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:
- •
- 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.
- •
- 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:
- •
- 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, the area parameter is ignored.
- •
- 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.
- •
- 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.
- •
- 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).
- •
- 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.
- •
- Normally the ncurses library is compiled with terminfo
support. In that case, tgoto uses tparm(3X) (a more capable
formatter).
- However, tparm is not a termcap feature, and
portable termcap applications should not rely upon its
availability.
The
tputs routine is described on the
terminfo(3NCURSES) 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
memleaks(3NCURSES)).
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.
If you call
tgetstr to fetch
ca or any other parameterized string,
be aware that it will be returned in terminfo notation, not the older and
not-quite-compatible termcap notation. This will not cause problems if all you
do with it is call
tgoto or
tparm, which both expand
terminfo-style strings as terminfo. (The
tgoto function, if configured
to support termcap, will check if the string is indeed terminfo-style by
looking for "%p" parameters or "$<..>" delays, and
invoke a termcap-style parser if the string does not appear to be terminfo).
Because terminfo conventions for representing padding in string capabilities
differ from termcap's, users can be surprised:
- •
-
tputs("50") in a terminfo system will put
out a literal “50” rather than busy-waiting for 50
milliseconds.
- •
- However, if ncurses is configured to support termcap, it
may also have been configured to support the 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 wait
for 50 milliseconds rather than put out a literal “50”
Note that termcap has nothing analogous to terminfo's
sgr string. One
consequence of this is that termcap applications assume
me (terminfo
sgr0) does not reset the alternate character set. This implementation
checks for, and modifies the data shown to the termcap interface to
accommodate termcap's limitation in this respect.
These functions are provided for supporting legacy applications, and should not
be used in new programs:
- •
- The XSI Curses standard, Issue 4 describes these functions.
However, they are marked TO BE WITHDRAWN and may be removed in future
versions.
- •
- 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:
- •
- 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.
- •
- 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:
- •
- One was used internally by the jove editor in 2BSD
through 4.4BSD. It defined global symbols for the termcap variables which
it used.
- •
- 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:
- •
- it provided declarations for a few global symbols used by
emacs
- •
- it provided function prototypes (using const).
- •
- 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.
ncurses(3NCURSES),
putc(3),
terminfo_variables(3NCURSES),
terminfo(5).
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