https://invisible-island.net/ncurses/man/
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
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.
o 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withdrawn.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.
o 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.
o It is assumed to be the address of a pointer to a buffer managed
by the calling application.
o 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.
o 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.
o 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.
o 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).
o 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.
o 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 term-
info 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 term-
info.
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).
The return values of tgetent, tgetflag, tgetname, and tgetstr are
documented above.
tgoto returns NULL on error. Error conditions include:
o uninitialized state (tgetent was not called successfully),
o cap being a null pointer,
o cap referring to a canceled capability,
o cap being a capability with string-valued parameters (a term-
info-only feature), and
o 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.
o X/Open Curses, Issue 4, Version 2 (1996), describes these
functions, marking them as "TO BE WITHDRAWN".
o X/Open Curses, Issue 7 (2009) withdrew the termcap interface (along
with the vwprintw and vwscanw functions).
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. 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 file for compatibility with
other implementations, but it is rarely used because the other
implementations are not mutually compatible; see below.
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). 4.4BSD-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.
o One was used internally by jove(1) from 4.3BSD onward. It declared
global symbols for the termcap variables that it used.
o 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.
NetBSD's termcap library added this header file in mid-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 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 bash(1) has bundled GNU termcap 1.3 since mid-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:
o global symbols used by emacs,
o const-qualified function prototypes, and
o 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 term-
info 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.
o tputs("50") in a terminfo system transmits "50" rather than busy-
waiting for 50 milliseconds.
o 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.
TCTEST -- A Termcap Test Utility <https://invisible-island.net/ncurses/
tctest.html>
curses(3x), curs_terminfo(3x), putc(3), term_variables(3x), terminfo(5)
ncurses 6.5 2024-09-21 curs_termcap(3x)