curs_termcap(3x) — Linux manual page


curs_termcap(3X)                                        curs_termcap(3X)

NAME         top

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

SYNOPSIS         top

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

       extern char PC;
       extern char * UP;
       extern char * BC;
       extern @NCURSES_OSPEED@ 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         top

       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.

       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:

          •   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.

          •   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:

          •   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, the area parameter is ignored.

          •   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.

          •   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

       •   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).

       •   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.

       •   Normally the ncurses library is compiled with terminfo
           support.  In that case, tgoto uses tparm(3X) (a more capable

           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

   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


       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

       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         top

       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.

BUGS         top

       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:

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

       •   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

PORTABILITY         top

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

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           However, they are marked TO BE WITHDRAWN and may be removed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

       •   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

   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:

       •   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.

       •   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:

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

       •   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:

       •   it provided declarations for a few global symbols used by

       •   it provided function prototypes (using const).

       •   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         top

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

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