curs\_termcap(3X) Library calls curs\_termcap(3X) # [NAME](#NAME) **PC**, **UP**, **BC**, **ospeed**, **tgetent**, **tgetflag**, **tgetnum**, **tgetstr**, **tgoto**, **tputs** - *ncurses* emulation of *termcap* # [SYNOPSIS](#SYNOPSIS) ``` #include #include ``` ``` 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 ** sbuf); char * tgoto(const char * cap, int col, int row); int tputs(const char * str, int affcnt, int (* putc)(int)); ``` # [DESCRIPTION](#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](#Initialization) **tgetent** loads the terminal database entry for *name*; see [**term**(7)](/man/os/ncurses/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ignores the buffer pointer *bp*, as do other *termcap* implementations conforming to portions of X/Open Curses now withdrawn.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 buffer referenced by this pointer. *terminfo* 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 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](#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 *sbuf* 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 *sbuf* is not *NULL*, and that the pointer obtained by dereferencing *sbuf* is also not *NULL*. If either check fails, *ncurses* ignores *sbuf*. - If the checks succeed, *ncurses* also copies the return value to the buffer pointed to by *sbuf*, and the library updates *sbuf* 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](#Applying_String_Capabilities) String capabilities can be parameterized; see subsection “Parameterized Strings” in [**terminfo**(5)](/man/os/ncurses/terminfo.5). **tgoto** applies its second and third arguments to the parametric placeholders in the capability stored in the first argument. - The capability may contain padding specifications; see subsection “Delays and Padding” of [**terminfo**(5)](/man/os/ncurses/terminfo.5). The output of **tgoto** should thus be passed to **tputs** rather than some other output function such as [*printf*(3)](/man/os/ncurses/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 its parameters. It does this even for calls requiring only a single parameter. In that case, the first parameter is merely a placeholder. • Normally the *ncurses* library is compiled without full *termcap* support. In that case, **tgoto** uses an internal version of [**tparm**(3X)](/man/os/ncurses/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)](/man/os/ncurses/curs_terminfo.3X). It can retrieve capabilities by either *termcap* or *terminfo* code. ## [Global Variables](#Global_Variables) **tgetent** sets the variables **PC**, **UP**, and **BC** to the *terminfo* entry's data for **pad\_char** (**pad**), **cursor\_up** (**cuu1**), and **backspace\_if\_not\_bs** (**OTbs**), respectively. *ncurses* does not employ **cuu1** internally. [**delay\_output**(3X)](/man/os/ncurses/delay_output.3X) uses **pad**, while **tgoto** emulation uses the obsolete *termcap* capability **bs**, represented in *ncurses* *terminfo* as “OTbs”. *ncurses* assigns the variable **ospeed** a system-specific value to encode the terminal's data rate. ## [Releasing Memory](#Releasing_Memory) The *termcap* functions provide no means of freeing memory, because legacy *termcap* implementations used only the storage provided by the caller via **tgetent** and **tgetstr**. Those buffers are unused in *terminfo*. By contrast, *terminfo* allocates memory. It uses [**setupterm**(3X)](/man/os/ncurses/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)](/man/os/ncurses/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)](/man/os/ncurses/del_curterm.3X), because the pool is freed using other functions; see [**curs\_memleaks**(3X)](/man/os/ncurses/curs_memleaks.3X). # [RETURN VALUE](#RETURN_VALUE) The return values of **tgetent**, **tgetflag**, **tgetname**, and **tgetstr** are documented above. **tgoto** returns *NULL* 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 *terminfo*-only feature), and - *cap* being a capability with more than two parameters. See [**curs\_terminfo**(3X)](/man/os/ncurses/curs_terminfo.3X) regarding **tputs**. # [NOTES](#NOTES) *ncurses* compares only the first two characters of the *id* parameter of **tgetflag**, **tgetnum**, and **tgetstr** to the capability names in the database. ## [Pitfalls of Long Terminal Type Descriptions](#Pitfalls_of_Long_Terminal_Type_Descriptions) Because terminal descriptions in *termcap* are limited to 1023 bytes, specifying many function key definitions or translating *terminfo* entries to *termcap* can cause problems. The man pages for 4.3BSD and older versions of **tgetent** instruct the user to allocate a 1024-byte buffer for the *termcap* entry. The entry gets null-terminated by the *termcap* library, so that makes the maximum safe length for a *termcap* entry 1KB-1 (1023) bytes. Depending on what the application and the *termcap* library being used does, and where in the *termcap* file the terminal type that **tgetent** is searching for is, several bad things can happen: - some *termcap* libraries print a warning message, - some exit if they find an entry longer than 1023 bytes, - some neither exit nor warn, doing nothing useful, and - some simply truncate the entries to 1023 bytes. Some application programs allocate more than the recommended 1K for the *termcap* entry; others do not. Each *termcap* entry has two important sizes associated with it: before “tc” expansion, and after “tc” expansion. “tc” is the capability that tacks on another *termcap* entry to the end of the current one, to supplement, redefine, or cancel capabilities. If a *termcap* entry does not use the “tc” capability, then of course the two lengths are the same. The “before tc expansion” length is the more important one, because it affects more than just users of that particular terminal. This is the length of the entry as it exists in */etc/termcap*, minus the backslash-newline pairs, which **tgetent** strips out while reading it. Some *termcap* libraries strip off the final newline, too (though GNU *termcap* does not). Now suppose: - a *termcap* entry before expansion is more than 1023 bytes long, - and the application has allocated only a 1KB buffer, - and the *termcap* library (like the one in BSD/OS 1.1 and GNU) reads the whole entry into the buffer, no matter what its length, to see if it is the entry it wants, - and **tgetent** is searching for a terminal type that either is the long entry, appears in the *termcap* file after the long entry, or does not appear in the file at all (so that **tgetent** has to search the whole *termcap* file). Then **tgetent** will overwrite memory, perhaps its stack, and probably core dump the program. Programs like [*telnet*(1)](/man/os/ncurses/telnet.1) are particularly vulnerable; modern *telnet*s pass along values like the terminal type automatically. The results are almost as undesirable with a *termcap* library, like those of SunOS 4.1.3 and Ultrix 4.4, that prints warning messages when it reads an overly long *termcap* entry. If a *termcap* library truncates long entries, like OSF/1 3.0, it is immune to dying here but returns incorrect data for the terminal. The “after tc expansion” length has a similar effect, but only for people who actually set *TERM* to that terminal type, since **tgetent** performs “tc” expansion only when it has found the terminal type it seeks — not while searching. In summary, a *termcap* entry that is longer than 1023 bytes can cause, with various combinations of *termcap* libraries and applications, a core dump, warnings, or incorrect operation. If it is too long even before “tc” expansion, it will have this effect even for users of some other terminal types and users whose *TERM* variable contents lack a corresponding *termcap* description. When in -C (translate to *termcap*) mode, the *ncurses* implementation of [**tic**(1M)](/man/os/ncurses/tic.1M) issues warning messages when the pre-tc length of a *termcap* translation is too long. The -c (check) option also validates resolved (after tc expansion) lengths. # [PORTABILITY](#PORTABILITY) These functions are no longer standardized (and the variables never were); see section “HISTORY” below. *ncurses* provides them to support legacy applications; they should not be used in new programs. SVr4 describes a successful return value only as “an integer value other than *ERR*”. 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*](#Compatibility_with_BSD_) *ncurses* provides externally visible variables to support 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 looked up in the terminal database escapes many application developers. The BSD *termcap* library did not require a trailing null character after 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. *ncurses* disallows matches by the *termcap* interface against extended capability names that are longer than two characters; see [**user\_caps**(5)](/man/os/ncurses/user_caps.5). The BSD *termcap* function *tgetent* returns the text of a *termcap* entry in the buffer passed as an argument. *ncurses*, like other *terminfo* implementations, does not store terminal type descriptions as text. It sets the buffer contents to a null-terminated string. ## [Header File](#Header_File) *ncurses* includes a *termcap.h* header file for compatibility with other implementations, but it is rarely used because the other implementations are not mutually compatible; see below. # [HISTORY](#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 through 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. - One was used internally by [*jove*(1)](/man/os/ncurses/jove.1) from 4.3BSD onward. It declared 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. NetBSD's *termcap* library added this header file in mid-1994. Meanwhile, GNU *termcap* began development in 1990. Its first release (1.0) in 1991 included a *termcap.h* header file. Its second (1.1) release in September 1992 modified the file to use *const* for the function prototypes in the header where one would expect 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 [*bash*(1)](/man/os/ncurses/bash.1) has bundled GNU *termcap* 1.3 since mid-1993 to support its [*readline*(3)](/man/os/ncurses/readline.3) library, and continues to use it if configured to do so. *ncurses* 1.8.1 (November 1993) provided a *termcap.h* file. It reflected influence from GNU *termcap* and [*emacs*(1)](/man/os/ncurses/emacs.1) (rather than [*jove*(1)](/man/os/ncurses/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. X/Open Curses Issue 4, Version 2 (1996), describes these functions, marking them as “TO BE WITHDRAWN”. X/Open Curses Issue 7 (2009) withdrew the *termcap* interface (along with the *vwprintw* and *vwscanw* functions). # [BUGS](#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. • **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 **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* inspects the data shared with the *termcap* interface and modifies it as necessary to accommodate the latter's limitation in this respect. # [SEE ALSO](#SEE_ALSO) [*TCTEST — A Termcap Test Utility*](https://invisible-island.net/ncurses/tctest.html) [**curses**(3X)](/man/os/ncurses/curses.3X), [**curs\_terminfo**(3X)](/man/os/ncurses/curs_terminfo.3X), [**putc**(3)](/man/os/ncurses/putc.3), [**term\_variables**(3X)](/man/os/ncurses/term_variables.3X), [**terminfo**(5)](/man/os/ncurses/terminfo.5) 2026-09-12 ncurses 6.6 --- Document: tgetnum(3x) from ncurses 6.6.20261017-r0 (Chainguard OS) Source: https://github.com/ThomasDickey/ncurses-snapshots