Provided by: ncurses-doc_6.3+20220423-2_all 
NAME
PC, UP, BC, ospeed, tgetent, tgetflag, tgetnum, tgetstr, tgoto, tputs - curses emulation
of termcap
SYNOPSIS
#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));
DESCRIPTION
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.
Initialization
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.
Capability Values
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.
Formatting Capabilities
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.
Global Variables
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.
Releasing Memory
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)).
RETURN VALUE
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.
BUGS
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.
PORTABILITY
Standards
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.
Compatibility with BSD Termcap
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.
Other Compatibility
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.
SEE ALSO
ncurses(3NCURSES), putc(3), terminfo_variables(3NCURSES), terminfo(5).
https://invisible-island.net/ncurses/tctest.html
termcap(3NCURSES)