# curs_termcap 3x 2023-10-07 ncurses 6.4 Library calls

    curs_termcap(3x)                 Library calls                curs_termcap(3x)

## NAME

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

## SYNOPSIS

           #include <curses.h>
           #include <term.h>

           extern char PC;
           extern char * UP;
           extern char * BC;
           extern 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

           These routines are included as a conversion aid for programs  that  use
           the  termcap  library.  Their parameters are the same, but the routines
           are emulated using the terminfo database.  Thus, they can only be  used
           to  query  the  capabilities  of entries for which a terminfo entry has
           been compiled.

### Initialization

           The tgetent routine loads the entry for name.  It returns:

              1  on success,

              0  if there is no such entry (or that it is a generic  type,  having
                 too little information for curses applications to run), and

              -1 if the terminfo database could not be found.

           This differs from the termcap library in two ways:

              o   The  emulation  ignores  the  buffer  pointer  bp.   The termcap
                  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.

              o   There is a difference in return codes.  The termcap library does
                  not check if the terminal description is marked with the generic
                  capability,   or   if   the  terminal  description  has  cursor-
                  addressing.

### Capability Values

           The tgetflag routine gets the boolean entry for id, or zero  if  it  is
           not available.

           The  tgetnum  routine gets the numeric entry for id, or -1 if it is not
           available.

           The tgetstr routine returns the string entry for id, or zero if  it  is
           not  available.   Use  tputs  to  output the returned string.  The area
           parameter is used as follows:

              o   It is assumed to be the address of a pointer to a buffer managed
                  by the calling application.

              o   However,  ncurses  checks  to  ensure that area is not NULL, and
                  also that the resulting buffer pointer is not NULL.   If  either
                  check fails, the area parameter is ignored.

              o   If  the  checks succeed, ncurses also copies the return value to
                  the buffer pointed to by  area,  and  the  area  value  will  be
                  updated to point past the null ending this value.

              o   The   return   value  itself  is  an  address  in  the  terminal
                  description which is loaded into memory.

           Only the first two characters of the id parameter of tgetflag,  tgetnum
           and tgetstr are compared in lookups.

### Formatting Capabilities

           The tgoto routine expands the given capability using the parameters.

           o   Because  the  capability may have padding characters, the output of
               tgoto should be passed to  tputs  rather  than  some  other  output
               function such as printf(3).

           o   While  tgoto  is  assumed  to  be used for the two-parameter cursor
               positioning  capability,  termcap  applications  also  use  it  for
               single-parameter capabilities.

               Doing  this  shows  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 column parameter first.  The
               tgoto 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.

           o   Normally the ncurses library is compiled with terminfo support.  In
               that  case,  tgoto  uses  an  internal version of tparm(3x) (a more
               capable formatter).

               With terminfo support, tgoto is able to use some  of  the  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.

           The  tputs  routine  is described on the curs_terminfo(3x) manual page.
           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 in the tdelay_output function.   BC
           is  used in the tgoto emulation.  The variable ospeed is set by ncurses
           in a system-specific coding to reflect the terminal speed.

### 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 tgetent  and  tgetstr.   Those  buffers  are  unused  in
           terminfo.

           On  the  other  hand,  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  variable when tgetent is called.  The screen(1)
           program relies upon this arrangement, to improve its performance.

           An application which uses only the low-level  termcap  functions  could
           free  the  memory  using  del_curterm,  because the pool is freed using
           other functions (see curs_memleaks(3x)).

## RETURN VALUE

           Except where explicitly noted, routines that return an  integer  return
           ERR  upon  failure  and OK (SVr4 only specifies "an integer value other
           than ERR") upon successful completion.

           Routines that return pointers return NULL on error.

           A few special cases apply:

           o   If the terminal database has not been initialized, these return  an
               error.

           o   The  calls  with  a  string  parameter  (tgoto, tputs) check if the
               string is null, or cancelled.  Those return an error.

           o   A call to tgoto using a capability with  string  parameters  is  an
               error.

           o   A call to tgoto using a capability with more than two parameters is
               an error.

## BUGS

           If you call tgetstr to fetch ca or any other parameterized  string,  be
           aware  that it will be returned in terminfo notation, not the older and
           not-quite-compatible termcap notation.  This will not cause problems if
           all  you do with it is call tgoto or tparm, which both expand terminfo-
           style strings as terminfo.   (The  tgoto  function,  if  configured  to
           support  termcap,  will check if the string is indeed terminfo-style by
           looking for "%p" parameters or "$<..>" delays, and  invoke  a  termcap-
           style parser if the string does not appear to be terminfo).

           Because   terminfo  conventions  for  representing  padding  in  string
           capabilities differ from termcap's, users can be surprised:

           o   tputs("50") in a terminfo system will put out a literal "50" rather
               than busy-waiting for 50 milliseconds.

           o   However,  if  ncurses is configured to support termcap, it may also
               have been configured to support the 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 wait for 50 milliseconds rather
               than put out a literal "50"

           Note that termcap has nothing analogous to terminfo's sgr string.   One
           consequence  of  this  is that termcap applications assume me (terminfo
           sgr0) does not reset the alternate character set.  This  implementation
           checks  for,  and  modifies  the data shown to the termcap interface to
           accommodate termcap's limitation in this respect.

## PORTABILITY

### Standards

           These functions are provided for supporting  legacy  applications,  and
           should not be used in new programs:

           o   The  XSI  Curses  standard,  Issue  4  describes  these  functions.
               However, they are marked TO BE WITHDRAWN  and  may  be  removed  in
               future versions.

           o   X/Open Curses, Issue 5 (December 2007) marked the termcap interface
               (along with vwprintw and vwscanw) as withdrawn.

           Neither the XSI Curses standard 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 XSI Curses
           documentation  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 XCurses,
           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, e.g., not distinguishing between input  and  output.
           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 original BSD 4.2 termcap
           library (and historical relics thereof) did not require a trailing null
           NUL  on  the  parameter  name  passed to tgetstr, tgetnum and tgetflag.
           Some applications assume that the termcap interface  does  not  require
           the  trailing  NUL  for  the parameter name.  Taking into account these
           issues:

           o   As a special case,  tgetflag  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.

           o   This implementation 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 descriptions as text.  It sets
           the buffer contents to a null-terminated string.

### 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.

           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:

           o   One was used internally by the jove editor in 2BSD through  4.4BSD.
               It defined global symbols for the termcap variables which it used.

           o   The  other  appeared in 4.4BSD Lite Release 2 (mid-1993) as part of
               libedit (also known as the  editline  library).   The  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.

           The header file from libedit was added to NetBSD's termcap  library  in
           mid-1994.

           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 const 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 tputs also  differed,  but
           in that instance, it was libedit which differed from BSD termcap.

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

           A termcap.h file was provided in ncurses 1.8.1 (November  1993).   That
           reflected influence by emacs(1) (rather than jove(1)) and GNU termcap:

           o   it provided declarations for a few global symbols used by emacs

           o   it provided function prototypes (using const).

           o   a prototype for tparam (a GNU termcap feature) was provided.

           Later (in mid-1996) the tparam 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.

## SEE ALSO

           curses(3x), putc(3), term_variables(3x), terminfo(5)

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

    ncurses 6.4                       2023-10-07                  curs_termcap(3x)