curs_termcap(3X) Library calls curs_termcap(3X)

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

#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 **area);
char *tgoto(const char *cap, int col, int row);
int tputs(const char *str, int affcnt, int (*putc)(int));

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.

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

on success,
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
if the terminfo database could not be found.

This implementation differs from those of historical termcap libraries.

  • ncurses ignores the buffer pointer bp, as do other termcap implementations conforming to portions of X/Open Curses now withdrawn.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 area referenced by this pointer. terminfo stores terminal type descriptions in compiled form, which is not the same thing.
  • 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.

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 area parameter is used as follows.

  • It is assumed to be the address of a pointer to a buffer managed by the calling application.
  • However, ncurses checks to ensure that area is not NULL, and also that the resulting buffer pointer is not NULL. If either check fails, area is ignored.
  • If the checks succeed, ncurses also copies the return value to the buffer pointed to by area, and the library updates area to point past the null character terminating this value.
  • The return value itself is an address in the terminal type description loaded into memory.

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.

  • 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).
  • 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.
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.

The variables PC, UP and BC are set by tgetent to the terminfo entry's data for pad_char, cursor_up and backspace_if_not_bs, respectively. UP is not used by ncurses. PC is used by delay_output(3X). BC is used by tgoto emulation. The variable ospeed is set by ncurses using a system-specific encoding to indicate the terminal's data rate.

The termcap functions provide no means of freeing memory, because legacy termcap implementations used only the buffer areas 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

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).

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

tgoto returns NULL on error. Error conditions include:

  • uninitialized state (tgetent was not called successfully),
  • cap being a null pointer,
  • cap referring to a canceled capability,
  • cap being a capability with string-valued parameters (a terminfo-only feature), and
  • cap being a capability with more than two parameters.

See curs_terminfo(3X) regarding tputs.

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

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

  • X/Open Curses, Issue 4, Version 2 (1996), describes these functions, marking them as “TO BE WITHDRAWN”.
  • X/Open Curses, Issue 7 (2009) marks the termcap interface (along with vwprintw and vwscanw) as withdrawn.

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.

Externally visible variables are provided for support of 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 used escapes many application developers. The BSD termcap library did not require a trailing null character on 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. This library, like other terminfo implementations, does not store terminal type descriptions as text. It sets the buffer contents to a null-terminated string.

This library includes a termcap.h header for compatibility with other implementations, but the header is rarely used because the other implementations are not strictly compatible.

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 though 4.3BSD (June 1986). 4.4BSD-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.

  • One was used internally by jove(1) from 4.3BSD onward. It declared global symbols for the termcap variables that it used.
  • The other appeared in 4.4BSD-Lite Release 2 (June 1995) as part of libedit (also known as the editline library). CSRG source history shows that this was added in mid-1992. The libedit header file was used internally as a convenience for compiling the editline library. It declared function prototypes, but no global variables. This header file was added to NetBSD's termcap library in mid-1994.

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

GNU termcap 1.3 was bundled with bash(1) in mid-1993 to support the readline(3) library.

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:

  • global symbols used by emacs,
  • const-qualified function prototypes, and
  • a prototype for tparam, a GNU termcap feature.

Later (in mid-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.

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 not-quite-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 busy-waiting for 50 milliseconds.
  • However, if ncurses is configured to support termcap, it may also have been configured to support BSD-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 busy-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 checks for, and modifies the data shared with, the termcap interface to accommodate the latter's limitation in this respect.

curses(3X), curs_terminfo(3X), putc(3), term_variables(3X), terminfo(5)

2024-04-20 ncurses 6.5