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<title>A Hacker's Guide to Ncurses Internals</title>
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<body>
 <h1>A Hacker's Guide to NCURSES</h1>

<h1>Contents</h1>

<ul>
 <li><a href="#abstract">Abstract</a></li>

<li>
 <a href="#objective">Objective of the Package</a>

<ul>
 <li><a href="#whysvr4">Why System V Curses?</a></li>

<li><a href="#extensions">How to Design Extensions</a></li>
 </ul>
 </li>

<li><a href="#portability">Portability and
 Configuration</a></li>

<li><a href="#documentation">Documentation Conventions</a></li>

<li><a href="#bugtrack">How to Report Bugs</a></li>

<li>
 <a href="#ncurslib">A Tour of the Ncurses Library</a>

<ul>
 <li><a href="#loverview">Library Overview</a></li>

<li><a href="#engine">The Engine Room</a></li>

<li><a href="#input">Keyboard Input</a></li>

<li><a href="#mouse">Mouse Events</a></li>

<li><a href="#output">Output and Screen Updating</a></li>
 </ul>
 </li>

<li><a href="#fmnote">The Forms and Menu Libraries</a></li>

<li>
 <a href="#tic">A Tour of the Terminfo Compiler</a>

<ul>
 <li><a href="#nonuse">Translation of
 Non-use Capabilities</a></li>

<li><a href="#uses">Use Capability Resolution</a></li>

<li><a href="#translation">Source-Form Translation</a></li>
 </ul>
 </li>

<li><a href="#utils">Other Utilities</a></li>

<li><a href="#style">Style Tips for Developers</a></li>

<li><a href="#port">Porting Hints</a></li>
 </ul>

<h1><a name="abstract" id="abstract">Abstract</a></h1>

This document is a hacker's tour of the
 ncurses library and utilities. It discusses
 design philosophy, implementation methods, and the conventions
 used for coding and documentation. It is recommended reading for
 anyone who is interested in porting, extending or improving the
 package.

<h1><a name="objective" id="objective">Objective of the
 Package</a></h1>

The objective of the ncurses package is to
 provide a free software API for character-cell terminals and
 terminal emulators with the following characteristics:

<ul>
 <li>Source-compatible with historical curses implementations
 (including the original BSD curses and System V curses.</li>

<li>Conformant with the XSI Curses standard issued as part of
 XPG4 by X/Open.</li>

<li>High-quality &mdash; stable and reliable code, wide
 portability, good packaging, superior documentation.</li>

<li>Featureful &mdash; should eliminate as much of the drudgery
 of C interface programming as possible, freeing programmers to
 think at a higher level of design.</li>
 </ul>

These objectives are in priority order. So, for example,
 source compatibility with older version must trump featurefulness
 &mdash; we cannot add features if it means breaking the portion
 of the API corresponding to historical curses versions.

<h2><a name="whysvr4" id="whysvr4">Why System V Curses?</a></h2>

We used System V curses as a model, reverse-engineering their
 API, in order to fulfill the first two objectives.

System V curses implementations can support BSD curses
 programs with just a recompilation, so by capturing the System V
 API we also capture BSD's.

More importantly for the future, the XSI Curses standard
 issued by X/Open is explicitly and closely modeled on System V.
 So conformance with System V took us most of the way to
 base-level XSI conformance.

<h2><a name="extensions" id="extensions">How to Design
 Extensions</a></h2>

The third objective (standards conformance) requires that it
 be easy to condition source code using ncurses
 so that the absence of nonstandard extensions does not break the
 code.

Accordingly, we have a policy of associating with each
 nonstandard extension a feature macro, so that ncurses client
 code can use this macro to condition in or out the code that
 requires the ncurses extension.

For example, there is a macro
 <code>NCURSES_MOUSE_VERSION</code> which XSI Curses does not
 define, but which is defined in the ncurses
 library header. You can use this to condition the calls to the
 mouse API calls.

<h1><a name="portability" id="portability">Portability and
 Configuration</a></h1>

Code written for ncurses may assume an
 ANSI-standard C compiler and POSIX-compatible OS interface. It
 may also assume the presence of a System-V-compatible
 select(2) call.

We encourage (but do not require) developers to make the code
 friendly to less-capable UNIX environments wherever possible.

We encourage developers to support OS-specific optimizations
 and methods not available under POSIX/ANSI, provided only
 that:

<ul>
 <li>All such code is properly conditioned so the build process
 does not attempt to compile it under a plain ANSI/POSIX
 environment.</li>

<li>Adding such implementation methods does not introduce
 incompatibilities in the ncurses API between
 platforms.</li>
 </ul>

We use GNU <code>autoconf(1)</code> as a tool to deal with
 portability issues. The right way to leverage an OS-specific
 feature is to modify the autoconf specification files
 (configure.in and aclocal.m4) to set up a new feature macro,
 which you then use to condition your code.

<h1><a name="documentation" id="documentation">Documentation
 Conventions</a></h1>

There are three kinds of documentation associated with this
 package. Each has a different preferred format:

<ul>
 <li>Package-internal files (README, INSTALL, TO-DO etc.)</li>

<li>Manual pages.</li>

<li>Everything else (i.e., narrative documentation).</li>
 </ul>

Our conventions are simple:

<ol>
 <li>Maintain package-internal files in plain
 text. The expected viewer for them more(1) or
 an editor window; there is no point in elaborate mark-up.</li>

<li>Mark up manual pages in the man macros.
 These have to be viewable through traditional man(1)
 programs.</li>

<li>Write everything else in HTML.</li>
 </ol>

When in doubt, HTMLize a master and use lynx(1) to
 generate plain ASCII (as we do for the announcement
 document).

The reason for choosing HTML is that it is (a) well-adapted
 for on-line browsing through viewers that are everywhere; (b)
 more easily readable as plain text than most other mark-ups, if
 you do not have a viewer; and (c) carries enough information that
 you can generate a nice-looking printed version from it. Also, of
 course, it make exporting things like the announcement document
 to WWW pretty trivial.

<h1><a name="bugtrack" id="bugtrack">How to Report Bugs</a></h1>

The <a name="bugreport" id="bugreport">reporting address for
 bugs</a> is <a href=
 "mailto:bug-ncurses@gnu.org">bug-ncurses@gnu.org</a>. This is a
 majordomo list; to join, write to
 <code>bug-ncurses-request@gnu.org</code> with a message
 containing the line:
 <pre>
 subscribe &lt;name&gt;@&lt;host.domain&gt;
</pre>

The <code>ncurses</code> code is maintained by a small group
 of volunteers. While we try our best to fix bugs promptly, we
 simply do not have a lot of hours to spend on elementary
 hand-holding. We rely on intelligent cooperation from our users.
 If you think you have found a bug in <code>ncurses</code>, there
 are some steps you can take before contacting us that will help
 get the bug fixed quickly.

In order to use our bug-fixing time efficiently, we put people
 who show us they have taken these steps at the head of our queue.
 This means that if you do not, you will probably end up at the
 tail end and have to wait a while.

<ol>
 <li>Develop a recipe to reproduce the bug.

Bugs we can reproduce are likely to be fixed very quickly,
 often within days. The most effective single thing you can do
 to get a quick fix is develop a way we can duplicate the bad
 behavior &mdash; ideally, by giving us source for a small,
 portable test program that breaks the library. (Even better
 is a keystroke recipe using one of the test programs provided
 with the distribution.)
 </li>

<li>Try to reproduce the bug on a different terminal type.

In our experience, most of the behaviors people report as
 library bugs are actually due to subtle problems in terminal
 descriptions. This is especially likely to be true if you are
 using a traditional asynchronous terminal or PC-based
 terminal emulator, rather than xterm or a UNIX console
 entry.

It is therefore extremely helpful if you can tell us
 whether or not your problem reproduces on other terminal
 types. Usually you will have both a console type and xterm
 available; please tell us whether or not your bug reproduces
 on both.

If you have xterm available, it is also good to collect
 xterm reports for different window sizes. This is especially
 true if you normally use an unusual xterm window size &mdash;
 a surprising number of the bugs we have seen are either
 triggered or masked by these.
 </li>

<li>Generate and examine a trace file for the broken behavior.

Recompile your program with the debugging versions of the
 libraries. Insert a <code>trace()</code> call with the
 argument set to <code>TRACE_UPDATE</code>. (See <a href=
 "ncurses-intro.html#debugging">"Writing Programs with
 NCURSES"</a> for details on trace levels.) Reproduce your
 bug, then look at the trace file to see what the library was
 actually doing.

Another frequent cause of apparent bugs is application
 coding errors that cause the wrong things to be put on the
 virtual screen. Looking at the virtual-screen dumps in the
 trace file will tell you immediately if this is happening,
 and save you from the possible embarrassment of being told
 that the bug is in your code and is your problem rather than
 ours.

If the virtual-screen dumps look correct but the bug
 persists, it is possible to crank up the trace level to give
 more and more information about the library's update actions
 and the control sequences it issues to perform them. The test
 directory of the distribution contains a tool for digesting
 these logs to make them less tedious to wade through.

Often you will find terminfo problems at this stage by
 noticing that the escape sequences put out for various
 capabilities are wrong. If not, you are likely to learn
 enough to be able to characterize any bug in the
 screen-update logic quite exactly.
 </li>

<li>Report details and symptoms, not just interpretations.

If you do the preceding two steps, it is very likely that
 you will discover the nature of the problem yourself and be
 able to send us a fix. This will create happy feelings all
 around and earn you good karma for the first time you run
 into a bug you really cannot characterize and fix
 yourself.

If you are still stuck, at least you will know what to
 tell us. Remember, we need details. If you guess about what
 is safe to leave out, you are too likely to be wrong.

If your bug produces a bad update, include a trace file.
 Try to make the trace at the least voluminous level
 that pins down the bug. Logs that have been through
 tracemunch are OK, it does not throw away any information
 (actually they are better than un-munched ones because they
 are easier to read).

If your bug produces a core-dump, please include a
 symbolic stack trace generated by gdb(1) or your local
 equivalent.

Tell us about every terminal on which you have reproduced
 the bug &mdash; and every terminal on which you cannot.
 Ideally, sent us terminfo sources for all of these (yours
 might differ from ours).

Include your ncurses version and your OS/machine type, of
 course! You can find your ncurses version in the
 <code>curses.h</code> file.
 </li>
 </ol>

If your problem smells like a logic error or in cursor
 movement or scrolling or a bad capability, there are a couple of
 tiny test frames for the library algorithms in the progs
 directory that may help you isolate it. These are not part of the
 normal build, but do have their own make productions.

The most important of these is <code>mvcur</code>, a test
 frame for the cursor-movement optimization code. With this
 program, you can see directly what control sequences will be
 emitted for any given cursor movement or scroll/insert/delete
 operations. If you think you have got a bad capability
 identified, you can disable it and test again. The program is
 command-driven and has on-line help.

If you think the vertical-scroll optimization is broken, or
 just want to understand how it works better, build
 <code>hashmap</code> and read the header comments of
 <code>hardscroll.c</code> and <code>hashmap.c</code>; then try it
 out. You can also test the hardware-scrolling optimization
 separately with <code>hardscroll</code>.

<h1><a name="ncurslib" id="ncurslib">A Tour of the Ncurses
 Library</a></h1>

<h2><a name="loverview" id="loverview">Library Overview</a></h2>

Most of the library is superstructure &mdash; fairly trivial
 convenience interfaces to a small set of basic functions and data
 structures used to manipulate the virtual screen (in particular,
 none of this code does any I/O except through calls to more
 fundamental modules described below). The files

<blockquote>
 <code>lib_addch.c lib_bkgd.c lib_box.c lib_chgat.c lib_clear.c
 lib_clearok.c lib_clrbot.c lib_clreol.c lib_colorset.c
 lib_data.c lib_delch.c lib_delwin.c lib_echo.c lib_erase.c
 lib_gen.c lib_getstr.c lib_hline.c lib_immedok.c lib_inchstr.c
 lib_insch.c lib_insdel.c lib_insstr.c lib_instr.c
 lib_isendwin.c lib_keyname.c lib_leaveok.c lib_move.c
 lib_mvwin.c lib_overlay.c lib_pad.c lib_printw.c lib_redrawln.c
 lib_scanw.c lib_screen.c lib_scroll.c lib_scrollok.c
 lib_scrreg.c lib_set_term.c lib_slk.c lib_slkatr_set.c
 lib_slkatrof.c lib_slkatron.c lib_slkatrset.c lib_slkattr.c
 lib_slkclear.c lib_slkcolor.c lib_slkinit.c lib_slklab.c
 lib_slkrefr.c lib_slkset.c lib_slktouch.c lib_touch.c
 lib_unctrl.c lib_vline.c lib_wattroff.c lib_wattron.c
 lib_window.c</code>
 </blockquote>

are all in this category. They are very unlikely to need
 change, barring bugs or some fundamental reorganization in the
 underlying data structures.

These files are used only for debugging support:

<blockquote>
 <code>lib_trace.c lib_traceatr.c lib_tracebits.c lib_tracechr.c
 lib_tracedmp.c lib_tracemse.c trace_buf.c</code>
 </blockquote>

It is rather unlikely you will ever need to change these,
 unless you want to introduce a new debug trace level for some
 reason.

There is another group of files that do direct I/O via
 tputs(), computations on the terminal capabilities, or
 queries to the OS environment, but nevertheless have only fairly
 low complexity. These include:

<blockquote>
 <code>lib_acs.c lib_beep.c lib_color.c lib_endwin.c
 lib_initscr.c lib_longname.c lib_newterm.c lib_options.c
 lib_termcap.c lib_ti.c lib_tparm.c lib_tputs.c lib_vidattr.c
 read_entry.c.</code>
 </blockquote>

They are likely to need revision only if ncurses is being
 ported to an environment without an underlying terminfo
 capability representation.

These files have serious hooks into the tty driver and signal
 facilities:

<blockquote>
 <code>lib_kernel.c lib_baudrate.c lib_raw.c lib_tstp.c
 lib_twait.c</code>
 </blockquote>

If you run into porting snafus moving the package to another
 UNIX, the problem is likely to be in one of these files. The file
 <code>lib_print.c</code> uses sleep(2) and also falls in this
 category.

Almost all of the real work is done in the files

<blockquote>
 <code>hardscroll.c hashmap.c lib_addch.c lib_doupdate.c
 lib_getch.c lib_mouse.c lib_mvcur.c lib_refresh.c lib_setup.c
 lib_vidattr.c</code>
 </blockquote>

Most of the algorithmic complexity in the library lives in
 these files. If there is a real bug in ncurses
 itself, it is probably here. We will tour some of these files in
 detail below (see <a href="#engine">The Engine Room</a>).

Finally, there is a group of files that is actually most of
 the terminfo compiler. The reason this code lives in the
 ncurses library is to support fallback to
 /etc/termcap. These files include

<blockquote>
 <code>alloc_entry.c captoinfo.c comp_captab.c comp_error.c
 comp_hash.c comp_parse.c comp_scan.c parse_entry.c
 read_termcap.c write_entry.c</code>
 </blockquote>

We will discuss these in the compiler tour.

<h2><a name="engine" id="engine">The Engine Room</a></h2>

<h3><a name="input" id="input">Keyboard Input</a></h3>

All <code>ncurses</code> input funnels through the function
 <code>wgetch()</code>, defined in <code>lib_getch.c</code>. This
 function is tricky; it has to poll for keyboard and mouse events
 and do a running match of incoming input against the set of
 defined special keys.

The central data structure in this module is a FIFO queue,
 used to match multiple-character input sequences against
 special-key capabilities; also to implement pushback via
 <code>ungetch()</code>.

The <code>wgetch()</code> code distinguishes between function
 key sequences and the same sequences typed manually by doing a
 timed wait after each input character that could lead a function
 key sequence. If the entire sequence takes less than 1 second, it
 is assumed to have been generated by a function key press.

Hackers bruised by previous encounters with variant
 <code>select(2)</code> calls may find the code in
 <code>lib_twait.c</code> interesting. It deals with the problem
 that some BSD selects do not return a reliable time-left value.
 The function <code>timed_wait()</code> effectively simulates a
 System V select.

<h3><a name="mouse" id="mouse">Mouse Events</a></h3>

If the mouse interface is active, <code>wgetch()</code> polls
 for mouse events each call, before it goes to the keyboard for
 input. It is up to <code>lib_mouse.c</code> how the polling is
 accomplished; it may vary for different devices.

Under xterm, however, mouse event notifications come in via
 the keyboard input stream. They are recognized by having the
 kmous capability as a prefix. This is kind of
 klugey, but trying to wire in recognition of a mouse key prefix
 without going through the function-key machinery would be just
 too painful, and this turns out to imply having the prefix
 somewhere in the function-key capabilities at terminal-type
 initialization.

This kluge only works because kmous is not
 actually used by any historic terminal type or curses
 implementation we know of. Best guess is it is a relic of some
 forgotten experiment in-house at Bell Labs that did not leave any
 traces in the publicly-distributed System V terminfo files. If
 System V or XPG4 ever gets serious about using it again, this
 kluge may have to change.

Here are some more details about mouse event handling:

The <code>lib_mouse()</code>code is logically split into a
 lower level that accepts event reports in a device-dependent
 format and an upper level that parses mouse gestures and filters
 events. The mediating data structure is a circular queue of event
 structures.

Functionally, the lower level's job is to pick up primitive
 events and put them on the circular queue. This can happen in one
 of two ways: either (a) <code>_nc_mouse_event()</code> detects a
 series of incoming mouse reports and queues them, or (b) code in
 <code>lib_getch.c</code> detects the kmous
 prefix in the keyboard input stream and calls _nc_mouse_inline to
 queue up a series of adjacent mouse reports.

In either case, <code>_nc_mouse_parse()</code> should be
 called after the series is accepted to parse the digested mouse
 reports (low-level events) into a gesture (a high-level or
 composite event).

<h3><a name="output" id="output">Output and Screen
 Updating</a></h3>

With the single exception of character echoes during a
 <code>wgetnstr()</code> call (which simulates cooked-mode line
 editing in an ncurses window), the library normally does all its
 output at refresh time.

The main job is to go from the current state of the screen (as
 represented in the <code>curscr</code> window structure) to the
 desired new state (as represented in the <code>newscr</code>
 window structure), while doing as little I/O as possible.

The brains of this operation are the modules
 <code>hashmap.c</code>, <code>hardscroll.c</code> and
 <code>lib_doupdate.c</code>; the latter two use
 <code>lib_mvcur.c</code>. Essentially, what happens looks like
 this:

The <code>hashmap.c</code> module tries to detect vertical
 motion changes between the real and virtual screens. This
 information is represented by the oldindex members in the newscr
 structure. These are modified by vertical-motion and clear
 operations, and both are re-initialized after each update. To
 this change-journalling information, the hashmap code adds
 deductions made using a modified Heckel algorithm on hash values
 generated from the line contents.

The <code>hardscroll.c</code> module computes an optimum set
 of scroll, insertion, and deletion operations to make the indices
 match. It calls <code>_nc_mvcur_scrolln()</code> in
 <code>lib_mvcur.c</code> to do those motions.

Then <code>lib_doupdate.c</code> goes to work. Its job is to
 do line-by-line transformations of <code>curscr</code> lines to
 <code>newscr</code> lines. Its main tool is the routine
 <code>mvcur()</code> in <code>lib_mvcur.c</code>. This routine
 does cursor-movement optimization, attempting to get from given
 screen location A to given location B in the fewest output
 characters possible.

If you want to work on screen optimizations, you should use
 the fact that (in the trace-enabled version of the library)
 enabling the <code>TRACE_TIMES</code> trace level causes a report
 to be emitted after each screen update giving the elapsed time
 and a count of characters emitted during the update. You can use
 this to tell when an update optimization improves efficiency.

In the trace-enabled version of the library, it is also
 possible to disable and re-enable various optimizations at
 runtime by tweaking the variable
 <code>_nc_optimize_enable</code>. See the file
 <code>include/curses.h.in</code> for mask values, near the
 end.

<h1><a name="fmnote" id="fmnote">The Forms and Menu
 Libraries</a></h1>

The forms and menu libraries should work reliably in any
 environment you can port ncurses to. The only portability issue
 anywhere in them is what flavor of regular expressions the
 built-in form field type TYPE_REGEXP will recognize.

The configuration code prefers the POSIX regex facility,
 modeled on System V's, but will settle for BSD regexps if the
 former is not available.

Historical note: the panels code was written primarily to
 assist in porting u386mon 2.0 (comp.sources.misc v14i001-4) to
 systems lacking panels support; u386mon 2.10 and beyond use it.
 This version has been slightly cleaned up for
 <code>ncurses</code>.

<h1><a name="tic" id="tic">A Tour of the Terminfo
 Compiler</a></h1>

The ncurses implementation of
 tic is rather complex internally; it has to do a
 trying combination of missions. This starts with the fact that,
 in addition to its normal duty of compiling terminfo sources into
 loadable terminfo binaries, it has to be able to handle termcap
 syntax and compile that too into terminfo entries.

The implementation therefore starts with a table-driven,
 dual-mode lexical analyzer (in <code>comp_scan.c</code>). The
 lexer chooses its mode (termcap or terminfo) based on the first
 &ldquo;,&rdquo; or &ldquo;:&rdquo; it finds in each entry. The
 lexer does all the work of recognizing capability names and
 values; the grammar above it is trivial, just "parse entries till
 you run out of file".

<h2><a name="nonuse" id="nonuse">Translation of
 Non-use Capabilities</a></h2>

Translation of most things besides use
 capabilities is pretty straightforward. The lexical analyzer's
 tokenizer hands each capability name to a hash function, which
 drives a table lookup. The table entry yields an index which is
 used to look up the token type in another table, and controls
 interpretation of the value.

One possibly interesting aspect of the implementation is the
 way the compiler tables are initialized. All the tables are
 generated by various awk/sed/sh scripts from a master table
 <code>include/Caps</code>; these scripts actually write C
 initializers which are linked to the compiler. Furthermore, the
 hash table is generated in the same way, so it doesn't have to be
 generated at compiler startup time (another benefit of this
 organization is that the hash table can be in shareable text
 space).

Thus, adding a new capability is usually pretty trivial, just
 a matter of adding one line to the <code>include/Caps</code>
 file. We will have more to say about this in the section on
 <a href="#translation">Source-Form Translation</a>.

<h2><a name="uses" id="uses">Use Capability Resolution</a></h2>

The background problem that makes tic tricky
 is not the capability translation itself, it is the resolution of
 use capabilities. Older versions would not
 handle forward use references for this reason
 (that is, a using terminal always had to follow its use target in
 the source file). By doing this, they got away with a simple
 implementation tactic; compile everything as it blows by, then
 resolve uses from compiled entries.

This will not do for ncurses. The problem is
 that that the whole compilation process has to be embeddable in
 the ncurses library so that it can be called by
 the startup code to translate termcap entries on the fly. The
 embedded version cannot go promiscuously writing everything it
 translates out to disk &mdash; for one thing, it will typically
 be running with non-root permissions.

So our tic is designed to parse an entire
 terminfo file into a doubly-linked circular list of entry
 structures in-core, and then do use resolution
 in-memory before writing everything out. This design has other
 advantages: it makes forward and back use-references equally easy
 (so we get the latter for free), and it makes checking for name
 collisions before they are written out easy to do.

And this is exactly how the embedded version works. But the
 stand-alone user-accessible version of tic
 partly reverts to the historical strategy; it writes to disk (not
 keeping in core) any entry with no use
 references.

This is strictly a core-economy kluge, implemented because the
 terminfo master file is large enough that some core-poor systems
 swap like crazy when you compile it all in memory...there have
 been reports of this process taking three hours,
 rather than the twenty seconds or less typical on the author's
 development box.

So. The executable tic passes the
 entry-parser a hook that immediately writes out the
 referenced entry if it has no use capabilities. The compiler main
 loop refrains from adding the entry to the in-core list when this
 hook fires. If some other entry later needs to reference an entry
 that got written immediately, that is OK; the resolution code
 will fetch it off disk when it cannot find it in core.

Name collisions will still be detected, just not as cleanly.
 The <code>write_entry()</code> code complains before overwriting
 an entry that postdates the time of tic's first
 call to <code>write_entry()</code>, Thus it will complain about
 overwriting entries newly made during the tic
 run, but not about overwriting ones that predate it.

<h2><a name="translation" id="translation">Source-Form
 Translation</a></h2>

Another use of tic is to do source
 translation between various termcap and terminfo formats. There
 are more variants out there than you might think; the ones we
 know about are described in the captoinfo(1)
 manual page.

The translation output code (<code>dump_entry()</code> in
 <code>ncurses/dump_entry.c</code>) is shared with the
 infocmp(1) utility. It takes the same internal
 representation used to generate the binary form and dumps it to
 standard output in a specified format.

The <code>include/Caps</code> file has a header comment
 describing ways you can specify source translations for
 nonstandard capabilities just by altering the master table. It is
 possible to set up capability aliasing or tell the compiler to
 plain ignore a given capability without writing any C code at
 all.

For circumstances where you need to do algorithmic
 translation, there are functions in <code>parse_entry.c</code>
 called after the parse of each entry that are specifically
 intended to encapsulate such translations. This, for example, is
 where the AIX box1 capability get translated to
 an acsc string.

<h1><a name="utils" id="utils">Other Utilities</a></h1>

The infocmp utility is just a wrapper around
 the same entry-dumping code used by tic for
 source translation. Perhaps the one interesting aspect of the
 code is the use of a predicate function passed in to
 <code>dump_entry()</code> to control which capabilities are
 dumped. This is necessary in order to handle both the ordinary
 De-compilation case and entry difference reporting.

The tput and clear utilities
 just do an entry load followed by a <code>tputs()</code> of a
 selected capability.

<h1><a name="style" id="style">Style Tips for Developers</a></h1>

See the TO-DO file in the top-level directory of the source
 distribution for additions that would be particularly useful.

The prefix <code>_nc_</code> should be used on library public
 functions that are not part of the curses API in order to prevent
 pollution of the application namespace. If you have to add to or
 modify the function prototypes in curses.h.in, read
 ncurses/MKlib_gen.sh first so you can avoid breaking XSI
 conformance. Please join the ncurses mailing list. See the
 INSTALL file in the top level of the distribution for details on
 the list.

Look for the string <code>FIXME</code> in source files to tag
 minor bugs and potential problems that could use fixing.

Do not try to auto-detect OS features in the main body of the
 C code. That is the job of the configuration system.

To hold down complexity, do make your code data-driven.
 Especially, if you can drive logic from a table filtered out of
 <code>include/Caps</code>, do it. If you find you need to augment
 the data in that file in order to generate the proper table, that
 is still preferable to ad-hoc code &mdash; that is why the fifth
 field (flags) is there.

Have fun!

<h1><a name="port" id="port">Porting Hints</a></h1>

The following notes are intended to be a first step towards
 DOS and Macintosh ports of the ncurses libraries.

The following library modules are &ldquo;pure curses&rdquo;;
 they operate only on the curses internal structures, do all
 output through other curses calls (not including
 <code>tputs()</code> and <code>putp()</code>) and do not call any
 other UNIX routines such as signal(2) or the stdio library. Thus,
 they should not need to be modified for single-terminal
 ports.

<blockquote>
 <code>lib_addch.c lib_addstr.c lib_bkgd.c lib_box.c lib_clear.c
 lib_clrbot.c lib_clreol.c lib_delch.c lib_delwin.c lib_erase.c
 lib_inchstr.c lib_insch.c lib_insdel.c lib_insstr.c
 lib_keyname.c lib_move.c lib_mvwin.c lib_newwin.c lib_overlay.c
 lib_pad.c lib_printw.c lib_refresh.c lib_scanw.c lib_scroll.c
 lib_scrreg.c lib_set_term.c lib_touch.c lib_tparm.c lib_tputs.c
 lib_unctrl.c lib_window.c panel.c</code>
 </blockquote>

This module is pure curses, but calls outstr():

<blockquote>
 <code>lib_getstr.c</code>
 </blockquote>

These modules are pure curses, except that they use
 <code>tputs()</code> and <code>putp()</code>:

<blockquote>
 <code>lib_beep.c lib_color.c lib_endwin.c lib_options.c
 lib_slk.c lib_vidattr.c</code>
 </blockquote>

This modules assist in POSIX emulation on non-POSIX
 systems:

<dl>
 <dt>sigaction.c</dt>

<dd>signal calls</dd>
 </dl>

The following source files will not be needed for a
 single-terminal-type port.

<blockquote>
 <code>alloc_entry.c captoinfo.c clear.c comp_captab.c
 comp_error.c comp_hash.c comp_main.c comp_parse.c comp_scan.c
 dump_entry.c infocmp.c parse_entry.c read_entry.c tput.c
 write_entry.c</code>
 </blockquote>

The following modules will use
 open()/read()/write()/close()/lseek() on files, but no other OS
 calls.

<dl>
 <dt>lib_screen.c</dt>

<dd>used to read/write screen dumps</dd>

<dt>lib_trace.c</dt>

<dd>used to write trace data to the logfile</dd>
 </dl>

Modules that would have to be modified for a port start
 here:

The following modules are &ldquo;pure curses&rdquo; but
 contain assumptions inappropriate for a memory-mapped port.

<dl>
 <dt>lib_longname.c</dt>

<dd>assumes there may be multiple terminals</dd>

<dd>assumes acs_map as a double indirection</dd>

<dt>lib_mvcur.c</dt>

<dd>assumes cursor moves have variable cost</dd>

<dt>lib_termcap.c</dt>

<dd>assumes there may be multiple terminals</dd>

<dd>assumes there may be multiple terminals</dd>
 </dl>

The following modules use UNIX-specific calls:

<dl>
 <dt>lib_doupdate.c</dt>

<dd>input checking</dd>

<dt>lib_getch.c</dt>

<dt>lib_initscr.c</dt>

<dd>getenv()</dd>

<dt>lib_newterm.c</dt>

<dt>lib_baudrate.c</dt>

<dt>lib_kernel.c</dt>

<dd>various tty-manipulation and system calls</dd>

<dd>various tty-manipulation calls</dd>

<dt>lib_setup.c</dt>

<dd>various tty-manipulation calls</dd>

<dt>lib_restart.c</dt>

<dd>various tty-manipulation calls</dd>

<dd>signal-manipulation calls</dd>

<dt>lib_twait.c</dt>

<dd>gettimeofday(), select().</dd>
 </dl>
 <hr>

<address>
 Eric S. Raymond &lt;esr@snark.thyrsus.com&gt;
 </address>(Note: This is not the <a href="#bugtrack">bug
 address</a>!)
</body>
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