ncurses/man/curs_termcap.3x

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.\" $Id: curs_termcap.3x,v 1.107 2025/08/16 19:09:12 tom Exp $
.TH curs_termcap 3X 2025-08-16 "ncurses @NCURSES_MAJOR@.@NCURSES_MINOR@" "Library calls"
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.SH NAME
\fB\%PC\fP,
\fB\%UP\fP,
\fB\%BC\fP,
\fB\%ospeed\fP,
\fB\%tgetent\fP,
\fB\%tgetflag\fP,
\fB\%tgetnum\fP,
\fB\%tgetstr\fP,
\fB\%tgoto\fP,
\fB\%tputs\fP \-
\fIcurses\fR emulation of \fItermcap\fR
.SH SYNOPSIS
.nf
\fB#include <curses.h>
\fB#include <term.h>
.PP
\fBchar PC;
\fBchar * UP;
\fBchar * BC;
\fB@NCURSES_OSPEED@ ospeed;
.PP
\fBint tgetent(char * \fIbp\fP, const char * \fIname\fP);
\fBint tgetflag(const char * \fIid\fP);
\fBint tgetnum(const char * \fIid\fP);
\fBchar * tgetstr(const char * \fIid\fP, char ** \fIsbuf\fP);
\fBchar * tgoto(const char * \fIcap\fP, int \fIcol\fP, int \fIrow\fP);
\fBint tputs(const char * \fIstr\fP, int \fIaffcnt\fP, int (* \fIputc\fP)(int));
.fi
.SH DESCRIPTION
.I \%ncurses
provides the foregoing variables and functions as a compatibility layer
for programs that use the \fItermcap\fP library.
The API is the same,
but behavior is emulated using the \fI\%term\%info\fP database.
Thus,
it can be used only to query the capabilities of terminal database
entries for which a \fI\%term\%info\fP entry has been compiled.
.SS Initialization
\fB\%tgetent\fP loads the terminal database entry for \fIname\fP;
see \fBterm\fP(7).
This must be done before calling any of the other functions.
It returns
.RS 3
.TP 5 \" "-1" + 2n + adjust for PDF
.B 1
on success,
.TP
.B 0
if there is no such entry
(or if the matching entry describes a generic terminal,
having too little information for
.I curses
applications to run),
and
.TP
.B \-1
if the
.I \%term\%info
database could not be found.
.RE
.PP
This implementation differs from those of historical \fItermcap\fP
libraries.
.bP
.I \%ncurses
ignores the buffer pointer \fIbp\fP,
as do other \fItermcap\fP implementations conforming to portions of
X/Open Curses now withdrawn.
The BSD \fItermcap\fP library would store a copy of the terminal type
description in the buffer referenced by this pointer.
\fI\%term\%info\fP stores terminal type descriptions in compiled form,
which is not the same thing.
.bP
The meanings of the return values differ.
The BSD \fItermcap\fP library does not check whether the terminal type
description includes the
.B \%generic
.RB ( gn )
capability,
nor whether the terminal type description supports an addressable
cursor,
a property essential for any \fIcurses\fP implementation to operate.
.SS "Retrieving Capability Values"
\fB\%tgetflag\fP reports the Boolean entry for \fIid\fP,
or zero if it is not available.
.PP
\fB\%tgetnum\fP obtains the numeric entry for \fIid\fP,
or \-1 if it is not available.
.PP
\fB\%tgetstr\fP returns the string entry for \fIid\fP,
or
.I NULL
if it is not available.
Use \fB\%tputs\fP to output the string returned.
The
.I sbuf
parameter is used as follows.
.bP
It is assumed to be the address of a pointer to a buffer managed by the
calling application.
.bP
However,
.I \%ncurses
checks to ensure that
.I sbuf
is not
.IR NULL ","
and that the pointer obtained by dereferencing
.I sbuf
is also not
.IR NULL "."
If either check fails,
.I \%ncurses
ignores
.IR sbuf .
.bP
If the checks succeed,
\fI\%ncurses\fP also copies the return value to the buffer pointed to by
\fIsbuf\fP,
and the library updates
.I sbuf
to point past the null character terminating this value.
.bP
The return value itself is an address in the terminal type description
loaded into memory.
.SS "Applying String Capabilities"
String capabilities can be parameterized;
see subsection \*(``Parameterized Strings\*('' in  \fB\%terminfo\fP(5).
\fB\%tgoto\fP applies its second and third arguments to the parametric
placeholders in the capability stored in the first argument.
.bP
The capability may contain padding specifications;
see subsection \*(``Delays and Padding\*('' of \fB\%terminfo\fP(5).
The output of \fB\%tgoto\fP should thus be passed to \fB\%tputs\fP
rather than some other output function such as \fI\%printf\fP(3).
.bP
While \fB\%tgoto\fP is assumed to be used for the two-parameter
cursor positioning capability,
\fItermcap\fP applications also use it for single-parameter
capabilities.
.IP
Doing so reveals a quirk in \fB\%tgoto\fP:
most hardware terminals use cursor addressing with \fIrow\fP first,
but the original developers of the \fItermcap\fP interface chose to
put the \fIcol\fP (column) parameter first.
The \fB\%tgoto\fP 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.
.bP
Normally the \fI\%ncurses\fP library is compiled without
full \fI\%termcap\fP support.
In that case,
\fB\%tgoto\fP uses an internal version of \fB\%tparm\fP(3X)
(a more capable function).
.IP
Because it uses \fB\%tparm\fP internally,
\fB\%tgoto\fP is able to use some \fI\%term\%info\fP features,
but not all.
In particular,
it allows only numeric parameters;
\fB\%tparm\fP supports string parameters.
.IP
However,
\fB\%tparm\fP is not a \fItermcap\fP feature,
and portable \fItermcap\fP applications should not rely upon its
availability.
.PP
\fB\%tputs\fP is described in \fB\%curs_terminfo\fP(3X).
It can retrieve capabilities by either \fItermcap\fP or
\fI\%term\%info\fP code.
.SS "Global Variables"
.B \%tgetent
sets the variables
.BR PC ","
.BR UP ","
and
.B BC
to the
.I \%term\%info
entry's data for
.B \%pad_char
.RB ( pad ),
.B \%cursor_up
.RB ( cuu1 ),
and
.B \%backspace_if_not_bs
.RB ( OTbs ),
respectively.
.I \%ncurses
does not employ
.B cuu1
internally.
\fB\%delay_output\fP(3X)
uses
.BR pad ","
while
.B \%tgoto
emulation uses the obsolete
.I termcap
capability
.BR bs ","
represented in
.I \%ncurses
.I \%term\%info
as \*(``OTbs\*(''.
.I \%ncurses
assigns the variable
.B \%ospeed
a system-specific value to encode the terminal's data rate.
.SS "Releasing Memory"
The \fItermcap\fP functions provide no means of freeing memory,
because legacy \fItermcap\fP implementations used only the storage
provided by the caller via \fB\%tgetent\fP and \fB\%tgetstr\fP.
Those buffers are unused in \fI\%term\%info\fP.
.PP
By contrast,
\fI\%term\%info\fP allocates memory.
It uses \fB\%setupterm\fP(3X) to obtain the data used by \fB\%tgetent\fP
and the functions that retrieve capability values.
One could use
.RS
.EX
del_curterm(cur_term);
.EE
.RE
to free this memory,
but there is an additional complication with \fI\%ncurses\fP.
It uses a fixed-size pool of storage locations,
one per value of the terminal name parameter given to \fB\%tgetent\fP.
The \fIscreen\fP(1) program relies upon this arrangement to improve its
performance.
.PP
An application that uses only the \fItermcap\fP functions,
not the higher-level
.I \%curses
API,
could release the memory using \fB\%del_curterm\fP(3X),
because the pool is freed using other functions;
see \fB\%curs_memleaks\fP(3X).
.SH "RETURN VALUE"
The return values of
\fB\%tgetent\fP,
\fB\%tgetflag\fP,
\fB\%tgetname\fP,
and
\fB\%tgetstr\fP
are documented above.
.PP
\fB\%tgoto\fP returns
.I NULL
on error.
Error conditions include:
.bP
uninitialized state
\%(\fBtgetent\fP was not called successfully),
.bP
.I cap
being a null pointer,
.bP
.I cap
referring to a canceled capability,
.bP
.I cap
being a capability with string-valued parameters
(a \fI\%term\%info\fP-only feature),
and
.bP
.I cap
being a capability with more than two parameters.
.PP
See \fB\%curs_terminfo\fP(3X) regarding \fB\%tputs\fP.
.SH NOTES
\fI\%ncurses\fP compares only the first two characters of the \fIid\fP
parameter of
\fB\%tgetflag\fP,
\fB\%tgetnum\fP,
and
\fB\%tgetstr\fP to the capability names in the database.
.SH PORTABILITY
These functions are no longer standardized
(and the variables never were);
see section \*(``HISTORY\*('' below.
.I \%ncurses
provides them to support legacy applications;
they should not be used in new programs.
.PP
SVr4 describes a successful return value only as
\*(``an integer value other than
.IR ERR \*(''. \" Courier roman in source; SVID 4, vol. 3, p. 536
.PP
Neither X/Open Curses nor the SVr4 man pages documented the return
values of
.I \%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
.I \%tgetent
returns
.I OK
or
.IR ERR "."
Because the purpose of these functions is to provide compatibility with
the
.I termcap
library,
that is a defect in X/Open Curses Issue\ 4 Version\ 2
rather than in
.IR \%ncurses "."
.SS "Compatibility with BSD \fItermcap\fP"
.I \%ncurses
provides externally visible variables to support certain
.I 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
.IR \%ospeed "."
.PP
The constraint that only the first two characters of the
.I id
parameter are looked up in the terminal database
escapes many application developers.
The BSD
.I termcap
library did not require a trailing null character
after the capability identifier passed to
.IR \%tgetstr ","
.IR \%tgetnum ","
and
.IR \%tgetflag "."
.\" See <https://minnie.tuhs.org/cgi-bin/utree.pl?file=2BSD/src/\
.\"   termlib/termcap.c>.
Some applications thus assume that the
.I termcap
interface does not require the trailing null character
for the capability identifier.
.I \%ncurses
disallows matches by the
.I termcap
interface against extended capability names
that are longer than two characters;
see \fB\%user_caps\fP(5).
.PP
The BSD
.I termcap
function
.I \%tgetent
returns the text of a
.I termcap
entry in the buffer passed as an argument.
.IR \%ncurses ","
like other
.I \%term\%info
implementations,
does not store terminal type descriptions as text.
It sets the buffer contents to a null-terminated string.
.SS "Header File"
.I \%ncurses
includes a
.I \%termcap.h
header file for compatibility with other implementations,
but it is rarely used because the other implementations
are not mutually compatible;
see below.
.SH HISTORY
.\" See https://www.oreilly.com/openbook/opensources/book/kirkmck.html
.\" for much BSD release history.
Bill Joy originated a forerunner of
.I termcap
called \*(``ttycap\*('',
dated September 1977,
and released in 1BSD
(March 1978).
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=1BSD/s7/ttycap.c
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=1BSD/man7/ttycap.7
It used many of the same function names as the later
.IR termcap ","
such as
.IR \%tgetent ","
.IR \%tgetflag ","
.IR \%tgetnum ","
and
.IR \%tgetstr "."
.PP
A clear descendant,
the
.I termlib
library,
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=2BSD/src/termlib/
followed in 2BSD
(May 1979),
adding
.I \%tgoto
and
.IR \%tputs "."
The former applied at that time only to cursor positioning capabilities,
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=2BSD/bin/etc/termcap
thus the overly specific name.
Little changed in 3BSD
(late 1979)
except the addition of test programs and a
.I termlib
man page,
which documented the API shown in section \*(``SYNOPSIS\*('' above.
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=3BSD/usr/src/lib/\
.\"   libtermlib/
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=3BSD/usr/man/man3/\
.\"   termlib.3
.PP
4BSD (November 1980)
renamed
.I termlib
to
.I termcap
.\" ...except in the source tree...
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=4BSD/usr/src/lib/\
.\"   libtermlib/makefile
and added another test program.
The library remained much the same through 4.3BSD
(June 1986).
4.4BSD-Lite
(June 1994)
refactored it,
.\" Observe the `tncktc()`, `tnamatch()`, `tskip()`, and `tdecode()`
.\" entry points disappearing from termcap.c.
leaving the API unchanged.
.PP
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
.I \%termcap.h
header files over time.
.bP
One was used internally by \fIjove\fP(1) from 4.3BSD onward.
.\" 2BSD became a branch retaining support for non-virtual memory
.\" systems (such as the PDP-11) whereas most BSD development focused on
.\" the VAX and other VM-enabled systems starting with 3BSD.
.\"
.\" This man page previously located a termcap.h in 2BSD, but that may
.\" be confusion arising from its backport to 2.9BSD (and still present
.\" in surviving sources for 2.11BSD, the "end of the line" for that
.\" branch's development).
.\"
.\" Observe the copyright notice in
.\"   https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD/usr/contrib/\
.\"     jove/Makefile
.\" --much too late for 2BSD (1979).
It declared global symbols for the
.I termcap
variables that it used.
.bP
The other appeared in 4.4BSD-Lite Release 2
(June 1995)
as part of
.I libedit
(also known as the
.I \%edit\%line
library).
CSRG source history shows that this was added in mid-1992.
The
.I libedit
header file was used internally as a convenience
for compiling the
.I \%edit\%line
library.
It declared function prototypes,
but no global variables.
NetBSD's
.I termcap
library added this header file in mid-1994.
.PP
Meanwhile,
GNU
.I termcap
began development in 1990.
Its first release (1.0) in 1991 included a
.I \%termcap.h
header file.
Its second (1.1) release in September 1992 modified the file to use
.I const
for the function prototypes in the header where one would
expect parameters to be read-only.
BSD
.I termcap
did not.
The prototype for
.I \%tputs
also differed,
but in that instance,
it was
.I libedit
that differed from BSD
.IR termcap "."
.PP
GNU \fIbash\fP(1) has bundled GNU
.I termcap
1.3 since mid-1993 to support its \fI\%readline\fP(3) library,
and continues to use it if configured to do so.
.PP
.I \%ncurses
1.8.1
(November 1993)
provided a
.I \%termcap.h
file.
It reflected influence from GNU
.I termcap
and \fI\%emacs\fP(1)
(rather than \fIjove\fP(1)),
providing the following interface:
.bP
global symbols used by
.IR \%emacs ","
.bP
.IR const -qualified
function prototypes,
and
.bP
a prototype for
.IR tparam ","
a GNU
.I termcap
feature.
.PP
Later
(in mid-1996)
the
.I tparam
function was removed from
.IR \%ncurses "."
Any two of the four implementations thus differ,
and programs that intend to work with all
.I termcap
library interfaces must account for that fact.
.PP
X/Open Curses Issue\ 4,
Version\ 2 (1996),
describes these functions,
marking them as
\*(``TO BE WITHDRAWN\*(''.
.PP
X/Open Curses Issue\ 7 (2009) withdrew the
.I termcap
interface
(along with the
.I \%vwprintw
and
.I \%vwscanw
functions).
.SH BUGS
If you call \fB\%tgetstr\fP to fetch
.B \%column_address
.RB ( ch )
or any other parameterized string capability,
be aware that it is returned in \fI\%term\%info\fP notation,
not the older and not-quite-compatible \fItermcap\fP notation.
This does not cause problems if all you do with it is call \fB\%tgoto\fP
or \fB\%tparm\fP,
which both parametrically expand \fI\%term\%info\fP-style string
capabilities as \fI\%term\%info\fP does.
(If
.I \%ncurses
is configured to support \fItermcap,\fP
\fB\%tgoto\fP checks whether the string is \fI\%term\%info\fP-style by
looking for \*(``\fB%p\fP\*('' parameters or
\*(``\fB<\fP.\|.\|.\fB>\fP\*('' delays,
and invokes a \fItermcap\fP-style parser if the string appears not to
use \fI\%term\%info\fP syntax.)
.PP
Because \fI\%term\%info\fP's syntax for padding in string capabilities
differs from \fItermcap\fP's,
users can be surprised.
.IP \(bu 4
\fB\%tputs("50")\fP in a \fI\%term\%info\fP system transmits
\*(``50\*('' rather than busy-waiting for 50 milliseconds.
.IP
However,
if \fI\%ncurses\fP is configured to support \fItermcap\fP,
it may also have been configured to support BSD-style padding.
In that case,
\fB\%tputs\fP inspects strings passed to it,
looking for digits at the beginning of the string.
.IP \(bu 4
\fB\%tputs("50")\fP in a \fItermcap\fP system may busy-wait for 50
milliseconds rather than transmitting \*(``50\*(''.
.PP
\fItermcap\fP has nothing analogous to \fI\%term\%info\fP's
.B \%set_attributes
.RB ( sgr )
capability.
One consequence is that \fItermcap\fP applications assume that
.RB \*(`` me \*(''
(equivalent to \fI\%term\%info\fP's
.B \%exit_attribute_mode
.RB ( sgr0 )
capability)
does not reset the alternate character set.
\fI\%ncurses\fP inspects the data shared with the
.I termcap
interface and modifies it as necessary
to accommodate the latter's limitation in this respect.
.SH "SEE ALSO"
.UR https://\*:invisible\-\*:island\*:.net/\*:ncurses/\*:tctest\*:.html
.I "TCTEST \(em A Termcap Test Utility"
.UE
.PP
\fB\%curses\fP(3X),
\fB\%curs_terminfo\fP(3X),
\fB\%putc\fP(3),
\fB\%term_variables\fP(3X),
\fB\%terminfo\fP(5)