ncurses 6.4 - patch 20231014

[ncurses.git] / man / curs_termcap.3x

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.\" $Id: curs_termcap.3x,v 1.70 2023/10/14 19:26:28 tom Exp $
.TH curs_termcap 3X 2023-10-14 "ncurses 6.4" "Library calls"
.ie \n(.g \{\
.ds `` \(lq
.ds '' \(rq
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.el \{\
.ie t .ds `` ``
.el   .ds `` ""
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.de bP
.ie n  .IP \(bu 4
.el    .IP \(bu 2
..
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.ds n 5
.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
\fBextern char PC;
\fBextern char * UP;
\fBextern char * BC;
\fBextern @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
These routines are included as a conversion aid for programs that use
the \fItermcap\fP library.
Their parameters are the same, but the
routines are emulated using the \fIterminfo\fP database.
Thus, they
can only be used to query the capabilities of entries for which a
terminfo entry has been compiled.
.SS Initialization
The \fBtgetent\fP routine loads the entry for \fIname\fP.
It returns:
.RS 3
.TP 3
1
on success,
.TP 3
0
if there is no such entry
(or that it is a generic type, having too little information for curses
applications to run), and
.TP 3
\-1
if the terminfo database could not be found.
.RE
.PP
This differs from the \fItermcap\fP library in two ways:
.RS 3
.bP
The emulation ignores the buffer pointer \fIbp\fP.
The \fItermcap\fP 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.
.bP
There is a difference in return codes.
The \fItermcap\fP library does not check if the terminal
description is marked with the \fIgeneric\fP capability,
or if the terminal description has cursor-addressing.
.RE
.SS Capability Values
The \fBtgetflag\fP routine gets the boolean entry for \fIid\fP,
or zero if it is not available.
.PP
The \fBtgetnum\fP routine gets the numeric entry for \fIid\fP,
or \-1 if it is not available.
.PP
The \fBtgetstr\fP routine returns the string entry for \fIid\fP,
or zero if it is not available.
Use \fBtputs\fP to output the returned string.
The \fIarea\fP 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, ncurses checks to ensure that \fBarea\fP is not NULL,
and also that the resulting buffer pointer is not NULL.
If either check fails, the \fIarea\fP parameter is ignored.
.bP
If the checks succeed, ncurses also copies the return value to
the buffer pointed to by \fIarea\fP,
and the \fIarea\fP value will be updated to point past the null ending
this value.
.bP
The return value itself is an address in the terminal description which
is loaded into memory.
.RE
.PP
Only the first two characters of the \fBid\fP parameter of
\fBtgetflag\fP,
\fBtgetnum\fP and
\fBtgetstr\fP are compared in lookups.
.SS Formatting Capabilities
The \fBtgoto\fP routine expands the given capability using the parameters.
.bP
Because the capability may have padding characters,
the output of \fBtgoto\fP should be passed to \fBtputs\fP
rather than some other output function such as \fBprintf\fP(3).
.bP
While \fBtgoto\fP is assumed to be used for the two-parameter
cursor positioning capability,
termcap applications also use it for single-parameter capabilities.
.IP
Doing this shows a quirk in \fBtgoto\fP: most hardware
terminals use cursor addressing with \fIrow\fP first,
but the original developers of the termcap interface chose to
put the \fIcolumn\fP parameter first.
The \fBtgoto\fP 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.
.bP
Normally the ncurses library is compiled with terminfo support.
In that case, \fBtgoto\fP uses an internal version of
\fBtparm\fP(3X) (a more capable formatter).
.IP
With terminfo support, \fBtgoto\fP is able to use some of the terminfo
features, but not all.
In particular, it allows only numeric parameters;
\fBtparm\fP supports string parameters.
.IP
However, \fBtparm\fP is not a \fItermcap\fP feature,
and portable \fItermcap\fP applications should not rely upon its availability.
.PP
The \fBtputs\fP routine is described on the \fBcurs_terminfo\fP(3X) manual
page.
It can retrieve capabilities by either termcap or terminfo name.
.SS Global Variables
The variables
\fBPC\fP,
\fBUP\fP and
\fBBC\fP
are set by \fBtgetent\fP to the terminfo entry's data for
\fBpad_char\fP,
\fBcursor_up\fP and
\fBbackspace_if_not_bs\fP,
respectively.
\fBUP\fP is not used by ncurses.
\fBPC\fP is used in the \fBtdelay_output\fP function.
\fBBC\fP is used in the \fBtgoto\fP emulation.
The variable \fBospeed\fP is set by ncurses in a system-specific coding
to reflect the terminal speed.
.SS 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 \fBtgetent\fP and \fBtgetstr\fP.
Those buffers are unused in terminfo.
.PP
On the other hand, terminfo allocates memory.
It uses \fBsetupterm\fP to retrieve the data used by \fBtgetent\fP
and the functions which return capability values such as \fBtgetstr\fP.
One could use
.sp
        \fBdel_curterm(cur_term);\fP
.sp
.PP
to free this memory, but there is an additional complication with ncurses.
It uses a fixed-size \fIpool\fP of storage locations,
one per setting of the \fBTERM\fP variable when \fBtgetent\fP is called.
The \fBscreen\fP(1) program relies upon this arrangement,
to improve its performance.
.PP
An application which uses only the low-level termcap functions could
free the memory using \fBdel_curterm\fP,
because the pool is freed using other functions
(see \fBcurs_memleaks\fP(3X)).
.
.SH RETURN VALUE
Except where explicitly noted,
routines that return an integer return \fBERR\fP upon failure and \fBOK\fP
(SVr4 only specifies "an integer value other than \fBERR\fP") upon successful
completion.
.PP
Routines that return pointers return \fBNULL\fP on error.
.PP
A few special cases apply:
.bP
If the terminal database has not been initialized,
these return an error.
.bP
The calls with a string parameter (\fBtgoto\fP, \fBtputs\fP)
check if the string is null, or cancelled.
Those return an error.
.bP
A call to \fBtgoto\fP using a capability with string parameters is an error.
.bP
A call to \fBtgoto\fP using a capability with more than two parameters
is an error.
.SH PORTABILITY
.SS Standards
These functions are provided for supporting legacy applications,
and should not be used in new programs:
.bP
The XSI Curses standard, Issue 4 describes these functions.
However, they
are marked TO BE WITHDRAWN and may be removed in future versions.
.bP
X/Open Curses, Issue 5 (December 2007) marked the termcap interface
(along with \fBvwprintw\fP and \fBvwscanw\fP) as withdrawn.
.PP
Neither the XSI Curses standard nor the SVr4 man pages documented the return
values of \fBtgetent\fP 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 \fBtgetent\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 XCurses, Issue 4, Version 2
rather than in ncurses.
.SS 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 \fBospeed\fP.
.PP
The comment that only the first two characters of the \fBid\fP 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 \fBtgetstr\fP, \fBtgetnum\fP and \fBtgetflag\fP.
Some applications assume that the termcap interface does not require
the trailing NUL for the parameter name.
Taking into account these issues:
.bP
As a special case,
\fBtgetflag\fP 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.
.bP
This implementation disallows matches by the termcap interface against
extended capability names which are longer than two characters.
.PP
The BSD termcap function \fBtgetent\fP 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.
.SS 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.
.PP
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:
.bP
One was used internally by the \fBjove\fP editor in 2BSD through 4.4BSD.
It defined global symbols for the termcap variables which it used.
.bP
The other appeared in 4.4BSD Lite Release 2 (mid-1993)
as part of \fIlibedit\fP (also known as the \fIeditline\fP library).
The 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 \fIeditline\fP library.
It declared function prototypes, but no global variables.
.PP
The header file from \fIlibedit\fP was added to NetBSD's termcap
library in mid-1994.
.PP
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 \fBconst\fP 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 \fBtputs\fP also differed,
but in that instance, it was \fIlibedit\fP which differed from BSD termcap.
.PP
A copy of GNU termcap 1.3 was bundled with \fIbash\fP in mid-1993,
to support the \fBreadline\fP(3) library.
.PP
A termcap.h file was provided in ncurses 1.8.1 (November 1993).
That reflected influence by \fBemacs\fP(1) (rather than \fBjove\fP(1))
and GNU termcap:
.bP
it provided declarations for a few global symbols used by \fBemacs\fP
.bP
it provided function prototypes (using \fBconst\fP).
.bP
a prototype for \fBtparam\fP (a GNU termcap feature) was provided.
.PP
Later (in mid-1996) the \fBtparam\fP 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.
.SH BUGS
If you call \fB\%tgetstr\fP to fetch \fB\%ca\fP or any other
parameterized string capability,
be aware that it is returned in \fI\%terminfo\fP notation,
not the older and not-quite-compatible \fI\%termcap\fP notation.
This does not cause problems if all you do with it is call \fB\%tgoto\fP
or \fB\%tparm\fP, which both expand
\fI\%terminfo\fP-style strings as \fI\%terminfo\fP does.
(The \fB\%tgoto\fP function,
if configured to support \fI\%termcap,\fP
checks if the string is indeed \fI\%terminfo\fP-style by looking for
\*(``\fB%p\fP\*('' parameters or \*(``\fB<\fP.\|.\|.\fB>\fP\*('' delays,
and invokes a \fI\%termcap\fP-style parser if the string appears not to
use \fI\%terminfo\fP syntax.)
.PP
Because \fI\%terminfo\fP's syntax for padding in string capabilities
differs from \fI\%termcap\fP's,
users can be surprised.
.IP \(bu 4
\fB\%tputs("50")\fP in a \fI\%terminfo\fP system transmits \*(``50\*(''
rather than busy-waiting for 50 milliseconds.
.IP \(bu 4
However,
if \fI\%ncurses\fP is configured to support \fI\%termcap\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 \fI\%termcap\fP system may busy-wait for 50
milliseconds rather than transmitting \*(``50\*(''.
.PP
\fI\%termcap\fP has nothing analogous to \fI\%terminfo\fP's \fBsgr\fP
string.
One consequence is that \fI\%termcap\fP applications assume that
\fBme\fP
(equivalent to \fI\%terminfo\fP's \fBsgr0\fP capability)
does not reset the alternate character set.
\fI\%ncurses\fP checks for,
and modifies the data shared with,
the \fI\%termcap\fP interface to accommodate the latter's limitation in
this respect.
.SH SEE ALSO
\fB\%curses\fP(3X),
\fB\%putc\fP(3),
\fB\%term_variables\fP(3X),
\fB\%terminfo\fP(\*n)
.PP
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