Ubuntu Manpage: PC, UP, BC, ospeed, tgetent, tgetflag, tgetnum, tgetstr, tgoto, tputs

Provided by: ncurses-doc_6.4+20240113-1ubuntu2_all

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 **area);
       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 compatibility 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 applications 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 implementations
conforming to portions of X/Open Curses now withdrawn. The BSD termcap library
would store a copy of the terminal type description in the area referenced by this
pointer. ncurses stores terminal 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 available.

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 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, area is ignored.

• If the checks succeed, ncurses also copies the return value to the buffer pointed
to by area, and the library updates area to point past the null character
terminating this value.

• The return value itself is an address in the terminal type description 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 argument.

• 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 positioning capability,
termcap applications also use it for single-parameter capabilities.

Doing so reveals 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 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 support. 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 parameters; tparm supports string
parameters.

However, tparm is not a termcap feature, and portable termcap applications should not
rely upon its availability.

tputs is described in terminfo(3NCURSES). It can retrieve capabilities by either termcap
or terminfo code.

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
by delay_output(3NCURSES). BC is used by tgoto emulation. The variable ospeed is set by
ncurses using a system-specific encoding to indicate the terminal's data rate.

Releasing Memory
The termcap functions provide no means of 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.

By contrast, terminfo allocates memory. It uses setupterm(3NCURSES) to obtain the data
used by tgetent and the functions that retrieve capability 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 documented 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 terminfo-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); ncurses
provides them to support legacy applications. They should not be used in new programs.

Standards
• X/Open Curses, Issue 4, Version 2 (1996), describes these functions, marking them as
“TO BE WITHDRAWN”.

• X/Open Curses, Issue 7 (2009) marks the termcap interface (along with vwprintw and
vwscanw) as withdrawn.

Neither X/Open Curses nor the SVr4 man pages documented the return values of tgetent
correctly, though all three shown here were in fact returned 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,
Version 2 rather than in ncurses.

Compatibility with BSD termcap
Externally visible variables are provided for support of 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 used escapes
many application developers. The BSD termcap library did not require a trailing null
character on 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 capability 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 implementations, 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 for compatibility with other implementations, but
the header is rarely used because the other implementations are not strictly compatible.

HISTORY

       Bill Joy originated a forerunner of termcap called “ttycap”,  dated  September  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  though  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 edit‐
           line library.  It declared function prototypes, but no global variables.  This  header
           file was added to NetBSD's termcap library in mid-1994.

       Meanwhile,  GNU  termcap  began  development  in  1990.   Its  first release (1.0) in 1991
       included a termcap.h header.  Its second (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.  BSD termcap did not.  The prototype for tputs also differed, but in that
       instance, it was libedit that differed from BSD termcap.

       GNU termcap 1.3 was bundled with bash(1) in mid-1993 to support the readline(3) library.

       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.

BUGS

If you call tgetstr to fetch column_address (ch) 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. (If
ncurses is configured to support termcap, tgoto checks whether the string is term‐
info-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.

• 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 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 set_attributes (sgr) capability. 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.