githubmirror/ncurses

misc/hackguide.html
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Thomas E. Dickey
ncurses 4.2
01 мар 1998, 07:21
01 мар 1998, 07:21661078d
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<H1>A Hacker's Guide to NCURSES</H1>

<H1>Contents</H1>
<UL>
<LI><A HREF="#abstract">Abstract</A>
<P>
<LI><A HREF="#objective">Objective of the Package</A>
<UL>
<LI><A HREF="#whysvr4">Why System V Curses?</A>
<LI><A HREF="#extensions">How to Design Extensions</A>
</UL>
<LI><A HREF="#portability">Portability and Configuration</A><UL>
<LI><A HREF="#fooup">If autoconf Fails</A>
</UL>
<LI><A HREF="#documentation">Documentation Conventions</A>
<P>
<LI><A HREF="#bugtrack">How to Report Bugs</A>
<P>
<LI><A HREF="#ncurslib">A Tour of the Ncurses Library</A>
<UL>
<LI><A HREF="#loverview">Library Overview</A>
<LI><A HREF="#engine">The Engine Room</A>
<LI><A HREF="#input">Keyboard Input</A>
<LI><A HREF="#mouse">Mouse Events</A>
<LI><A HREF="#output">Output and Screen Updating</A>
</UL>
<LI><A HREF="#fmnote">The Forms and Menu Libraries</A>
<P>
<LI><A HREF="#tic">A Tour of the Terminfo Compiler</A>
<UL>
<LI><A HREF="#nonuse">Translation of Non-<STRONG>use</STRONG> Capabilities</A>
<LI><A HREF="#uses">Use Capability Resolution</A>
<LI><A HREF="#translation">Source-Form Translation</A>
</UL>
<LI><A HREF="#utils">Other Utilities</A>
<P>
<LI><A HREF="#style">Style Tips for Developers</A>
<P>
<LI><A HREF="#port">Porting Hints</A>
</UL>

<H1><A NAME="abstract">Abstract</A></H1>

This document is a hacker's tour of the <STRONG>ncurses</STRONG> 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. <P>

<H1><A NAME="objective">Objective of the Package</A></H1>

The objective of the <STRONG>ncurses</STRONG> package is to provide a freeware API for
character-cell terminals and terminal emulators with the following 
characteristics: <P>

<UL>
<LI>Source-compatible with historical curses implementations (including
     the original BSD curses and System V curses.
<P>
<LI>Conformant with the XSI Curses standard issued as part of XPG4 by
     X/Open. 
<P>
<LI>High-quality -- stable and reliable code, wide portability, good
     packaging, superior documentation.
<P>
<LI>Featureful -- should eliminate as much of the drudgery of C interface
     programming as possible, freeing programmers to think at a higher
     level of design.
</UL>

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

<H2><A NAME="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. <P>

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. <P>

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. <P>

<H2><A NAME="extensions">How to Design Extensions</A></H2>

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

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 <STRONG>ncurses</STRONG> extension. <P>

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

<H1><A NAME="portability">Portability and Configuration</A></H1>

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

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

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

<UL>
<LI>All such code is properly conditioned so the build process does not
     attempt to compile it under a plain ANSI/POSIX environment.
<P>
<LI>Adding such implementation methods does not introduce incompatibilities
     in the <STRONG>ncurses</STRONG> API between platforms.
</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. <P>

<H2><A NAME="fooup">If autoconf Fails</A></H2>

The 'configure' script usually gets your system environment right
automatically.  Here are some -D options you might need to compile
with if it fails: <P>
<DL>
<DT>-DHAVE_UNISTD_H
<DD>if &lt;unistd.h&gt; is present
<DT>-DHAVE_SIGACTION
<DD>if the sigaction function is present 
<DT>-DHAVE_USLEEP
<DD>if the usleep function is present
<DT>-DSVR4_ACTION
<DD>if (e.g., svr4) you need _POSIX_SOURCE to have sigaction
<DT>-DHAVE_TERMIOS_H
<DD>if you have &lt;termios.h&gt;
<DT>-DHAVE_TERMIO_H
<DD>if you have &lt;termio.h&gt;; otherwise it uses &lt;sgtty.h&gt;
<DT>-DBROKEN_TIOCGETWINSZ
<DD>on SVR4 and HPUX, the get window size ioctl is broken.
</DL>

<H1><A NAME="documentation">Documentation Conventions</A></H1>

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

<UL>
<LI>Package-internal files (README, INSTALL, TO-DO etc.)
<LI>Manual pages.
<LI>Everything else (i.e., narrative documentation).
</UL>

Our conventions are simple: <P>
<OL>
<LI><STRONG>Maintain package-internal files in plain text.</STRONG>
     The expected viewer for them <EM>more(1)</EM> or an editor window; there's
     no point in elaborate mark-up. <P>

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

<LI><STRONG>Write everything else in HTML.</STRONG>
</OL>

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

The reason for choosing HTML is that it's (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 don't 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.<P>

<H1><A NAME="bugtrack">How to Report Bugs</A></H1>

The <A NAME="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>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
don't 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. <P>

In order to use our bug-fixing time efficiently, we put people who
show us they've taken these steps at the head of our queue.  This
means that if you don't, you'll probably end up at the tail end and
have to wait a while. <P>

<OL>
<LI>Develop a recipe to reproduce the bug. <P>

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 --
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.) <P>

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

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're using a traditional
asynchronous terminal or PC-based terminal emulator, rather than xterm
or a UNIX console entry. <P>

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

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 -- a surprising number of the bugs we've seen
are either triggered or masked by these.  <P>

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

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. <P>

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. <P>

If the virtual-screen dumps look correct but the bug persists, it's
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. <P>

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

<LI>Report details and symptoms, not just interpretations. <P>

If you do the preceding two steps, it is very likely that you'll 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 can't characterize and fix yourself. <P>

If you're still stuck, at least you'll 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. <P>

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

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

Tell us about every terminal on which you've reproduced the bug -- and
every terminal on which you can't.  Ideally, sent us terminfo sources
for all of these (yours might differ from ours). <P>

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

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've got
a bad capability identified, you can disable it and test again. The
program is command-driven and has on-line help. <P>

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>. <P>

There's one other interactive tester, <CODE>tctest</CODE>, that exercises
translation between termcap and terminfo formats.  If you have a serious
need to run this, you probably belong on our development team! <P>

<H1><A NAME="ncurslib">A Tour of the Ncurses Library</A></H1>

<H2><A NAME="loverview">Library Overview</A></H2>

Most of the library is superstructure -- 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 <CODE>lib_addch.c</CODE>,
<CODE>lib_bkgnd.c</CODE>, <CODE>lib_box.c</CODE>, <CODE>lib_clear.c</CODE>,
<CODE>lib_clrbot.c</CODE>, <CODE>lib_clreol.c</CODE>, <CODE>lib_data.c</CODE>,
<CODE>lib_delch.c</CODE>, <CODE>lib_delwin.c</CODE>, <CODE>lib_erase.c</CODE>,
<CODE>lib_getstr.c</CODE>, <CODE>lib_inchstr.c</CODE>, <CODE>lib_insch.c</CODE>,
<CODE>lib_insdel.c</CODE>, <CODE>lib_insstr.c</CODE>, <CODE>lib_instr.c</CODE>,
<CODE>lib_isendwin.c</CODE>, <CODE>lib_keyname.c</CODE>, <CODE>lib_move.c</CODE>,
<CODE>lib_mvwin.c</CODE>, <CODE>lib_overlay.c</CODE>, <CODE>lib_pad.c</CODE>,
<CODE>lib_printw.c</CODE>, <CODE>lib_scanw.c</CODE>, <CODE>lib_screen.c</CODE>,
<CODE>lib_scroll.c</CODE>, <CODE>lib_scrreg.c</CODE>, <CODE>lib_set_term.c</CODE>,
<CODE>lib_slk.c</CODE>, <CODE>lib_touch.c</CODE>, <CODE>lib_unctrl.c</CODE>, and
<CODE>lib_window.c</CODE> are all in this category.  They are very
unlikely to need change, barring bugs or some fundamental
reorganization in the underlying data structures. <P>

The <CODE>lib_trace.c</CODE>, <CODE>lib_traceatr.c</CODE>, and
<CODE>lib_tracechr.c</CODE> file are used only for debugging support. 
It is rather unlikely you will ever need to change these, unless
you want to introduce a new debug trace level for some reasoon.<P>

There is another group of files that do direct I/O via <EM>tputs()</EM>,
computations on the terminal capabilities, or queries to the OS
environment, but nevertheless have only fairly low complexity.  These
include: <CODE>lib_acs.c</CODE>, <CODE>lib_beep.c</CODE>,
<CODE>lib_color.c</CODE>, <CODE>lib_endwin.c</CODE>, <CODE>lib_initscr.c</CODE>,
<CODE>lib_longname.c</CODE>, <CODE>lib_newterm.c</CODE>,
<CODE>lib_options.c</CODE>, <CODE>lib_termcap.c</CODE>, <CODE>lib_ti.c</CODE>,
<CODE>lib_tparm.c</CODE>, <CODE>lib_tputs.c</CODE>, <CODE>lib_vidattr.c</CODE>,
and <CODE>read_entry.c</CODE>.  These are likely to need revision only if
ncurses is being ported to an environment without an underlying
terminfo capability representation. <P>

The files <CODE>lib_kernel.c</CODE>, <CODE>lib_baudrate.c</CODE>, <CODE>lib_raw.c</CODE>,
<CODE>lib_tstp.c</CODE>, and <CODE>lib_twait.c</CODE> have serious hooks into
the tty driver and signal facilities.  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.<P>

Almost all of the real work is done in the files 
<CODE>hashmap.c</CODE>, <CODE>hardscroll.c</CODE>,
<CODE>lib_addch.c</CODE>, <CODE>lib_doupdate.c</CODE>, <CODE>lib_mvcur.c</CODE>,
<CODE>lib_getch.c</CODE>, <CODE>lib_mouse.c</CODE>, <CODE>lib_refresh.c</CODE>,
and <CODE>lib_setup.c</CODE>.  Most of the algorithmic complexity in the
library lives in these files.  If there is a real bug in <STRONG>ncurses</STRONG>
itself, it's probably here.  We'll tour some of these files in detail
below (see <A HREF="#engine">The Engine Room</A>). <P>

Finally, there is a group of files that is actually most of the
terminfo compiler.  The reason this code lives in the <STRONG>ncurses</STRONG>
library is to support fallback to /etc/termcap.  These files include
<CODE>alloc_entry.c</CODE>, <CODE>captoinfo.c</CODE>, <CODE>comp_captab.c</CODE>,
<CODE>comp_error.c</CODE>, <CODE>comp_hash.c</CODE>, <CODE>comp_parse.c</CODE>,
<CODE>comp_scan.c</CODE>, and <CODE>parse_entry.c</CODE>,
<CODE>read_termcap.c</CODE>, and <CODE>write_entry.c</CODE>.  We'll discuss these
in the compiler tour. <P>

<H2><A NAME="engine">The Engine Room</A></H2>

<H3><A NAME="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. <P>

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>. <P>

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. <P>

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 don't return a reliable
time-left value.  The function <CODE>timed_wait()</CODE> effectively
simulates a System V select. <P>

<H3><A NAME="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. <P>

Under xterm, however, mouse event notifications come in via the keyboard
input stream.  They are recognized by having the <STRONG>kmous</STRONG> 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. <P>

This kluge only works because <STRONG>kmous</STRONG> isn't actually used by any
historic terminal type or curses implementation we know of.  Best
guess is it's a relic of some forgotten experiment in-house at Bell
Labs that didn't 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. <P>

Here are some more details about mouse event handling: <P>

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. <P>

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
<STRONG>kmous</STRONG> prefix in the keyboard input stream and calls _nc_mouse_inline
to queue up a series of adjacent mouse reports. <P>

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). <P>

<H3><A NAME="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. <P>

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. <P>

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: <P>

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. <P>

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. <P>

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 posible. <P>

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. <P>

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. <P>

<H1><A NAME="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. <P>

The configuration code prefers the POSIX regex facility, modeled on
System V's, but will settle for BSD regexps if the former isn't available. <P>

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>. <P>

<H1><A NAME="tic">A Tour of the Terminfo Compiler</A></H1>

The <STRONG>ncurses</STRONG> implementation of <STRONG>tic</STRONG> 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. <P>

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 `,' or `:' 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". <P>

<H2><A NAME="nonuse">Translation of Non-<STRONG>use</STRONG> Capabilities</A></H2>

Translation of most things besides <STRONG>use</STRONG> 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. <P>

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). <P>

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

<H2><A NAME="uses">Use Capability Resolution</A></H2>

The background problem that makes <STRONG>tic</STRONG> tricky isn't the capability
translation itself, it's the resolution of <STRONG>use</STRONG> capabilities.  Older
versions would not handle forward <STRONG>use</STRONG> 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. <P>

This won't do for <STRONG>ncurses</STRONG>.  The problem is that that the whole
compilation process has to be embeddable in the <STRONG>ncurses</STRONG> library
so that it can be called by the startup code to translate termcap
entries on the fly.  The embedded version can't go promiscuously writing
everything it translates out to disk -- for one thing, it will typically
be running with non-root permissions. <P> 

So our <STRONG>tic</STRONG> is designed to parse an entire terminfo file into a
doubly-linked circular list of entry structures in-core, and then do
<STRONG>use</STRONG> 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're written out easy to do. <P>

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

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 <STRONG>three hours</STRONG>, rather than the twenty seconds
or less typical on the author's development box. <P>

So.  The executable <STRONG>tic</STRONG> passes the entry-parser a hook that
<EM>immediately</EM> 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's OK;
the resolution code will fetch it off disk when it can't find it in
core. <P>

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 <STRONG>tic</STRONG>'s first call to
<CODE>write_entry()</CODE>, Thus it will complain about overwriting
entries newly made during the <STRONG>tic</STRONG> run, but not about
overwriting ones that predate it. <P>

<H2><A NAME="translation">Source-Form Translation</A></H2>

Another use of <STRONG>tic</STRONG> 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 <STRONG>captoinfo(1)</STRONG>
manual page. <P>

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

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's possible to set up capability aliasing 
or tell the compiler to plain ignore a given capability without writing
any C code at all. <P>

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 <STRONG>box1</STRONG> capability
get translated to an <STRONG>acsc</STRONG> string.<P>

<H1><A NAME="utils">Other Utilities</A></H1>

The <STRONG>infocmp</STRONG> utility is just a wrapper around the same
entry-dumping code used by <STRONG>tic</STRONG> 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. <P>

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

<H1><A NAME="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. <P>

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. <P>

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

Don't try to auto-detect OS features in the main body of the C code.
That's the job of the configuration system. <P>

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's still
preferable to ad-hoc code -- that's why the fifth field (flags) is
there. <P>

Have fun! <P>

<H1><A NAME="port">Porting Hints</A></H1>

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

The following library modules are `pure curses'; 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. <P>

<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>
<P>

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

<CODE>
lib_getstr.c
</CODE>
<P>

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

<CODE>
lib_beep.c
lib_endwin.c
lib_color.c
lib_options.c
lib_slk.c
lib_vidattr.c
</CODE>
<P>

This modules assist in POSIX emulation on non-POSIX systems: <P>
<DL>
<DT> sigaction.c
<DD> signal calls
</DL>

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

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

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

<DL>
<DT> lib_screen.c
<DD> used to read/write screen dumps
<DT> lib_trace.c
<DD> used to write trace data to the logfile
</DL>

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

The following modules are `pure curses' but contain assumptions inappropriate
for a memory-mapped port. <P>

<PRE>
lib_longname.c	-- assumes there may be multiple terminals
	longname()		-- return long name of terminal
lib_acs.c	-- assumes acs_map as a double indirection
	init_acs()		-- initialize acs map
lib_mvcur.c	-- assumes cursor moves have variable cost
	mvcur_init()		-- initialize
	mvcur()			-- do physical cursor move
	mvcur_wrap()		-- wrap
	scrolln()		-- do physical scrolling
lib_termcap.c	-- assumes there may be multiple terminals
	tgetent()		-- load entry
	tgetflag()		-- get boolean capability
	tgetnum()		-- get numeric capability
	tgetstr()		-- get string capability
lib_ti.c	-- assumes there may be multiple terminals
	tigetent()		-- load entry
	tigetflag()		-- get boolean capability
	tigetnum()		-- get numeric capability
	tigetstr()		-- get string capability

The following modules use UNIX-specific calls:

lib_doupdate.c	-- input checking
	doupdate()		-- repaint real screen to match virtual
	_nc_outch()		-- put out a single character
lib_getch.c	-- read()
	wgetch()		-- get single character
	wungetch()		-- push back single character
lib_initscr.c	-- getenv()
	initscr()		-- initialize curses functions
lib_newterm.c
	newterm()		-- set up new terminal screen
lib_baudrate.c
	baudrate()		-- return the baudrate
lib_kernel.c	-- various tty-manipulation and system calls
	reset_prog_mode()	-- reset ccurses-raw mode
	reset_shell_mode()	-- reset cooked mode
	erasechar()		-- return the erase char
	killchar()		-- return the kill character
	flushinp()		-- flush pending input
	savetty()		-- save tty state
	resetty()		-- reset tty to state at last savetty() 
lib_raw.c	-- various tty-manipulation calls
	raw()
	echo()
	nl()
	qiflush()
	cbreak()
	noraw()
	noecho()
	nonl()
	noqiflush()
	nocbreak()
lib_setup.c	-- various tty-manipulation calls
	use_env()
	setupterm()
lib_restart.c	-- various tty-manipulation calls
	def_shell_mode()
	def_prog_mode()
	set_curterm()
	del_curterm()
lib_tstp.c	-- signal-manipulation calls
	_nc_signal_handler()	-- enable/disable window-mode signal catching
lib_twait.c	-- gettimeofday(), select().
	usleep()		-- microsecond sleep
	_nc_timed_wait()	-- timed wait for input
</PRE>

The package kernel could be made smaller. <P>

<HR>
<ADDRESS>Eric S. Raymond &lt;esr@snark.thyrsus.com&gt;</ADDRESS>
(Note: This is <EM>not</EM> the <A HREF="#bugtrack">bug address</A>!)
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