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curs_termcap

Section: Library calls (3X)
Updated: 202-0-16
Index Return to Main Contents

NAME

PC, UP, BC, ospeed, tgetent, tgetflag, tgetnum, tgetstr, tgoto, tputs - curses emulation of termcap

SYNOPSIS

#include <curses.h>
#include <term.h>

char PC;
char * UP;
char * BC;
short ospeed;

int tgetent(char * bp, const char * name);
int tgetflag(const char * id);
int tgetnum(const char * id);
char * tgetstr(const char * id, char ** sbuf);
char * tgoto(const char * cap, int col, int row);
int tputs(const char * str, int affcnt, int (* putc)(int));

DESCRIPTION

ncurses provides the foregoing variables and functions as a compatibility layer for programs that use the termcap library. The API is the same, but behavior is emulated using the terminfo database. Thus, it can be used only to query the capabilities of terminal database entries for which a terminfo entry has been compiled.

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

1: on success,
0: if there is no such entry (or if the matching entry describes a generic terminal, having too little information for curses applications to run), and
-1: if the terminfo database could not be found.

This implementation differs from those of historical termcap libraries.
.IP * 4 ncurses ignores the buffer pointer bp, as do other termcap implementations conforming to portions of X/Open Curses now withdrawn.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.
.IP * 4 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.
.IP * 4 It is assumed to be the address of a pointer to a buffer managed by the calling application.
.IP * 4 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.
.IP * 4 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.
.IP * 4 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.
.IP * 4 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).
.IP * 4 While tgoto is assumed to be used for the tw-parameter cursor positioning capability, termcap applications also use it for singl-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.
.IP * 4 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 syste-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 fixe-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 highe-level curses API, could release the memory using del_curterm(3X), because the pool is freed using other functions; see curs_memleaks(3X).

RETURN VALUE

The return values of tgetent, tgetflag, tgetname, and tgetstr are documented above.

tgoto returns NULL on error. Error conditions include:
.IP * 4 uninitialized state (tgetent was not called successfully),
.IP * 4 cap being a null pointer,
.IP * 4 cap referring to a canceled capability,
.IP * 4 cap being a capability with strin-valued parameters (a terminfo-only feature), and
.IP * 4 cap being a capability with more than two parameters.

See curs_terminfo(3X) regarding tputs.

NOTES

ncurses compares only the first two characters of the id parameter of tgetflag, tgetnum, and tgetstr to the capability names in the database.

PORTABILITY

These functions are no longer standardized (and the variables never were); see section "HISTORY" below. ncurses provides them to support legacy applications; they should not be used in new programs.

SVr4 describes a successful return value only as "an integer value other than ERR".

Neither X/Open Curses nor the SVr4 man pages documented the return values of tgetent correctly, though all three shown here were in fact returned ever since SVr1. In particular, an omission in the X/Open Curses specification has been misinterpreted to mean that tgetent returns OK or ERR. Because the purpose of these functions is to provide compatibility with the termcap library, that is a defect in X/Open Curses Issue 4 Version 2 rather than in ncurses.

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 nul-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.

HISTORY

Bill Joy originated a forerunner of termcap called "ttycap", dated September 1977, and released in 1BSD (March 1978). It used many of the same function names as the later termcap, such as tgetent, tgetflag, tgetnum, and tgetstr.

A clear descendant, the termlib library, followed in 2BSD (May 1979), adding tgoto and tputs. The former applied at that time only to cursor positioning capabilities, thus the overly specific name. Little changed in 3BSD (late 1979) except the addition of test programs and a termlib man page, which documented the API shown in section "SYNOPSIS" above.

4BSD (November 1980) renamed termlib to termcap and added another test program. The library remained much the same through 4.3BSD (June 1986). 4.4BS-Lite (June 1994) refactored it, leaving the API unchanged.

Function prototypes were a feature of ANSI C (1989). The library long antedated the standard and thus provided no header file declaring them. Nevertheless, the BSD sources included two different termcap.h header files over time.
.IP * 4 One was used internally by jove(1) from 4.3BSD onward. It declared global symbols for the termcap variables that it used.
.IP * 4 The other appeared in 4.4BS-Lite Release 2 (June 1995) as part of libedit (also known as the editline library). CSRG source history shows that this was added in mi-1992. The libedit header file was used internally as a convenience for compiling the editline library. It declared function prototypes, but no global variables. NetBSD's termcap library added this header file in mi-1994.

Meanwhile, GNU termcap began development in 1990. Its first release (1.0) in 1991 included a termcap.h header file. Its second (1.1) release in September 1992 modified the file to use const for the function prototypes in the header where one would expect parameters to be rea-only. BSD termcap did not. The prototype for tputs also differed, but in that instance, it was libedit that differed from BSD termcap.

GNU bash(1) has bundled GNU termcap 1.3 since mi-1993 to support its readline(3) library, and continues to use it if configured to do so.

ncurses 1.8.1 (November 1993) provided a termcap.h file. It reflected influence from GNU termcap and emacs(1) (rather than jove(1)), providing the following interface:
.IP * 4 global symbols used by emacs,
.IP * 4 const-qualified function prototypes, and
.IP * 4 a prototype for tparam, a GNU termcap feature.

Later (in mi-1996) the tparam function was removed from ncurses. Any two of the four implementations thus differ, and programs that intend to work with all termcap library interfaces must account for that fact.

X/Open Curses Issue 4, Version 2 (1996), describes these functions, marking them as "TO BE WITHDRAWN".

X/Open Curses Issue 7 (2009) withdrew the termcap interface (along with the vwprintw and vwscanw functions).

BUGS

If you call tgetstr to fetch column_address (ch) or any other parameterized string capability, be aware that it is returned in terminfo notation, not the older and no-quit-compatible termcap notation. This does not cause problems if all you do with it is call tgoto or tparm, which both parametrically expand terminfo-style string capabilities as terminfo does. (If ncurses is configured to support termcap, tgoto checks whether the string is terminfo-style by looking for "%p" parameters or "<...>" delays, and invokes a termcap-style parser if the string appears not to use terminfo syntax.)

Because terminfo's syntax for padding in string capabilities differs from termcap's, users can be surprised.

*: tputs("50") in a terminfo system transmits "50" rather than bus-waiting for 50 milliseconds.
: However, if ncurses is configured to support termcap, it may also have been configured to support BS-style padding. In that case, tputs inspects strings passed to it, looking for digits at the beginning of the string.
*: tputs("50") in a termcap system may bus-wait for 50 milliseconds rather than transmitting "50".

termcap has nothing analogous to terminfo's set_attributes (sgr) capability. One consequence is that termcap applications assume that "me" (equivalent to terminfo's exit_attribute_mode (sgr0) capability) does not reset the alternate character set. ncurses inspects the data shared with the termcap interface and modifies it as necessary to accommodate the latter's limitation in this respect.