PC, UP, BC, ospeed, tgetent, tgetflag,
tgetnum, tgetstr, tgoto, tputs - curses
emulation of termcap
#include <ncurses/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
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.
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.