wchar_t values.
  • Solaris xcurses uses a single array of 6 wchar_t values.

ncurses defined its cchar_t in 1995 using 5 as the total of spacing and non-spacing characters (CCHARW_MAX). That was probably due to a misreading of the AIX 4 header files, because the X/Open Curses document was not generally available at that time. Later (in 2002), this detail was overlooked when work began to implement the functions using the structure.

In practice, a mere four non-spacing characters may seem adequate. X/Open Curses documents possible applications of non-spacing characters, including their use as ligatures (a feature apparently not supported by any curses implementation). Unicode does not limit the (analogous) number of combining characters in a grapheme cluster; some applications may be affected. ncurses can be compiled with a different CCHARW_MAX value; doing so alters the library's ABI.

HISTORY

These functions were initially specified by X/Open Curses Issue 4 (1995).

SEE ALSO

ncurses(3NCURSES), attr(3NCURSES), color(3NCURSES), wcwidth(3)

2025-01-18 ncurses 6.5