man/curs_termcap.3x

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.\" $Id: curs_termcap.3x,v 1.85 2024/04/20 19:13:12 tom Exp $
.TH curs_termcap 3X 2024-04-20 "ncurses @NCURSES_MAJOR@.@NCURSES_MINOR@" "Library calls"
.ie \n(.g \{\
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.el \{\
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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 **\fIarea\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
1
on success,
.TP
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
\-1
if the \fI\%term\%info\fP database could not be found.
.RE
.PP
This implementation differs from those of historical \fItermcap\fP
libraries.
.RS 3
.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X/Open Curses now withdrawn.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 area 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.
.RE
.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
.B NULL
if it is not available.
Use \fB\%tputs\fP to output the string returned.
The
.I area
parameter is used as follows.
.RS 3
.bP
It is assumed to be the address of a pointer to a buffer managed by the
calling application.
.bP
However,
\fI\%ncurses\fP checks to ensure that
.I area
is not
.BR NULL ,
and also that the resulting buffer pointer is not
.BR NULL .
If either check fails,
.I area
is ignored.
.bP
If the checks succeed,
\fI\%ncurses\fP also copies the return value to the buffer pointed to by
\fIarea\fP,
and the library updates
.I area
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.
.RE
.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"
The variables
\fBPC\fP,
\fBUP\fP and
\fBBC\fP
are set by \fB\%tgetent\fP to the \fI\%term\%info\fP entry's data for
\fB\%pad_char\fP,
\fB\%cursor_up\fP and
\fB\%backspace_if_not_bs\fP,
respectively.
\fBUP\fP is not used by \fI\%ncurses\fP.
\fBPC\fP is used by \fB\%delay_output\fP(3X).
\fBBC\fP is used by \fB\%tgoto\fP emulation.
The variable \fB\%ospeed\fP is set by \fI\%ncurses\fP using a
system-specific encoding to indicate 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 buffer
areas 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
.B NULL
on error.
Error conditions include:
.bP
uninitialized state
(\fB\%tgetent\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);
\fI\%ncurses\fP provides them to support legacy applications.
They should not be used in new programs.
.SS Standards
.bP
X/Open Curses, Issue 4, Version 2 (1996),
describes these functions,
marking them as
\*(``TO BE WITHDRAWN\*(''.
.bP
X/Open Curses, Issue 7 (2009) marks the \fItermcap\fP interface
(along with \fB\%vwprintw\fP and \fB\%vwscanw\fP) as withdrawn.
.PP
Neither X/Open Curses nor the SVr4 man pages documented the return
values of \fB\%tgetent\fP 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 \fB\%tgetent\fP returns \fBOK\fP or \fBERR\fP.
Because the purpose of these functions is to provide compatibility with
the \fItermcap\fP library,
that is a defect in X/Open Curses, Issue 4, Version 2
rather than in \fI\%ncurses\fP.
.SS "Compatibility with BSD \fItermcap\fP"
Externally visible variables are provided for support of certain
\fItermcap\fP 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 \fB\%ospeed\fP.
.PP
The constraint that only the first two characters of the \fIid\fP
parameter are used escapes many application developers.
The BSD \fItermcap\fP library did not require a trailing null character
on the capability identifier passed to \fB\%tgetstr\fP,
\fB\%tgetnum\fP,
and
\fB\%tgetflag\fP.
.\" See <https://minnie.tuhs.org/cgi-bin/utree.pl?file=2BSD/src/\
.\"   termlib/termcap.c>.
Some applications thus assume that the \fItermcap\fP interface does not
require the trailing null character for the capability identifier.
.bP
.I \%ncurses
disallows matches by the \fItermcap\fP interface against extended
capability names that are longer than two characters;
see \fB\%user_caps\fP(5).
.PP
The BSD \fItermcap\fP function \fB\%tgetent\fP returns the text of a
\fItermcap\fP entry in the buffer passed as an argument.
This library,
like other \fI\%term\%info\fP implementations,
does not store terminal type descriptions as text.
It sets the buffer contents to a null-terminated string.
.SS "Header File"
This library includes a \fI\%termcap.h\fP header for compatibility with
other implementations,
but the header is rarely used because the other implementations are not
strictly compatible.
.SH HISTORY
.\" See https://www.oreilly.com/openbook/opensources/book/kirkmck.html
.\" for much BSD release history.
Bill Joy originated a forerunner of \fItermcap\fP 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 \fItermcap\fP,
such as
\fB\%tgetent\fP,
\fB\%tgetflag\fP,
\fB\%tgetnum\fP,
and
\fB\%tgetstr\fP.
.PP
A clear descendant,
the \fItermlib\fP library,
.\" https://minnie.tuhs.org/cgi-bin/utree.pl?file=2BSD/src/termlib/
followed in 2BSD
(May 1979),
adding \fB\%tgoto\fP and \fB\%tputs\fP.
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 \fI\%termlib\fP 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 \fItermlib\fP to \fItermcap\fP
.\" ...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 though 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 \fI\%termcap.h\fP header files
over time.
.bP
One was used internally by \fBjove\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 \fItermcap\fP variables that it used.
.bP
The other appeared in 4.4BSD-Lite Release 2
(June 1995)
as part of \fIlibedit\fP
(also known as the \fI\%edit\%line\fP library).
CSRG source history shows that this was added in mid-1992.
The \fIlibedit\fP header file was used internally as a convenience for
compiling the \fI\%edit\%line\fP library.
It declared function prototypes,
but no global variables.
This header file was added to NetBSD's \fItermcap\fP library in
mid-1994.
.PP
Meanwhile,
GNU \fItermcap\fP began development in 1990.
Its first release (1.0) in 1991 included a \fI\%termcap.h\fP header.
Its second (1.1) in September 1992 modified the header to use
\fIconst\fP for the function prototypes in the header where one would
expect the parameters to be read-only.
BSD \fItermcap\fP did not.
The prototype for \fB\%tputs\fP also differed,
but in that instance,
it was \fIlibedit\fP that differed from BSD \fItermcap\fP.
.PP
GNU \fItermcap\fP 1.3 was bundled with \fIbash\fP(1) in mid-1993 to
support the \fI\%readline\fP(3) library.
.PP
\fI\%ncurses\fP 1.8.1
(November 1993)
provided a \fI\%termcap.h\fP file.
It reflected influence from GNU \fItermcap\fP and \fBemacs\fP(1)
(rather than \fBjove\fP(1)),
providing the following interface:
.bP
global symbols used by \fIemacs\fP,
.bP
\fIconst\fP-qualified function prototypes,
and
.bP
a prototype for \fBtparam\fP,
a GNU \fItermcap\fP feature.
.PP
Later
(in mid-1996)
the \fB\%tparam\fP function was removed from \fI\%ncurses\fP.
Any two of the four implementations thus differ,
and programs that intend to work with all \fItermcap\fP library
interfaces must account for that fact.
.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 \(bu 4
However,
if \fI\%ncurses\fP is configured to support \fItermcap\fP,
it may also have been configured to support BSD-style padding.
.IP
In that case,
\fB\%tputs\fP inspects strings passed to it,
looking for digits at the beginning of the string.
.IP
\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 checks for,
and modifies the data shared with,
the \fItermcap\fP interface to accommodate the latter's limitation in
this respect.
.SH "SEE ALSO"
\fB\%curses\fP(3X),
\fB\%curs_terminfo\fP(3X),
\fB\%putc\fP(3),
\fB\%term_variables\fP(3X),
\fB\%terminfo\fP(5)
.PP
https://invisible\-island.net/ncurses/tctest.html