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curs_termcap(3)                 Library calls                curs_termcap(3)


NAME

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


SYNOPSIS

       #include <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.

          o   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              withdrawn.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.  terminfo stores terminal type descriptions in compiled
              form, which is not the same thing.

          o   The meanings of the return values differ.  The BSD termcap
              library does not check whether the terminal type description
              includes the generic (gn) 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 obtains the numeric entry for id, or -1 if it is not available.

       tgetstr returns the string entry for id, or NULL if it is not
       available.  Use tputs to output the string returned.  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, area is ignored.

          o   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.

          o   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 stored in the first
       argument.

       o   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).

       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 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 its parameters.  It
           does this even for calls requiring only a single parameter.  In
           that case, the first parameter is merely a placeholder.

       o   Normally the ncurses library is compiled without full termcap
           support.  In that case, tgoto uses an internal version of tparm(3X)
           (a more capable function).

           Because it uses tparm internally, tgoto is able to use some
           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.

       tputs is described in curs_terminfo(3X).  It can retrieve capabilities
       by either termcap or terminfo code.

   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(3X).  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(3X) to
       obtain the data used by tgetent and the functions that retrieve
       capability values.  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 value
       of the terminal name parameter given to tgetent.  The screen(1) program
       relies upon this arrangement to improve its performance.

       An application that uses only the termcap functions, not the higher
       level curses API, could release the memory using del_curterm(3X),
       because the pool is freed using other functions; see curs_memleaks(3X).


RETURN VALUE

       The return values of tgetent, tgetflag, tgetname, and tgetstr are
       documented above.

       tgoto returns NULL on error.  Error conditions include:

       o   uninitialized state (tgetent was not called successfully),

       o   cap being a null pointer,

       o   cap referring to a canceled capability,

       o   cap being a capability with string-valued parameters (a
           terminfo-only feature), and

       o   cap being a capability with more than two parameters.

       See curs_terminfo(3X) 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 (and the variables never
       were); ncurses provides them to support legacy applications.  They
       should not be used in new programs.

   Standards
       o   X/Open Curses, Issue 4, Version 2 (1996), describes these
           functions, marking them as "TO BE WITHDRAWN".

       o   X/Open Curses, Issue 7 (2009) marks 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 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 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
       Externally visible variables are provided for support of certain
       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 ospeed.

       The constraint 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.

       o   ncurses disallows matches by the termcap interface against extended
           capability names that are longer than two characters; see
           user_caps(5).

       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, leaving the API unchanged.

       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 termcap.h header
       files over time.

       o   One was used internally by jove(1) from 4.3BSD onward.  It declared
           global symbols for the termcap variables that it used.

       o   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(1) 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:

       o   global symbols used by emacs,

       o   const-qualified function prototypes, and

       o   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 column_address (ch) 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 parametrically expand terminfo-style string
       capabilities 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.

       o   tputs("50") in a terminfo system transmits "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 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 set_attributes (sgr)
       capability.  One consequence is that termcap applications assume that
       "me" (equivalent to terminfo's exit_attribute_mode (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

       curses(3X), curs_terminfo(3X), putc(3), term_variables(3X), terminfo(5)

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

ncurses 6.5                       2024-04-20                  curs_termcap(3)

ncurses 6.5 - Generated Wed May 1 18:29:27 CDT 2024
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