termcap(3NCURSES) Library calls termcap(3NCURSES)
NAME
PC, UP, BC, ospeed, tgetent, tgetflag, tgetnum, tgetstr, tgoto, tputs -
curses emulation of termcap
SYNOPSIS
#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 ** sbuf);
char * tgoto(const char * cap, int col, int row);
int tputs(const char * str, int affcnt, int (* putc)(int));
DESCRIPTION
ncurses provides the foregoing variables and functions as a compatibil-
ity 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.
Initialization
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 ap-
plications to run), and
-1 if the terminfo database could not be found.
This implementation differs from those of historical termcap libraries.
• ncurses ignores the buffer pointer bp, as do other termcap implemen-
tations conforming to portions of X/Open Curses now withdrawn. The
BSD termcap library would store a copy of the terminal type descrip-
tion in the buffer referenced by this pointer. terminfo stores ter-
minal type descriptions in compiled form, which is not the same
thing.
• 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.
Retrieving Capability Values
tgetflag reports the Boolean entry for id, or zero if it is not avail-
able.
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 sbuf 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 sbuf is not NULL, and that
the pointer obtained by dereferencing sbuf is also not NULL. If ei-
ther check fails, ncurses ignores sbuf.
• If the checks succeed, ncurses also copies the return value to the
buffer pointed to by sbuf, and the library updates sbuf to point
past the null character terminating this value.
• The return value itself is an address in the terminal type descrip-
tion loaded into memory.
Applying String Capabilities
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 ar-
gument.
• 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).
• While tgoto is assumed to be used for the two-parameter cursor posi-
tioning capability, termcap applications also use it for single-pa-
rameter capabilities.
Doing so reveals a quirk in tgoto: most hardware terminals use cur-
sor 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.
• Normally the ncurses library is compiled without full termcap sup-
port. In that case, tgoto uses an internal version of
tparm(3NCURSES) (a more capable function).
Because it uses tparm internally, tgoto is able to use some terminfo
features, but not all. In particular, it allows only numeric para-
meters; tparm supports string parameters.
However, tparm is not a termcap feature, and portable termcap appli-
cations should not rely upon its availability.
tputs is described in terminfo(3NCURSES). It can retrieve capabilities
by either termcap or terminfo code.
Global Variables
tgetent sets the variables PC, UP, and BC to the terminfo entry's data
for pad_char (pad), cursor_up (cuu1), and backspace_if_not_bs (OTbs),
respectively. ncurses does not employ cuu1 internally.
delay_output(3NCURSES) uses pad, while tgoto emulation uses the obsolete
termcap capability bs, represented in ncurses terminfo as “OTbs”.
ncurses assigns the variable ospeed a system-specific value to encode
the terminal's data rate.
Releasing Memory
The termcap functions provide no means of freeing memory, because legacy
termcap implementations used only the storage provided by the caller via
tgetent and tgetstr. Those buffers are unused in terminfo.
By contrast, terminfo allocates memory. It uses setupterm(3NCURSES) to
obtain the data used by tgetent and the functions that retrieve capabil-
ity 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(3NCURSES),
because the pool is freed using other functions; see memleaks(3NCURSES).
RETURN VALUE
The return values of tgetent, tgetflag, tgetname, and tgetstr are docu-
mented above.
tgoto returns NULL on error. Error conditions include:
• uninitialized state (tgetent was not called successfully),
• cap being a null pointer,
• cap referring to a canceled capability,
• cap being a capability with string-valued parameters (a term-
info-only feature), and
• cap being a capability with more than two parameters.
See terminfo(3NCURSES) regarding tputs.
NOTES
ncurses compares only the first two characters of the id parameter of
tgetflag, tgetnum, and tgetstr to the capability names in the database.
PORTABILITY
These functions are no longer standardized (and the variables never
were); see section “HISTORY” below. ncurses provides them to support
legacy applications; they should not be used in new programs.
SVr4 describes a successful return value only as “an integer value other
than ERR”.
Neither X/Open Curses nor the SVr4 man pages documented the return val-
ues of tgetent correctly, though all three shown here were in fact re-
turned 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 Ver-
sion 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 develop-
ers. 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 ca-
pability 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 im-
plementations, does not store terminal type descriptions as text. It
sets the buffer contents to a null-terminated string.
Header File
This library includes a termcap.h header file for compatibility with
other implementations, but it is rarely used because the other implemen-
tations are not mutually compatible; see below.
HISTORY
Bill Joy originated a forerunner of termcap called “ttycap”, dated Sep-
tember 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.
• One was used internally by jove(1) from 4.3BSD onward. It declared
global symbols for the termcap variables that it used.
• 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) re-
lease 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:
• global symbols used by emacs,
• const-qualified function prototypes, and
• 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.
X/Open Curses Issue 4, Version 2 (1996), describes these functions,
marking them as “TO BE WITHDRAWN”.
X/Open Curses Issue 7 (2009) withdrew the termcap interface (along with
the vwprintw and vwscanw functions).
BUGS
If you call tgetstr to fetch column_address (ch) or any other parameter-
ized string capability, be aware that it is returned in terminfo nota-
tion, 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” parame-
ters 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.
• tputs("50") in a terminfo system transmits “50” rather than busy-
waiting for 50 milliseconds.
• 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 dig-
its 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) capa-
bility. 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.
SEE ALSO
]8;;https://invisible-island.net/ncurses/tctest.html\TCTEST — A Termcap Test Utility]8;;\
ncurses(3NCURSES), terminfo(3NCURSES), putc(3),
terminfo_variables(3NCURSES), terminfo(5)
ncurses 6.5 2025-02-01 termcap(3NCURSES)
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