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>
extern char PC;
extern char * UP;
extern char * BC;
extern 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));
These routines are included as a conversion aid for programs that use
the termcap library. Their parameters are the same, but the routines
are emulated using the terminfo database. Thus, they can only be used
to query the capabilities of entries for which a terminfo entry has
been compiled.
The tgetent routine loads the entry for name. It returns:
1 on success,
0 if there is no such entry (or that it is a generic type, having
too little information for curses applications to run), and
-1 if the terminfo database could not be found.
This differs from the termcap library in two ways:
o The emulation ignores the buffer pointer bp. The termcap
library would store a copy of the terminal description in the
area referenced by this pointer. However, ncurses stores its
terminal descriptions in compiled binary form, which is not the
same thing.
o There is a difference in return codes. The termcap library does
not check if the terminal description is marked with the generic
capability, or if the terminal description has cursor-
addressing.
The tgetflag routine gets the boolean entry for id, or zero if it is
not available.
The tgetnum routine gets the numeric entry for id, or -1 if it is not
available.
The tgetstr routine returns the string entry for id, or zero if it is
not available. Use tputs to output the returned string. 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, the area parameter is ignored.
o If the checks succeed, ncurses also copies the return value to
the buffer pointed to by area, and the area value will be
updated to point past the null ending this value.
o The return value itself is an address in the terminal
description which is loaded into memory.
Only the first two characters of the id parameter of tgetflag, tgetnum
and tgetstr are compared in lookups.
The tgoto routine expands the given capability using the parameters.
o Because the capability may have padding characters, the output of
tgoto should 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 this shows 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 column parameter first. The
tgoto function swaps the order of parameters. It does this also
for calls requiring only a single parameter. In that case, the
first parameter is merely a placeholder.
o Normally the ncurses library is compiled with terminfo support. In
that case, tgoto uses an internal version of tparm(3x) (a more
capable formatter).
With terminfo support, tgoto is able to use some of the 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.
The tputs routine is described on the curs_terminfo(3x) manual page.
It can retrieve capabilities by either termcap or terminfo name.
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 in the tdelay_output function. BC
is used in the tgoto emulation. The variable ospeed is set by ncurses
in a system-specific coding to reflect the terminal speed.
The termcap functions provide no means for 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.
On the other hand, terminfo allocates memory. It uses setupterm to
retrieve the data used by tgetent and the functions which return
capability values such as tgetstr. 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
setting of the TERM variable when tgetent is called. The screen(1)
program relies upon this arrangement, to improve its performance.
An application which uses only the low-level termcap functions could
free the memory using del_curterm, because the pool is freed using
other functions (see curs_memleaks(3x)).
Except where explicitly noted, routines that return an integer return
ERR upon failure and OK (SVr4 only specifies "an integer value other
than ERR") upon successful completion.
Routines that return pointers return NULL on error.
A few special cases apply:
o If the terminal database has not been initialized, these return an
error.
o The calls with a string parameter (tgoto, tputs) check if the
string is null, or cancelled. Those return an error.
o A call to tgoto using a capability with string parameters is an
error.
o A call to tgoto using a capability with more than two parameters is
an error.
These functions are provided for supporting legacy applications, and
should not be used in new programs:
o The XSI Curses standard, Issue 4 describes these functions.
However, they are marked TO BE WITHDRAWN and may be removed in
future versions.
o X/Open Curses, Issue 5 (December 2007) marked the termcap interface
(along with vwprintw and vwscanw) as withdrawn.
Neither the XSI Curses standard nor the SVr4 man pages documented the
return values of tgetent correctly, though all three were in fact
returned ever since SVr1. In particular, an omission in the XSI Curses
documentation 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 XCurses,
Issue 4, Version 2 rather than in ncurses.
External variables are provided for support of certain termcap
applications. However, termcap applications' use of those variables is
poorly documented, e.g., not distinguishing between input and output.
In particular, some applications are reported to declare and/or modify
ospeed.
The comment that only the first two characters of the id parameter are
used escapes many application developers. The original BSD 4.2 termcap
library (and historical relics thereof) did not require a trailing null
NUL on the parameter name passed to tgetstr, tgetnum and tgetflag.
Some applications assume that the termcap interface does not require
the trailing NUL for the parameter name. Taking into account these
issues:
o As a special case, tgetflag matched against a single-character
identifier provided that was at the end of the terminal
description. You should not rely upon this behavior in portable
programs. This implementation disallows matches against single-
character capability names.
o This implementation disallows matches by the termcap interface
against extended capability names which are longer than two
characters.
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 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.
The original BSD termcap (through 4.3BSD) had no header file which gave
function prototypes, because that was a feature of ANSI C. BSD termcap
was written several years before C was standardized. However, there
were two different termcap.h header files in the BSD sources:
o One was used internally by the jove editor in 2BSD through 4.4BSD.
It defined global symbols for the termcap variables which it used.
o The other appeared in 4.4BSD Lite Release 2 (mid-1993) as part of
libedit (also known as the editline library). The 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.
The header file from libedit was added to NetBSD's termcap library in
mid-1994.
Meanwhile, GNU termcap was under development, starting in 1990. The
first release (termcap 1.0) in 1991 included a termcap.h header. The
second release (termcap 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. This was a difference versus
the original BSD termcap. The prototype for tputs also differed, but
in that instance, it was libedit which differed from BSD termcap.
A copy of GNU termcap 1.3 was bundled with bash in mid-1993, to support
the readline(3) library.
A termcap.h file was provided in ncurses 1.8.1 (November 1993). That
reflected influence by emacs(1) (rather than jove(1)) and GNU termcap:
o it provided declarations for a few global symbols used by emacs
o it provided function prototypes (using const).
o a prototype for tparam (a GNU termcap feature) was provided.
Later (in mid-1996) the tparam function was removed from ncurses. As a
result, there are differences between any of the four implementations,
which must be taken into account by programs which can work with all
termcap library interfaces.
If you call tgetstr to fetch ca 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
expand terminfo-style strings as terminfo does. (The tgoto function,
if configured to support termcap, checks if the string is indeed
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 sgr string. One
consequence is that termcap applications assume that me (equivalent to
terminfo's 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), putc(3), term_variables(3x), terminfo(5)
https://invisible-island.net/ncurses/tctest.html
ncurses 6.4 2023-10-14 curs_termcap(3x)