a question about termcap support

[G. Branden Robinson]

I'm preparing a heavy revision (attached) of ncurses's termcap man page,
and I ran into some difficult material.  It starts with "Normally the
ncurses library is compiled with terminfo support."

That implies that under some circumstances, ncurses can be compiled
WITHOUT terminfo support, but I don't think that is possible.

Here is the chunk of man(7) source in question.  I commented out the
confusing material and added "XXX" annotations.

.\" XXX: This is pretty confusing.  It appears to refer to the
.\" `PURE_TERMINFO` Autoconf variable; defining it drops the
.\" `is_termcap()` and `tgoto_internal()` function definitions.  But in
.\" that case why support the X/Open-withdrawn `tgoto()` at all?
.\" "PURE_TERMINFO" appears to mean "partial termcap support".  If
.\" terminfo is not "pure", it offers full termcap support.
.\" .bP
.\" Normally the \fI\%ncurses\fP library is compiled with
.\" \fI\%term\%info\fP support.
.\" XXX: Isn't it rather that ncurses is always compiled with terminfo
.\" support, and with either partial or full termcap database/API
.\" support?
.\" In that case,
.\" \fB\%tgoto\fP uses an internal version of \fB\%tparm\fP(3X)
.\" (a more capable function).
.\" .IP
.\" With \fI\%term\%info\fP support,
.\" XXX: Doesn't that mean "partial termcap support" instead?
.\" \fB\%tgoto\fP is able to use some \fI\%term\%info\fP 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.

Can Thomas or someone else put me in the picture?

This is what I mean by "extensive revision".

$ COLUMNS=72 git diff --stat man/curs_termcap.3x
 man/curs_termcap.3x | 592 ++++++++++++++++++++++++++++----------------
 1 file changed, 376 insertions(+), 216 deletions(-)

Here is a dump of the page as of my current revision.

$ nroff -rLL=72n -rHY=0 -dAD=l -man -P -cbou man/curs_termcap.3x
termcap(3NCURSES)             Library calls            termcap(3NCURSES)

NAME
       PC, UP, BC, ospeed, tgetent, tgetflag, tgetnum, tgetstr, tgoto,
       tputs - curses emulation of termcap

SYNOPSIS
       #include <ncursesw/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 is marked with the gn (generic) 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 gets the numeric entry for id, or -1 if it is not
       available.

       tgetstr returns the string entry for id, or a null pointer 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 named 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 parameters.  It does this even for calls requiring only a
           single parameter.  In that case, the first parameter is
           merely a placeholder.

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

   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 to
       retrieve the data used by tgetent and the functions which return
       capability values such as tgetstr.  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
       setting of the TERM environment variable when tgetent is called.
       The screen(1) program relies upon this arrangement to improve its
       performance.

       An application that uses only the low‐level termcap functions
       could free 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 a null pointer 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; 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.  However, they are marked “TO BE WITHDRAWN”.

       •   X/Open Curses, Issue 7 (2009) marked 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 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
       External variables are provided for support of certain termcap
       applications.  However, termcap applications’ use of those
       variables is poorly documented; for example, by failing to
       establish when reading and writing them is meaningful.  In
       particular, some applications are reported to declare and/or
       modify ospeed.

       The comment 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.

       •   As a special case, tgetflag matched a single‐character
           identifier provided that it was at the end of the terminal
           type description.  You should not rely upon this behavior in
           portable programs.  This implementation disallows matches
           against single‐character capability names.

       •   ncurses disallows matches by the termcap interface against
           extended capability names which are longer than two
           characters.

       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 but left the
       API unchanged.

       Function prototypes were a feature of the forthcoming ANSI C
       (1989).  Thus the library 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
           delcared 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.  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 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 ca 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 terminfo‐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 sgr string.  One
       consequence is that termcap applications assume that me
       (equivalent to terminfo’s 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
       ncurses(3NCURSES), putc(3), terminfo_variables(3NCURSES),
       terminfo(5)

       https://invisible-island.net/ncurses/tctest.html

ncurses 6.4                    2023‐12‐02              termcap(3NCURSES)

Regards,
Branden

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