tgetflag3x

Initialization

tgetent loads the terminal database entry for name; see term(7). This must be done before calling any of the other functions. It returns

This implementation differs from those of historical termcap libraries.

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ncurses ignores the buffer pointer bp, as do other termcap implementations conforming to portions of X/Open Curses now withdrawn. The BSD termcap library would store a copy of the terminal type description in the buffer referenced by this pointer. terminfo stores terminal type descriptions in compiled form, which is not the same thing.

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The meanings of the return values differ. The BSD termcap library does not check whether the terminal type description includes the generic (gn) capability, nor whether the terminal type description supports an addressable cursor, a property essential for any curses implementation to operate.

Retrieving Capability Values

tgetflag reports the Boolean entry for id, or zero if it is not available.

tgetnum obtains the numeric entry for id, or -1 if it is not available.

tgetstr returns the string entry for id, or NULL if it is not available. Use tputs to output the string returned. The sbuf parameter is used as follows.

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It is assumed to be the address of a pointer to a buffer managed by the calling application.

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However, ncurses checks to ensure that sbuf is not NULL, and that the pointer obtained by dereferencing sbuf is also not NULL. If either check fails, ncurses ignores sbuf.

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If the checks succeed, ncurses also copies the return value to the buffer pointed to by sbuf, and the library updates sbuf to point past the null character terminating this value.

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The return value itself is an address in the terminal type description loaded into memory.

Applying String Capabilities

String capabilities can be parameterized; see subsection “Parameterized Strings” in terminfo(5). tgoto applies its second and third arguments to the parametric placeholders in the capability stored in the first argument.

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The capability may contain padding specifications; see subsection “Delays and Padding” of terminfo(5). The output of tgoto should thus be passed to tputs rather than some other output function such as printf(3).

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While tgoto is assumed to be used for the two-parameter cursor positioning capability, termcap applications also use it for single-parameter capabilities.

Doing so reveals a quirk in tgoto: most hardware terminals use cursor addressing with row first, but the original developers of the termcap interface chose to put the col (column) parameter first. The tgoto function swaps the order of its parameters. It does this even for calls requiring only a single parameter. In that case, the first parameter is merely a placeholder.

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Normally the ncurses library is compiled without full termcap support. In that case, tgoto uses an internal version of tparm(3X) (a more capable function).

Because it uses tparm internally, tgoto is able to use some terminfo features, but not all. In particular, it allows only numeric parameters; tparm supports string parameters.

However, tparm is not a termcap feature, and portable termcap applications should not rely upon its availability.

tputs is described in curs_terminfo(3X). It can retrieve capabilities by either termcap or terminfo code.

Global Variables

tgetent sets the variables PC, UP, and BC to the terminfo entry's data for pad_char (pad), cursor_up (cuu1), and backspace_if_not_bs (OTbs), respectively. ncurses does not employ cuu1 internally. delay_output(3X) uses pad, while tgoto emulation uses the obsolete termcap capability bs, represented in ncurses terminfo as “OTbs”. ncurses assigns the variable ospeed a system-specific value to encode the terminal's data rate.

Releasing Memory

The termcap functions provide no means of freeing memory, because legacy termcap implementations used only the storage provided by the caller via tgetent and tgetstr. Those buffers are unused in terminfo.

By contrast, terminfo allocates memory. It uses setupterm(3X) to obtain the data used by tgetent and the functions that retrieve capability values. One could use

del_curterm(cur_term);

to free this memory, but there is an additional complication with ncurses. It uses a fixed-size pool of storage locations, one per value of the terminal name parameter given to tgetent. The screen(1) program relies upon this arrangement to improve its performance.

An application that uses only the termcap functions, not the higher-level curses API, could release the memory using del_curterm(3X), because the pool is freed using other functions; see curs_memleaks(3X).

Compatibility with BSD termcap

ncurses provides externally visible variables to support certain termcap applications. However, their correct usage is poorly documented; for example, it is unclear when reading and writing them is meaningful. In particular, some applications are reported to declare and/or modify ospeed.

The constraint that only the first two characters of the id parameter are looked up in the terminal database escapes many application developers. The BSD termcap library did not require a trailing null character after the capability identifier passed to tgetstr, tgetnum, and tgetflag. Some applications thus assume that the termcap interface does not require the trailing null character for the capability identifier. ncurses disallows matches by the termcap interface against extended capability names that are longer than two characters; see user_caps(5).

The BSD termcap function tgetent returns the text of a termcap entry in the buffer passed as an argument. ncurses, like other terminfo implementations, does not store terminal type descriptions as text. It sets the buffer contents to a null-terminated string.

Header File

ncurses includes a termcap.h header file for compatibility with other implementations, but it is rarely used because the other implementations are not mutually compatible; see below.