evo/Gtk4-tutorial: GTK 4 tutorial for beginners

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Toshio Sekiya
Section 22, 24 and 25 are updated.
05 янв 2023, 14:41
05 янв 2023, 14:41a8cf717
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<h1 id="tiny-turtle-graphics-interpreter">Tiny turtle graphics
interpreter</h1>
<p>A program <code>turtle</code> is an example with the combination of
TfeTextView and GtkDrawingArea objects. It is a very small interpreter
but you can draw fractal curves with it. The following diagram is a Koch
curve, which is one of famous fractal curves.</p>
<figure>
<img src="image/turtle_koch.png" alt="Koch curve" />
<figcaption aria-hidden="true">Koch curve</figcaption>
</figure>
<p>The following is a snow-crystal-shaped curve. It is composed of six
Koch curves.</p>
<figure>
<img src="image/turtle_snow.png" alt="Snow" />
<figcaption aria-hidden="true">Snow</figcaption>
</figure>
<p>This program uses flex and bison. Flex is a lexical analyzer. Bison
is a parser generator. These two programs are similar to lex and yacc
which are proprietary software developed in Bell Laboratory. However,
flex and bison are open source software. I will write about how to use
those software, but they are not topics about GTK 4. So, readers can
skip this section.</p>
<h2 id="how-to-use-turtle">How to use turtle</h2>
<p>The turtle document is <a
href="https://toshiocp.github.io/Gtk4-tutorial/turtle_doc.html">here</a>.
I’ll show you a simple example.</p>
<pre><code>fc (1,0,0) # Foreground color is red, rgb = (1,0,0).
pd         # Pen down.
rp (4) {   # Repeat four times.
  fd 100   # Go forward by 100 pixels.
  tr 90    # Turn right by 90 degrees.
}</code></pre>
<ol type="1">
<li>Compile and install <code>turtle</code> (See the documentation
above). Then, run <code>turtle</code>.</li>
<li>Type the program above in the editor (left part of the window).</li>
<li>Click on the <code>Run</code> button, then a red square appears on
the right part of the window. The side of the square is 100 pixels
long.</li>
</ol>
<p>In the same way, you can draw other curves. The turtle document
includes some fractal curves such as tree, snow and square-koch. The
source codes are located at src/turtle/example directory. You can read
these files into <code>turtle</code> editor by clicking on the
<code>Open</code> button.</p>
<h2
id="combination-of-tfetextview-and-gtkdrawingarea-objects">Combination
of TfeTextView and GtkDrawingArea objects</h2>
<p>Turtle uses TfeTextView and GtkDrawingArea. It is similar to
<code>color</code> program in the previous section.</p>
<ol type="1">
<li>A user inputs/reads a turtle program into the buffer in the
TfeTextView instance.</li>
<li>The user clicks on the “Run” button.</li>
<li>The parser reads the program and generates tree-structured
data.</li>
<li>The interpriter reads the data and executes it step by step. And it
draws shapes on a surface. The surface isn’t the one in the
GtkDrawingArea widget.</li>
<li>The widget is added to the queue. It will be redrawn with the
drawing function, which just copies the surface into the one in the
GtkDrawingArea.</li>
</ol>
<figure>
<img src="image/turtle.png"
alt="Parser, interpreter and drawing function" />
<figcaption aria-hidden="true">Parser, interpreter and drawing
function</figcaption>
</figure>
<p>The body of the interpreter is written with flex and bison. The codes
are not thread safe. So the handler of “clicked” signal of the
<code>Run</code> button prevents from reentering.</p>
<div class="sourceCode" id="cb2"><pre
class="sourceCode numberSource C numberLines"><code class="sourceCode c"><span id="cb2-1"><a href="#cb2-1"></a><span class="dt">void</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>run_cb <span class="op">(</span>GtkWidget <span class="op">*</span>btnr<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a>  GtkTextBuffer <span class="op">*</span>tb <span class="op">=</span> gtk_text_view_get_buffer <span class="op">(</span>GTK_TEXT_VIEW <span class="op">(</span>tv<span class="op">));</span></span>
<span id="cb2-4"><a href="#cb2-4"></a>  GtkTextIter start_iter<span class="op">;</span></span>
<span id="cb2-5"><a href="#cb2-5"></a>  GtkTextIter end_iter<span class="op">;</span></span>
<span id="cb2-6"><a href="#cb2-6"></a>  <span class="dt">char</span> <span class="op">*</span>contents<span class="op">;</span></span>
<span id="cb2-7"><a href="#cb2-7"></a>  <span class="dt">int</span> stat<span class="op">;</span></span>
<span id="cb2-8"><a href="#cb2-8"></a>  <span class="dt">static</span> gboolean busy <span class="op">=</span> FALSE<span class="op">;</span> <span class="co">/* initialized only once */</span></span>
<span id="cb2-9"><a href="#cb2-9"></a></span>
<span id="cb2-10"><a href="#cb2-10"></a>  <span class="co">/* yyparse() and run() are NOT thread safe. */</span></span>
<span id="cb2-11"><a href="#cb2-11"></a>  <span class="co">/* The variable busy avoids reentrance. */</span></span>
<span id="cb2-12"><a href="#cb2-12"></a>  <span class="cf">if</span> <span class="op">(</span>busy<span class="op">)</span></span>
<span id="cb2-13"><a href="#cb2-13"></a>    <span class="cf">return</span><span class="op">;</span></span>
<span id="cb2-14"><a href="#cb2-14"></a>  busy <span class="op">=</span> TRUE<span class="op">;</span></span>
<span id="cb2-15"><a href="#cb2-15"></a>  gtk_text_buffer_get_bounds <span class="op">(</span>tb<span class="op">,</span> <span class="op">&amp;</span>start_iter<span class="op">,</span> <span class="op">&amp;</span>end_iter<span class="op">);</span></span>
<span id="cb2-16"><a href="#cb2-16"></a>  contents <span class="op">=</span> gtk_text_buffer_get_text <span class="op">(</span>tb<span class="op">,</span> <span class="op">&amp;</span>start_iter<span class="op">,</span> <span class="op">&amp;</span>end_iter<span class="op">,</span> FALSE<span class="op">);</span></span>
<span id="cb2-17"><a href="#cb2-17"></a>  <span class="cf">if</span> <span class="op">(</span>surface<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-18"><a href="#cb2-18"></a>    init_flex <span class="op">(</span>contents<span class="op">);</span></span>
<span id="cb2-19"><a href="#cb2-19"></a>    stat <span class="op">=</span> yyparse <span class="op">();</span></span>
<span id="cb2-20"><a href="#cb2-20"></a>    <span class="cf">if</span> <span class="op">(</span>stat <span class="op">==</span> <span class="dv">0</span><span class="op">)</span> <span class="co">/* No error */</span> <span class="op">{</span></span>
<span id="cb2-21"><a href="#cb2-21"></a>      run <span class="op">();</span></span>
<span id="cb2-22"><a href="#cb2-22"></a>    <span class="op">}</span></span>
<span id="cb2-23"><a href="#cb2-23"></a>    finalize_flex <span class="op">();</span></span>
<span id="cb2-24"><a href="#cb2-24"></a>  <span class="op">}</span></span>
<span id="cb2-25"><a href="#cb2-25"></a>  g_free <span class="op">(</span>contents<span class="op">);</span></span>
<span id="cb2-26"><a href="#cb2-26"></a>  gtk_widget_queue_draw <span class="op">(</span>GTK_WIDGET <span class="op">(</span>da<span class="op">));</span></span>
<span id="cb2-27"><a href="#cb2-27"></a>  busy <span class="op">=</span> FALSE<span class="op">;</span></span>
<span id="cb2-28"><a href="#cb2-28"></a><span class="op">}</span></span>
<span id="cb2-29"><a href="#cb2-29"></a></span>
<span id="cb2-30"><a href="#cb2-30"></a><span class="dt">static</span> <span class="dt">void</span></span>
<span id="cb2-31"><a href="#cb2-31"></a>resize_cb <span class="op">(</span>GtkDrawingArea <span class="op">*</span>drawing_area<span class="op">,</span> <span class="dt">int</span> width<span class="op">,</span> <span class="dt">int</span> height<span class="op">,</span> gpointer user_data<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-32"><a href="#cb2-32"></a></span>
<span id="cb2-33"><a href="#cb2-33"></a>  <span class="cf">if</span> <span class="op">(</span>surface<span class="op">)</span></span>
<span id="cb2-34"><a href="#cb2-34"></a>    cairo_surface_destroy <span class="op">(</span>surface<span class="op">);</span></span>
<span id="cb2-35"><a href="#cb2-35"></a>  surface <span class="op">=</span> cairo_image_surface_create <span class="op">(</span>CAIRO_FORMAT_ARGB32<span class="op">,</span> width<span class="op">,</span> height<span class="op">);</span></span>
<span id="cb2-36"><a href="#cb2-36"></a>  run_cb <span class="op">(</span>NULL<span class="op">);</span> <span class="co">// NULL is a fake (run button).</span></span>
<span id="cb2-37"><a href="#cb2-37"></a><span class="op">}</span></span></code></pre></div>
<ul>
<li>8-13: The static value <code>busy</code> holds a status of the
interpreter. If it is <code>TRUE</code>, the interpreter is running and
it is not possible to call the interpreter again because it’s not a
re-entrant program. If it is <code>FALSE</code>, it is safe to call the
interpreter.</li>
<li>14: Changes <code>busy</code> to TRUE to avoid reentrance.</li>
<li>15-16: Gets the contents of <code>tb</code>.</li>
<li>17: The variable <code>surface</code> is a static variable. It
points to a <code>cairo_surface_t</code> instance. It is created when
the GtkDrawingArea instance is realized and whenever it is resized.
Therefore, <code>surface</code> isn’t NULL usually. But if it is NULL,
the interpreter won’t be called.</li>
<li>18: Initializes lexical analyzer.</li>
<li>19: Calls parser. Parser analyzes the program codes syntactically
and generates a tree structured data.</li>
<li>20-22: If the parser successfully parsed, it calls <code>run</code>
(runtime routine).</li>
<li>23: finalizes the lexical analyzer.</li>
<li>25: frees <code>contents</code>.</li>
<li>26: Adds the drawing area widget to the queue to draw.</li>
<li>27: The interpreter program has finished so <code>busy</code> is now
changed to FALSE.</li>
<li>30-37: A “resized” signal handler. If the <code>surface</code> isn’t
NULL, it is destroyed. A new surface is created. Its size is the same as
the surface of the GtkDrawingArea instance. Run_cb is called to redraw
the shape on the drawing area.</li>
</ul>
<p>Other part of <code>turtleapplication.c</code> is almost same as the
codes of <code>colorapplication.c</code> in the previous section. The
codes of <code>turtleapplication.c</code> is in the turtle
directory.</p>
<h2 id="what-does-the-interpreter-do">What does the interpreter do?</h2>
<p>Suppose that the turtle runs with the following program.</p>
<pre><code>distance = 100
fd distance*2</code></pre>
<p>The turtle recognizes the program above and works as follows.</p>
<ul>
<li>Generally, a program consists of tokens. Tokens are “distance”, “=”,
“100”, “fd”, “*” and “2” in the above example..</li>
<li>The parser calls a function <code>yylex</code> to read a token in
the source file. <code>yylex</code> returns a code which is called
“token kind” and sets a global variable <code>yylval</code> with a
value, which is called a semantic value. The type of <code>yylval</code>
is union and <code>yylval.ID</code> is string and
<code>yylval.NUM</code> is double. There are seven tokens in the program
so <code>yylex</code> is called seven times.</li>
</ul>
<table>
<thead>
<tr class="header">
<th style="text-align: center;"></th>
<th style="text-align: center;">token kind</th>
<th style="text-align: center;">yylval.ID</th>
<th style="text-align: center;">yylval.NUM</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: center;">1</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">2</td>
<td style="text-align: center;">=</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">3</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">100</td>
</tr>
<tr class="even">
<td style="text-align: center;">4</td>
<td style="text-align: center;">FD</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">5</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">6</td>
<td style="text-align: center;">*</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">7</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">2</td>
</tr>
</tbody>
</table>
<ul>
<li><code>yylex</code> returns a token kind every time, but it doesn’t
set <code>yylval.ID</code> or <code>yylval.NUM</code> every time. It is
because keywords (<code>FD</code>) and symbols (<code>=</code> and
<code>*</code>) don’t have any semantic values. The function
<code>yylex</code> is called lexical analyzer or scanner.</li>
<li><code>turtle</code> makes a tree structured data. This part of
<code>turtle</code> is called parser.</li>
</ul>
<figure>
<img src="image/turtle_parser_tree.png" alt="turtle parser tree" />
<figcaption aria-hidden="true">turtle parser tree</figcaption>
</figure>
<ul>
<li><code>turtle</code> analyzes the tree and executes it. This part of
<code>turtle</code> is called runtime routine or interpreter. The tree
consists of rectangles and line segments between the rectangles. The
rectangles are called nodes. For example, N_PROGRAM, N_ASSIGN, N_FD and
N_MUL are nodes.
<ol type="1">
<li>Goes down from N_PROGRAM to N_ASSIGN.</li>
<li>N_ASSIGN node has two children, ID and NUM. This node comes from
“distance = 100” which is “ID = NUM” syntactically. First,
<code>turtle</code> checks if the first child is ID. If it’s ID, then
<code>turtle</code> looks for the variable in the variable table. If it
doesn’t exist, it registers the ID (<code>distance</code>) to the table.
Then go back to the N_ASSIGN node.</li>
<li><code>turtle</code> calculates the second child. In this case its a
number 100. Saves 100 to the variable table at the <code>distance</code>
record.</li>
<li><code>turtle</code> goes back to N_PROGRAM then go to the next node
N_FD. It has only one child. Goes down to the child N_MUL.</li>
<li>The first child is ID (distance). Searches the variable table for
the variable <code>distance</code> and gets the value 100. The second
child is a number 2. Multiplies 100 by 2 and gets 200. Then
<code>turtle</code> goes back to N_FD.</li>
<li>Now <code>turtle</code> knows the distance is 200. It moves the
cursor forward by 200 pixels. The segment is drawn on the surface
(<code>surface</code>).</li>
<li>There are no node follows. Runtime routine returns to the function
<code>run_cb</code>.</li>
</ol></li>
<li><code>run_cb</code> calls <code>gtk_widget_queue_draw</code> and put
the GtkDrawingArea widget to the queue.</li>
<li>The system redraws the widget. At that time drawing function
<code>draw_func</code> is called. The function copies the surface
(<code>surface</code>) to the surface in the GtkDrawingArea.</li>
</ul>
<p>Actual turtle program is more complicated than the example above.
However, what turtle does is basically the same. Interpretation consists
of three parts.</p>
<ul>
<li>Lexical analysis</li>
<li>Syntax Parsing and tree generation</li>
<li>Interpretation and execution of the tree.</li>
</ul>
<h2 id="compilation-flow">Compilation flow</h2>
<p>The source files are:</p>
<ul>
<li>flex source file =&gt; <code>turtle.lex</code></li>
<li>bison source file =&gt; <code>turtle.y</code></li>
<li>C header file =&gt; <code>turtle_lex.h</code></li>
<li>C source file =&gt; <code>turtleapplication.c</code></li>
<li>other files =&gt; <code>turtle.ui</code>,
<code>turtle.gresources.xml</code> and <code>meson.build</code></li>
</ul>
<p>The compilation process is a bit complicated.</p>
<ol type="1">
<li>glib-compile-resources compiles <code>turtle.ui</code> to
<code>resources.c</code> according to <code>turtle.gresource.xml</code>.
It also generates <code>resources.h</code>.</li>
<li>bison compiles <code>turtle.y</code> to <code>turtle_parser.c</code>
and generates <code>turtle_parser.h</code></li>
<li>flex compiles <code>turtle.lex</code> to
<code>turtle_lex.c</code>.</li>
<li>gcc compiles <code>application.c</code>, <code>resources.c</code>,
<code>turtle_parser.c</code> and <code>turtle_lex.c</code> with
<code>turtle_lex.h</code>, <code>resources.h</code> and
<code>turtle_parser.h</code>. It generates an executable file
<code>turtle</code>.</li>
</ol>
<figure>
<img src="image/turtle_compile_process.png" alt="compile process" />
<figcaption aria-hidden="true">compile process</figcaption>
</figure>
<p>Meson controls the process. The instruction is described in
<code>meson.build</code>.</p>
<div class="sourceCode" id="cb4"><pre
class="sourceCode numberSource numberLines"><code class="sourceCode"><span id="cb4-1"><a href="#cb4-1"></a>project(&#39;turtle&#39;, &#39;c&#39;)</span>
<span id="cb4-2"><a href="#cb4-2"></a></span>
<span id="cb4-3"><a href="#cb4-3"></a>compiler = meson.get_compiler(&#39;c&#39;)</span>
<span id="cb4-4"><a href="#cb4-4"></a>mathdep = compiler.find_library(&#39;m&#39;, required : true)</span>
<span id="cb4-5"><a href="#cb4-5"></a></span>
<span id="cb4-6"><a href="#cb4-6"></a>gtkdep = dependency(&#39;gtk4&#39;)</span>
<span id="cb4-7"><a href="#cb4-7"></a></span>
<span id="cb4-8"><a href="#cb4-8"></a>gnome=import(&#39;gnome&#39;)</span>
<span id="cb4-9"><a href="#cb4-9"></a>resources = gnome.compile_resources(&#39;resources&#39;,&#39;turtle.gresource.xml&#39;)</span>
<span id="cb4-10"><a href="#cb4-10"></a></span>
<span id="cb4-11"><a href="#cb4-11"></a>flex = find_program(&#39;flex&#39;)</span>
<span id="cb4-12"><a href="#cb4-12"></a>bison = find_program(&#39;bison&#39;)</span>
<span id="cb4-13"><a href="#cb4-13"></a>turtleparser = custom_target(&#39;turtleparser&#39;, input: &#39;turtle.y&#39;, output: [&#39;turtle_parser.c&#39;, &#39;turtle_parser.h&#39;], command: [bison, &#39;-d&#39;, &#39;-o&#39;, &#39;turtle_parser.c&#39;, &#39;@INPUT@&#39;])</span>
<span id="cb4-14"><a href="#cb4-14"></a>turtlelexer = custom_target(&#39;turtlelexer&#39;, input: &#39;turtle.lex&#39;, output: &#39;turtle_lex.c&#39;, command: [flex, &#39;-o&#39;, &#39;@OUTPUT@&#39;, &#39;@INPUT@&#39;])</span>
<span id="cb4-15"><a href="#cb4-15"></a></span>
<span id="cb4-16"><a href="#cb4-16"></a>sourcefiles=files(&#39;turtleapplication.c&#39;, &#39;../tfetextview/tfetextview.c&#39;)</span>
<span id="cb4-17"><a href="#cb4-17"></a></span>
<span id="cb4-18"><a href="#cb4-18"></a>executable(&#39;turtle&#39;, sourcefiles, resources, turtleparser, turtlelexer, turtleparser[1], dependencies: [mathdep, gtkdep], export_dynamic: true, install: true)</span></code></pre></div>
<ul>
<li>3: Gets C compiler. It is usually <code>gcc</code> in linux.</li>
<li>4: Gets math library. This program uses trigonometric functions.
They are defined in the math library, but the library is optional. So,
it is necessary to include it by <code>#include &lt;math.h&gt;</code>
and also link the library with the linker.</li>
<li>6: Gets gtk4 library.</li>
<li>8: Gets gnome module.See <a
href="https://mesonbuild.com/Gnome-module.html#gnome-module">Meson build
system website – GNUME module</a> for further information.</li>
<li>9: Compiles ui file to C source file according to the XML file
<code>turtle.gresource.xml</code>.</li>
<li>11: Gets flex.</li>
<li>12: Gets bison.</li>
<li>13: Compiles <code>turtle.y</code> to <code>turtle_parser.c</code>
and <code>turtle_parser.h</code> by bison. The function
<code>custom_target</code> creates a custom top level target. See <a
href="https://mesonbuild.com/Reference-manual.html#custom_target">Meson
build system website – custom target</a> for further information.</li>
<li>14: Compiles <code>turtle.lex</code> to <code>turtle_lex.c</code> by
flex.</li>
<li>16: Specifies C source files.</li>
<li>18: Compiles C source files including generated files by
glib-compile-resources, bison and flex. The argument
<code>turtleparser[1]</code> refers to <code>tirtle_parser.h</code>
which is the second output in the line 13.</li>
</ul>
<h2 id="turtle.lex">Turtle.lex</h2>
<h3 id="what-does-flex-do">What does flex do?</h3>
<p>Flex creates lexical analyzer from flex source file. Flex source file
is a text file. Its syntactic rule will be explained later. Generated
lexical analyzer is a C source file. It is also called scanner. It reads
a text file, which is a source file of a program language, and gets
variable names, numbers and symbols. Suppose here is a turtle source
file.</p>
<pre><code>fc (1,0,0) # Foreground color is red, rgb = (1,0,0).
pd         # Pen down.
distance = 100
angle = 90
fd distance    # Go forward by distance (100) pixels.
tr angle     # Turn right by angle (90) degrees.</code></pre>
<p>The content of the text file is separated into <code>fc</code>,
<code>(</code>, <code>1</code> and so on. The words <code>fc</code>,
<code>pd</code>, <code>distance</code>, <code>angle</code>,
<code>tr</code>, <code>1</code>, <code>0</code>, <code>100</code> and
<code>90</code> are called tokens. The characters ‘<code>(</code>’ (left
parenthesis), ‘<code>,</code>’ (comma), ‘<code>)</code>’ (right
parenthesis) and ‘<code>=</code>’ (equal sign) are called symbols. (
Sometimes those symbols called tokens, too.)</p>
<p>Flex reads <code>turtle.lex</code> and generates the C source file of
a scanner. The file <code>turtle.lex</code> specifies tokens, symbols
and the behavior which corresponds to each token or symbol. Turtle.lex
isn’t a big program.</p>
<div class="sourceCode" id="cb6"><pre
class="sourceCode numberSource lex numberLines"><code class="sourceCode lex"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">%top{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a>#include &lt;string.h&gt;</span>
<span id="cb6-3"><a href="#cb6-3"></a>#include &lt;stdlib.h&gt;</span>
<span id="cb6-4"><a href="#cb6-4"></a>#include &lt;glib.h&gt;</span>
<span id="cb6-5"><a href="#cb6-5"></a>#include &quot;turtle_parser.h&quot;</span>
<span id="cb6-6"><a href="#cb6-6"></a></span>
<span id="cb6-7"><a href="#cb6-7"></a>  <span class="at">static</span> <span class="dt">int</span> nline <span class="op">=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb6-8"><a href="#cb6-8"></a>  <span class="at">static</span> <span class="dt">int</span> ncolumn <span class="op">=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb6-9"><a href="#cb6-9"></a>  <span class="at">static</span> <span class="dt">void</span> get_location <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>text<span class="op">);</span></span>
<span id="cb6-10"><a href="#cb6-10"></a></span>
<span id="cb6-11"><a href="#cb6-11"></a>  <span class="co">/* Dinamically allocated memories are added to the single list. They will be freed in the finalize function. */</span></span>
<span id="cb6-12"><a href="#cb6-12"></a>  <span class="at">extern</span> GSList <span class="op">*</span>list<span class="op">;</span></span>
<span id="cb6-13"><a href="#cb6-13"></a>}</span>
<span id="cb6-14"><a href="#cb6-14"></a></span>
<span id="cb6-15"><a href="#cb6-15"></a><span class="kw">%option noyywrap</span></span>
<span id="cb6-16"><a href="#cb6-16"></a></span>
<span id="cb6-17"><a href="#cb6-17"></a><span class="dt">REAL_NUMBER </span><span class="st">(0|[1-9][0-9]*)(\.[0-9]+)?</span></span>
<span id="cb6-18"><a href="#cb6-18"></a><span class="dt">IDENTIFIER </span><span class="st">[a-zA-Z][a-zA-Z0-9]*</span></span>
<span id="cb6-19"><a href="#cb6-19"></a><span class="bn">%%</span></span>
<span id="cb6-20"><a href="#cb6-20"></a>  <span class="co">/* rules */</span></span>
<span id="cb6-21"><a href="#cb6-21"></a><span class="st">#.*</span>               <span class="op">;</span> <span class="co">/* comment. Be careful. Dot symbol (.) matches any character but new line. */</span></span>
<span id="cb6-22"><a href="#cb6-22"></a><span class="st">[ ]</span>               ncolumn<span class="op">++;</span></span>
<span id="cb6-23"><a href="#cb6-23"></a><span class="st">\t</span>                ncolumn <span class="op">+=</span> <span class="dv">8</span><span class="op">;</span> <span class="co">/* assume that tab is 8 spaces. */</span></span>
<span id="cb6-24"><a href="#cb6-24"></a><span class="st">\n</span>                nline<span class="op">++;</span> ncolumn <span class="op">=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb6-25"><a href="#cb6-25"></a>  <span class="co">/* reserved keywords */</span></span>
<span id="cb6-26"><a href="#cb6-26"></a><span class="st">pu</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> PU<span class="op">;</span> <span class="co">/* pen up */</span></span>
<span id="cb6-27"><a href="#cb6-27"></a><span class="st">pd</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> PD<span class="op">;</span> <span class="co">/* pen down */</span></span>
<span id="cb6-28"><a href="#cb6-28"></a><span class="st">pw</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> PW<span class="op">;</span> <span class="co">/* pen width = line width */</span></span>
<span id="cb6-29"><a href="#cb6-29"></a><span class="st">fd</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> FD<span class="op">;</span> <span class="co">/* forward */</span></span>
<span id="cb6-30"><a href="#cb6-30"></a><span class="st">tr</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> TR<span class="op">;</span> <span class="co">/* turn right */</span></span>
<span id="cb6-31"><a href="#cb6-31"></a><span class="st">tl</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> TL<span class="op">;</span> <span class="co">/* turn left ver 0.5 */</span></span>
<span id="cb6-32"><a href="#cb6-32"></a><span class="st">bc</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> BC<span class="op">;</span> <span class="co">/* background color */</span></span>
<span id="cb6-33"><a href="#cb6-33"></a><span class="st">fc</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> FC<span class="op">;</span> <span class="co">/* foreground color */</span></span>
<span id="cb6-34"><a href="#cb6-34"></a><span class="st">dp</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> DP<span class="op">;</span> <span class="co">/* define procedure */</span></span>
<span id="cb6-35"><a href="#cb6-35"></a><span class="st">if</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> IF<span class="op">;</span> <span class="co">/* if statement */</span></span>
<span id="cb6-36"><a href="#cb6-36"></a><span class="st">rt</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> RT<span class="op">;</span> <span class="co">/* return statement */</span></span>
<span id="cb6-37"><a href="#cb6-37"></a><span class="st">rs</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> RS<span class="op">;</span> <span class="co">/* reset the status */</span></span>
<span id="cb6-38"><a href="#cb6-38"></a><span class="st">rp</span>                get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> RP<span class="op">;</span> <span class="co">/* repeat ver 0.5 */</span></span>
<span id="cb6-39"><a href="#cb6-39"></a>  <span class="co">/* constant */</span></span>
<span id="cb6-40"><a href="#cb6-40"></a><span class="st">{REAL_NUMBER}</span>     get_location <span class="op">(</span>yytext<span class="op">);</span> yylval<span class="op">.</span>NUM <span class="op">=</span> atof <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> NUM<span class="op">;</span></span>
<span id="cb6-41"><a href="#cb6-41"></a>  <span class="co">/* identifier */</span></span>
<span id="cb6-42"><a href="#cb6-42"></a><span class="st">{IDENTIFIER}</span>      { get_location <span class="op">(</span>yytext<span class="op">);</span> yylval<span class="op">.</span>ID <span class="op">=</span> <span class="va">g_strdup</span><span class="op">(</span>yytext<span class="op">);</span></span>
<span id="cb6-43"><a href="#cb6-43"></a>                    list <span class="op">=</span> <span class="va">g_slist_prepend</span> <span class="op">(</span>list<span class="op">,</span> yylval<span class="op">.</span>ID<span class="op">);</span></span>
<span id="cb6-44"><a href="#cb6-44"></a>                    <span class="cf">return</span> ID<span class="op">;</span></span>
<span id="cb6-45"><a href="#cb6-45"></a>                  }</span>
<span id="cb6-46"><a href="#cb6-46"></a><span class="st">&quot;=&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;=&#39;</span><span class="op">;</span></span>
<span id="cb6-47"><a href="#cb6-47"></a><span class="st">&quot;&gt;&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;&gt;&#39;</span><span class="op">;</span></span>
<span id="cb6-48"><a href="#cb6-48"></a><span class="st">&quot;&lt;&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;&lt;&#39;</span><span class="op">;</span></span>
<span id="cb6-49"><a href="#cb6-49"></a><span class="st">&quot;+&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;+&#39;</span><span class="op">;</span></span>
<span id="cb6-50"><a href="#cb6-50"></a><span class="st">&quot;-&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;-&#39;</span><span class="op">;</span></span>
<span id="cb6-51"><a href="#cb6-51"></a><span class="st">&quot;*&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;*&#39;</span><span class="op">;</span></span>
<span id="cb6-52"><a href="#cb6-52"></a><span class="st">&quot;/&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;/&#39;</span><span class="op">;</span></span>
<span id="cb6-53"><a href="#cb6-53"></a><span class="st">&quot;(&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;(&#39;</span><span class="op">;</span></span>
<span id="cb6-54"><a href="#cb6-54"></a><span class="st">&quot;)&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;)&#39;</span><span class="op">;</span></span>
<span id="cb6-55"><a href="#cb6-55"></a><span class="st">&quot;{&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;{&#39;</span><span class="op">;</span></span>
<span id="cb6-56"><a href="#cb6-56"></a><span class="st">&quot;}&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;}&#39;</span><span class="op">;</span></span>
<span id="cb6-57"><a href="#cb6-57"></a><span class="st">&quot;,&quot;</span>               get_location <span class="op">(</span>yytext<span class="op">);</span> <span class="cf">return</span> <span class="ch">&#39;,&#39;</span><span class="op">;</span></span>
<span id="cb6-58"><a href="#cb6-58"></a><span class="st">.</span>                 ncolumn<span class="op">++;</span>             <span class="cf">return</span> YYUNDEF<span class="op">;</span></span>
<span id="cb6-59"><a href="#cb6-59"></a><span class="bn">%%</span></span>
<span id="cb6-60"><a href="#cb6-60"></a></span>
<span id="cb6-61"><a href="#cb6-61"></a><span class="at">static</span> <span class="dt">void</span></span>
<span id="cb6-62"><a href="#cb6-62"></a>get_location <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>text<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-63"><a href="#cb6-63"></a>  yylloc<span class="op">.</span>first_line <span class="op">=</span> yylloc<span class="op">.</span>last_line <span class="op">=</span> nline<span class="op">;</span></span>
<span id="cb6-64"><a href="#cb6-64"></a>  yylloc<span class="op">.</span>first_column <span class="op">=</span> ncolumn<span class="op">;</span></span>
<span id="cb6-65"><a href="#cb6-65"></a>  yylloc<span class="op">.</span>last_column <span class="op">=</span> <span class="op">(</span>ncolumn <span class="op">+=</span> strlen<span class="op">(</span>text<span class="op">))</span> <span class="op">-</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb6-66"><a href="#cb6-66"></a><span class="op">}</span></span>
<span id="cb6-67"><a href="#cb6-67"></a></span>
<span id="cb6-68"><a href="#cb6-68"></a><span class="at">static</span> YY_BUFFER_STATE state<span class="op">;</span></span>
<span id="cb6-69"><a href="#cb6-69"></a></span>
<span id="cb6-70"><a href="#cb6-70"></a><span class="dt">void</span></span>
<span id="cb6-71"><a href="#cb6-71"></a>init_flex <span class="op">(</span><span class="at">const</span> <span class="dt">char</span> <span class="op">*</span>text<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-72"><a href="#cb6-72"></a>  state <span class="op">=</span> yy_scan_string <span class="op">(</span>text<span class="op">);</span></span>
<span id="cb6-73"><a href="#cb6-73"></a><span class="op">}</span></span>
<span id="cb6-74"><a href="#cb6-74"></a></span>
<span id="cb6-75"><a href="#cb6-75"></a><span class="dt">void</span></span>
<span id="cb6-76"><a href="#cb6-76"></a>finalize_flex <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-77"><a href="#cb6-77"></a>  yy_delete_buffer <span class="op">(</span>state<span class="op">);</span></span>
<span id="cb6-78"><a href="#cb6-78"></a><span class="op">}</span></span></code></pre></div>
<p>The file consists of three sections which are separated by “%%” (line
18 and 56). They are definitions, rules and user code sections.</p>
<h3 id="definitions-section">Definitions section</h3>
<ul>
<li>1-12: Lines between “%top{” and “}” are C source codes. They will be
copied to the top of the generated C source file.</li>
<li>2-3: The function <code>strlen</code>, in line 65, is defined in
<code>string.h</code> The function <code>atof</code>, in line 40, is
defined in <code>stdlib.h</code>.</li>
<li>7-9: The current input position is pointed by <code>nline</code> and
<code>ncolumn</code>. The function <code>get_location</code> (line
61-66) sets <code>yylloc</code>to point the start and end point of
<code>yytext</code> in the buffer. This function is declared here so
that it can be called before the function is defined.</li>
<li>12: GSlist is used to keep allocated memories.</li>
<li>15: This option (<code>%option noyywrap</code>) must be specified
when you have only single source file to the scanner. Refer to “9 The
Generated Scanner” in the flex documentation in your distribution for
further information. (The documentation is not on the internet.)</li>
<li>17-18: <code>REAL_NUMBER</code> and <code>IDENTIFIER</code> are
names. A name begins with a letter or an underscore followed by zero or
more letters, digits, underscores (<code>_</code>) or dashes
(<code>-</code>). They are followed by regular expressions which are
their definitions. They will be used in rules section and will expand to
the definition. You can leave out such definitions here and use regular
expressions in rules section directly.</li>
</ul>
<h3 id="rules-section">Rules section</h3>
<p>This section is the most important part. Rules consist of patterns
and actions. The patterns are regular expressions or names surrounded by
braces. The names must be defined in the definitions section. The
definition of the regular expression is written in the flex
documentation.</p>
<p>For example, line 40 is a rule.</p>
<ul>
<li><code>{REAL_NUMBER}</code> is a pattern</li>
<li><code>get_location (yytext); yylval.NUM = atof (yytext); return NUM;</code>
is an action.</li>
</ul>
<p><code>{REAL_NUMBER}</code> is defined in the line 17, so it expands
to <code>(0|[1-9][0-9]*)(\.[0-9]+)?</code>. This regular expression
matches numbers like <code>0</code>, <code>12</code> and
<code>1.5</code>. If an input is a number, it matches the pattern in
line 40. Then the matched text is assigned to <code>yytext</code> and
corresponding action is executed. A function <code>get_location</code>
changes the location variables to the position at the text. It assigns
<code>atof (yytext)</code>, which is double sized number converted from
<code>yytext</code>, to <code>yylval.NUM</code> and return
<code>NUM</code>. <code>NUM</code> is a token kind and it represents
integer. It is defined in <code>turtle.y</code>.</p>
<p>The scanner generated by flex has <code>yylex</code> function. If
<code>yylex</code> is called and the input is “123.4”, then it works as
follows.</p>
<ol type="1">
<li>A string “123.4” matches <code>{REAL_NUMBER}</code>.</li>
<li>Update the location variable <code>ncolumn</code> and
<code>yylloc</code>with <code>get_location</code>.</li>
<li>The function <code>atof</code> converts the string “123.4” to double
type number 123.4.</li>
<li>It is assigned to <code>yylval.NUM</code>.</li>
<li><code>yylex</code> returns <code>NUM</code> to the caller.</li>
</ol>
<p>Then the caller knows the input is a number (<code>NUM</code>), and
its value is 123.4.</p>
<ul>
<li>20-58: Rules section.</li>
<li>21: The symbol <code>.</code> (dot) matches any character except
newline. Therefore, a comment begins <code>#</code> followed by any
characters except newline. No action happens.</li>
<li>22: White space just increases the variable <code>ncolumn</code> by
one.</li>
<li>23: Tab is assumed to be equal to eight spaces.</li>
<li>24: New line increases a variable <code>nline</code> by one and
resets <code>ncolumn</code>.</li>
<li>26-38: Keywords just updates the location variables
<code>ncolumn</code> and <code>yylloc</code>, and return the token kinds
of the keywords.</li>
<li>40: Real number constant.</li>
<li>42: <code>IDENTIFIER</code> is defined in line 18. The location
variables are updated and the name of the identifier is assigned to
<code>yylval.ID</code>. The memory of the name is allocated by the
function <code>g_strdup</code>. The memory is registered to the list
(GSlist type list). The memory will be freed after the runtime routine
finishes. A token kind <code>ID</code> is returned.</li>
<li>46-56: Symbols just update the location variable and return the
token kinds. The token kind is the same as the symbol itself.</li>
<li>58: If the input doesn’t match the patterns, then it is an error. A
special token kind <code>YYUNDEF</code> is returned.</li>
</ul>
<h3 id="user-code-section">User code section</h3>
<p>This section is just copied to C source file.</p>
<ul>
<li>61-66: A function <code>get_location</code>. The location of the
input is recorded to <code>nline</code> and <code>ncolumn</code>. A
variable <code>yylloc</code> is referred by the parser. It is a C
structure and has four members, <code>first_line</code>,
<code>first_column</code>, <code>last_line</code> and
<code>last_column</code>. They point the start and end of the current
input text.</li>
<li>68: <code>YY_BUFFER_STATE</code> is a pointer points the input
buffer.</li>
<li>70-73: A function <code>init_flex</code> is called by
<code>run_cb</code> which is a “clicked” signal handler on the
<code>Run</code> button. It has one string type parameter. The caller
assigns it with the content of the GtkTextBuffer instance. A function
<code>yy_scan_string</code> sets the input buffer for the scanner.</li>
<li>75-78: A function <code>finalize_flex</code> is called after runtime
routine finishes. It deletes the input buffer.</li>
</ul>
<h2 id="turtle.y">Turtle.y</h2>
<p>Turtle.y has more than 800 lines so it is difficult to explain all
the source code. So I will explain the key points and leave out other
less important parts.</p>
<h3 id="what-does-bison-do">What does bison do?</h3>
<p>Bison creates C source file of a parser from a bison source file. The
bison source file is a text file. A parser analyzes a program source
code according to its grammar. Suppose here is a turtle source file.</p>
<pre><code>fc (1,0,0) # Foreground color is red, rgb = (1,0,0).
pd         # Pen down.
distance = 100
angle = 90
fd distance    # Go forward by distance (100) pixels.
tr angle     # Turn right by angle (90) degrees.</code></pre>
<p>The parser calls <code>yylex</code> to get a token. The token
consists of its type (token kind) and value (semantic value). So, the
parser gets items in the following table whenever it calls
<code>yylex</code>.</p>
<table>
<thead>
<tr class="header">
<th style="text-align: center;"></th>
<th style="text-align: center;">token kind</th>
<th style="text-align: center;">yylval.ID</th>
<th style="text-align: center;">yylval.NUM</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: center;">1</td>
<td style="text-align: center;">FC</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">2</td>
<td style="text-align: center;">(</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">3</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">1.0</td>
</tr>
<tr class="even">
<td style="text-align: center;">4</td>
<td style="text-align: center;">,</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">5</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">0.0</td>
</tr>
<tr class="even">
<td style="text-align: center;">6</td>
<td style="text-align: center;">,</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">7</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">0.0</td>
</tr>
<tr class="even">
<td style="text-align: center;">8</td>
<td style="text-align: center;">)</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">9</td>
<td style="text-align: center;">PD</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">10</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">11</td>
<td style="text-align: center;">=</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">12</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">100.0</td>
</tr>
<tr class="odd">
<td style="text-align: center;">13</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">angle</td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">14</td>
<td style="text-align: center;">=</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">15</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">90.0</td>
</tr>
<tr class="even">
<td style="text-align: center;">16</td>
<td style="text-align: center;">FD</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">17</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
</tr>
<tr class="even">
<td style="text-align: center;">18</td>
<td style="text-align: center;">TR</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
</tr>
<tr class="odd">
<td style="text-align: center;">19</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">angle</td>
<td style="text-align: center;"></td>
</tr>
</tbody>
</table>
<p>Bison source code specifies the grammar rules of turtle language. For
example, <code>fc (1,0,0)</code> is called primary procedure. A
procedure is like a void type C function. It doesn’t return any values.
Programmers can define their own procedures. On the other hand,
<code>fc</code> is a built-in procedure. Such procedures are called
primary procedures. It is described in bison source code like:</p>
<pre><code>primary_procedure: FC &#39;(&#39; expression &#39;,&#39; expression &#39;,&#39; expression &#39;)&#39;;
expression: ID | NUM;</code></pre>
<p>This means:</p>
<ul>
<li>Primary procedure is FC followed by ‘(’, expression, ‘,’,
expression, ‘,’, expression and ‘)’.</li>
<li>expression is ID or NUM.</li>
</ul>
<p>The description above is called BNF (Backus-Naur form). Precisely
speaking, it is not exactly the same as BNF. But the difference is
small.</p>
<p>The first line is:</p>
<pre><code>FC &#39;(&#39; NUM &#39;,&#39; NUM &#39;,&#39; NUM &#39;)&#39;;</code></pre>
<p>The parser analyzes the turtle source code and if the input matches
the definition above, the parser recognizes it as a primary
procedure.</p>
<p>The grammar of turtle is described in the <a
href="https://toshiocp.github.io/Gtk4-tutorial/turtle_doc.html">Turtle
manual</a>. The following is an extract from the document.</p>
<pre><code>program:
  statement
| program statement
;

statement:
  primary_procedure
| procedure_definition
;

primary_procedure:
  PU
| PD
| PW expression
| FD expression
| TR expression
| TL expression
| BC &#39;(&#39; expression &#39;,&#39; expression &#39;,&#39; expression &#39;)&#39;
| FC &#39;(&#39; expression &#39;,&#39; expression &#39;,&#39; expression &#39;)&#39;
| ID &#39;=&#39; expression
| IF &#39;(&#39; expression &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;
| RT
| RS
| RP &#39;(&#39; expression &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;
| ID &#39;(&#39; &#39;)&#39;
| ID &#39;(&#39; argument_list &#39;)&#39;
;

procedure_definition:
  DP ID &#39;(&#39;  &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;
| DP ID &#39;(&#39; parameter_list &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;
;

parameter_list:
  ID
| parameter_list &#39;,&#39; ID
;

argument_list:
  expression
| argument_list &#39;,&#39; expression
;

primary_procedure_list:
  primary_procedure
| primary_procedure_list primary_procedure
;

expression:
  expression &#39;=&#39; expression
| expression &#39;&gt;&#39; expression
| expression &#39;&lt;&#39; expression
| expression &#39;+&#39; expression
| expression &#39;-&#39; expression
| expression &#39;*&#39; expression
| expression &#39;/&#39; expression
| &#39;-&#39; expression %prec UMINUS
| &#39;(&#39; expression &#39;)&#39;
| ID
| NUM
;</code></pre>
<p>The grammar rule defines <code>program</code> first.</p>
<ul>
<li>program is a statement or a program followed by a statement.</li>
</ul>
<p>The definition is recursive.</p>
<ul>
<li><code>statement</code> is program.</li>
<li><code>statement statement</code> is <code>program statement</code>.
Therefore, it is program.</li>
<li><code>statement statement statement</code> is
<code>program statement</code>. Therefore, it is program.</li>
</ul>
<p>You can find that a sequence of statements is program like this.</p>
<p><code>program</code> and <code>statement</code> aren’t tokens. They
don’t appear in the input. They are called non terminal symbols. On the
other hand, tokens are called terminal symbols. The word “token” used
here has wide meaning, it includes tokens and symbols which appear in
the input. Non terminal symbols are often shortened to nterm.</p>
<p>Let’s analyze the program above as bison does.</p>
<table>
<colgroup>
<col style="width: 4%" />
<col style="width: 14%" />
<col style="width: 12%" />
<col style="width: 14%" />
<col style="width: 50%" />
<col style="width: 4%" />
</colgroup>
<thead>
<tr class="header">
<th style="text-align: center;"></th>
<th style="text-align: center;">token kind</th>
<th style="text-align: center;">yylval.ID</th>
<th style="text-align: center;">yylval.NUM</th>
<th style="text-align: left;">parse</th>
<th style="text-align: center;">S/R</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: center;">1</td>
<td style="text-align: center;">FC</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">2</td>
<td style="text-align: center;">(</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;">3</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">1.0</td>
<td style="text-align: left;">FC(NUM</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">4</td>
<td style="text-align: center;">,</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression,</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">5</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">0.0</td>
<td style="text-align: left;">FC(expression,NUM</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression,expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;">6</td>
<td style="text-align: center;">,</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression,expression,</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;">7</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">0.0</td>
<td style="text-align: left;">FC(expression,expression,NUM</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression,expression,expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">8</td>
<td style="text-align: center;">)</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">FC(expression,expression,expression)</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">9</td>
<td style="text-align: center;">PD</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program PD</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">10</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">11</td>
<td style="text-align: center;">=</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID=</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;">12</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">100.0</td>
<td style="text-align: left;">program ID=NUM</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID=expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;">13</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">angle</td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;">14</td>
<td style="text-align: center;">=</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID=</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">15</td>
<td style="text-align: center;">NUM</td>
<td style="text-align: center;"></td>
<td style="text-align: center;">90.0</td>
<td style="text-align: left;">program ID=NUM</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program ID=expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">16</td>
<td style="text-align: center;">FD</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program FD</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">17</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">distance</td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program FD ID</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program FD expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;">18</td>
<td style="text-align: center;">TR</td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program TR</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="even">
<td style="text-align: center;">19</td>
<td style="text-align: center;">ID</td>
<td style="text-align: center;">angle</td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program TR ID</td>
<td style="text-align: center;">S</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program TR expression</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program primary_procedure</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program statement</td>
<td style="text-align: center;">R</td>
</tr>
<tr class="even">
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: center;"></td>
<td style="text-align: left;">program</td>
<td style="text-align: center;">R</td>
</tr>
</tbody>
</table>
<p>The right most column shows shift/reduce. Shift is appending an input
to the buffer. Reduce is substituting a higher nterm for the pattern in
the buffer. For example, NUM is replaced by expression in the forth row.
This substitution is “reduce”.</p>
<p>Bison repeats shift and reduction until the end of the input. If the
result is reduced to <code>program</code>, the input is syntactically
valid. Bison executes an action whenever reduction occurs. Actions build
a tree. The tree is analyzed and executed by runtime routine later.</p>
<p>Bison source files are called bison grammar files. A bison grammar
file consists of four sections, prologue, declarations, rules and
epilogue. The format is as follows.</p>
<pre><code>%{
prologue
%}
declarations
%%
rules
%%
epilogue</code></pre>
<h3 id="prologue">Prologue</h3>
<p>Prologue section consists of C codes and the codes are copied to the
parser implementation file. You can use <code>%code</code> directives to
qualify the prologue and identifies the purpose explicitly. The
following is an extract from <code>turtle.y</code>.</p>
<pre class="bison"><code>%code top{
  #include &lt;stdarg.h&gt;
  #include &lt;setjmp.h&gt;
  #include &lt;math.h&gt;
  #include &lt;glib.h&gt;
  #include &lt;cairo.h&gt;
  #include &quot;turtle_parser.h&quot;

  /* The following line defines &#39;debug&#39; so that debug information is printed out during the run time. */
  /* However it makes the program slow. */
  /* If you want to debug on, uncomment the line. */

  /* #define debug 1 */

  extern cairo_surface_t *surface;

  /* error reporting */
  static void yyerror (char const *s) { /* for syntax error */
    g_print (&quot;%s from line %d, column %d to line %d, column %d\n&quot;,s, yylloc.first_line, yylloc.first_column, yylloc.last_line, yylloc.last_column);
  }
  /* Node type */
  enum {
    N_PU,
    N_PD,
    N_PW,
 ... ... ...
  };
}</code></pre>
<p>The directive <code>%code top</code> copies its contents to the top
of the parser implementation file. It usually includes
<code>#include</code> directives, declarations of functions and
definitions of constants. A function <code>yyerror</code> reports a
syntax error and is called by the parser. Node type identifies a node in
the tree.</p>
<p>Another directive <code>%code requires</code> copies its contents to
both the parser implementation file and header file. The header file is
read by the scanner C source file and other files.</p>
<pre class="bison"><code>%code requires {
  int yylex (void);
  int yyparse (void);
  void run (void);

  /* semantic value type */
  typedef struct _node_t node_t;
  struct _node_t {
    int type;
    union {
      struct {
        node_t *child1, *child2, *child3;
      } child;
      char *name;
      double value;
    } content;
  };
}</code></pre>
<ul>
<li><code>yylex</code> is shared by the parser implementation file and
scanner file.</li>
<li><code>yyparse</code> and <code>run</code> is called by
<code>run_cb</code> in <code>turtleapplication.c</code>.</li>
<li><code>node_t</code> is the type of the semantic value of nterms. The
header file defines <code>YYSTYPE</code>, which is the semantic value
type, with all the token and nterm value types. The following is
extracted from the header file.</li>
</ul>
<pre><code>/* Value type.  */
#if ! defined YYSTYPE &amp;&amp; ! defined YYSTYPE_IS_DECLARED
union YYSTYPE
{
  char * ID;                               /* ID  */
  double NUM;                              /* NUM  */
  node_t * program;                        /* program  */
  node_t * statement;                      /* statement  */
  node_t * primary_procedure;              /* primary_procedure  */
  node_t * primary_procedure_list;         /* primary_procedure_list  */
  node_t * procedure_definition;           /* procedure_definition  */
  node_t * parameter_list;                 /* parameter_list  */
  node_t * argument_list;                  /* argument_list  */
  node_t * expression;                     /* expression  */
};</code></pre>
<p>Other useful macros and declarations are put into the
<code>%code</code> directive.</p>
<pre><code>%code {
/* The following macro is convenient to get the member of the node. */
  #define child1(n) (n)-&gt;content.child.child1
  #define child2(n) (n)-&gt;content.child.child2
  #define child3(n) (n)-&gt;content.child.child3
  #define name(n) (n)-&gt;content.name
  #define value(n) (n)-&gt;content.value

  /* start of nodes */
  static node_t *node_top = NULL;
  /* functions to generate trees */
  static node_t *tree1 (int type, node_t *child1, node_t *child2, node_t *child3);
  static node_t *tree2 (int type, double value);
  static node_t *tree3 (int type, char *name);
}</code></pre>
<h3 id="bison-declarations">Bison declarations</h3>
<p>Bison declarations defines terminal and non-terminal symbols. It also
specifies some directives.</p>
<pre><code>%locations
%define api.value.type union /* YYSTYPE, the type of semantic values, is union of following types */
 /* key words */
%token PU
%token PD
%token PW
%token FD
%token TR
%token TL
%token BC
%token FC
%token DP
%token IF
%token RT
%token RS
%token RP
 /* constant */
%token &lt;double&gt; NUM
 /* identirier */
%token &lt;char *&gt; ID
 /* non terminal symbol */
%nterm &lt;node_t *&gt; program
%nterm &lt;node_t *&gt; statement
%nterm &lt;node_t *&gt; primary_procedure
%nterm &lt;node_t *&gt; primary_procedure_list
%nterm &lt;node_t *&gt; procedure_definition
%nterm &lt;node_t *&gt; parameter_list
%nterm &lt;node_t *&gt; argument_list
%nterm &lt;node_t *&gt; expression
 /* logical relation symbol */
%left &#39;=&#39; &#39;&lt;&#39; &#39;&gt;&#39;
 /* arithmetic symbol */
%left &#39;+&#39; &#39;-&#39;
%left &#39;*&#39; &#39;/&#39;
%precedence UMINUS /* unary minus */</code></pre>
<p><code>%locations</code> directive inserts the location structure into
the header file. It is like this.</p>
<pre><code>typedef struct YYLTYPE YYLTYPE;
struct YYLTYPE
{
  int first_line;
  int first_column;
  int last_line;
  int last_column;
};</code></pre>
<p>This type is shared by the scanner file and the parser implementation
file. The error report function <code>yyerror</code> uses it so that it
can inform the location that error occurs.</p>
<p><code>%define api.value.type union</code> generates semantic value
type with tokens and nterms and inserts it to the header file. The
inserted part is shown in the previous subsection as the extracts that
shows the value type (YYSTYPE).</p>
<p><code>%token</code> and <code>%nterm</code> directives define tokens
and non terminal symbols respectively.</p>
<pre><code>%token PU
... ...
%token &lt;double&gt; NUM</code></pre>
<p>These directives define a token <code>PU</code> and <code>NUM</code>.
The values of token kinds <code>PU</code> and <code>NUM</code> are
defined as an enumeration constant in the header file.</p>
<pre><code>  enum yytokentype
  {
  ... ... ...
    PU = 258,                      /* PU  */
  ... ... ...
    NUM = 269,                     /* NUM  */
  ... ... ...
  };
  typedef enum yytokentype yytoken_kind_t;</code></pre>
<p>In addition, the type of the semantic value of <code>NUM</code> is
defined as double in the header file because of
<code>&lt;double&gt;</code> tag.</p>
<pre><code>union YYSTYPE
{
  char * ID;                               /* ID  */
  double NUM;                              /* NUM  */
  ... ...
}</code></pre>
<p>All the nterm symbols have the same type <code>* node_t</code> of the
semantic value.</p>
<p><code>%left</code> and <code>%precedence</code> directives define the
precedence of operation symbols.</p>
<pre><code> /* logical relation symbol */
%left &#39;=&#39; &#39;&lt;&#39; &#39;&gt;&#39;
 /* arithmetic symbol */
%left &#39;+&#39; &#39;-&#39;
%left &#39;*&#39; &#39;/&#39;
%precedence UMINUS /* unary minus */</code></pre>
<p><code>%left</code> directive defines the following symbols as
left-associated operators. If an operator <code>+</code> is
left-associated, then</p>
<pre><code>A + B + C = (A + B) + C</code></pre>
<p>That is, the calculation is carried out the left operator first, then
the right operator. If an operator <code>*</code> is right-associated,
then:</p>
<pre><code>A * B * C = A * (B * C)</code></pre>
<p>The definition above decides the behavior of the parser. Addition and
multiplication hold associative law so the result of
<code>(A+B)+C</code> and <code>A+(B+C)</code> are equal in terms of
mathematics. However, the parser will be confused if left (or right)
associativity is not specified.</p>
<p><code>%left</code> and <code>%precedence</code> directives show the
precedence of operators. Later declared operators have higher precedence
than former declared ones. The declaration above says, for example,</p>
<pre><code>v=w+z*5+7 is the same as v=((w+(z*5))+7)</code></pre>
<p>Be careful. The operator <code>=</code> above is an assignment.
Assignment is not expression in turtle language. It is
primary_procedure. But if <code>=</code> appears in an expression, it is
a logical operator, not an assignment. The logical equal
‘<code>=</code>’ usually used in the conditional expression, for
example, in <code>if</code> statement. (Turtle language uses ‘=’ instead
of ‘==’ in C language).</p>
<h3 id="grammar-rules">Grammar rules</h3>
<p>Grammar rules section defines the syntactic grammar of the language.
It is similar to BNF form.</p>
<pre><code>result: components { action };</code></pre>
<ul>
<li>result is a nterm.</li>
<li>components are list of tokens or nterms.</li>
<li>action is C codes. It is executed whenever the components are
reduced to the result. Action can be left out.</li>
</ul>
<p>The following is a part of the grammar rule in
<code>turtle.y</code>.</p>
<pre><code>program:
  statement { node_top = $$ = $1; }
;
statement:
  primary_procedure
;
primary_procedure:
  FD expression    { $$ = tree1 (N_FD, $2, NULL, NULL); }
;
expression:
  NUM   { $$ = tree2 (N_NUM, $1); }
;</code></pre>
<ul>
<li>The first two lines tell that <code>program</code> is
<code>statement</code>.</li>
<li>Whenever <code>statement</code> is reduced to <code>program</code>,
an action <code>node_top=$$=$1;</code> is executed.</li>
<li><code>node_top</code> is a static variable. It points the top node
of the tree.</li>
<li>A symbol <code>$$</code> is a semantic value of the result. For
example, <code>$$</code> in line 2 is the semantic value of
<code>program</code>. It is a pointer to a <code>node_t</code> type
structure.</li>
<li><code>$1</code> is a semantic value of the first component. For
example, <code>$1</code> in line 2 is the semantic value of
<code>statement</code>. It is also a pointer to
<code>node_t</code>.</li>
<li>The next rule is that <code>statement</code> is
<code>primary_procedure</code>. There’s no action specified. Then, the
default action <code>$$ = $1</code> is executed.</li>
<li>The next rule is that <code>primary_procedure</code> is
<code>FD</code> followed by expression. The action calls
<code>tree1</code> and assigns its return value to <code>$$</code>. The
function <code>tree1</code> makes a tree node. The tree node has type
and union of three pointers to children nodes, string or double.</li>
</ul>
<pre><code>node --+-- type
       +-- union contents
                    +---struct {node_t *child1, *child2, *child3;};
                    +---char *name
                    +---double value</code></pre>
<ul>
<li><code>tree1</code> assigns the four arguments to type, child1,
child2 and child3 members.</li>
<li>The last rule is that <code>expression</code> is
<code>NUM</code>.</li>
<li><code>tree2</code> makes a tree node. The paremeters of
<code>tree2</code> are a type and a semantic value.</li>
</ul>
<p>Suppose the parser reads the following program.</p>
<pre><code>fd 100</code></pre>
<p>What does the parser do?</p>
<ol type="1">
<li>The parser recognizes the input is <code>FD</code>. Maybe it is the
start of <code>primary_procedure</code>, but parser needs to read the
next token.</li>
<li><code>yylex</code> returns the token kind <code>NUM</code> and sets
<code>yylval.NUM</code> to 100.0 (the type is double). The parser
reduces <code>NUM</code> to <code>expression</code>. At the same time,
it sets the semantic value of the <code>expression</code> to point a new
node. The node has an type <code>N_NUM</code> and a semantic value
100.0.</li>
<li>After the reduction, the buffer has <code>FD</code> and
<code>expression</code>. The parser reduces it to
<code>primary_procedure</code>. And it sets the semantic value of the
<code>primary_procedure</code> to point a new node. The node has an type
<code>N_FD</code> and its member child1 points the node of
<code>expression</code>, whose type is <code>N_NUM</code>.</li>
<li>The parser reduces <code>primary_procedure</code> to
<code>statement</code>. The semantic value of <code>statement</code> is
the same as the one of <code>primary_procedure</code>, which points to
the node <code>N_FD</code>.</li>
<li>The parser reduces <code>statement</code> to <code>program</code>.
The semantic value of <code>statement</code> is assigned to the one of
<code>program</code> and the static variable <code>node_top</code>.</li>
<li>Finally <code>node_top</code> points the node <code>N_FD</code> and
the node <code>N_FD</code> points the node <code>N_NUM</code>.</li>
</ol>
<figure>
<img src="image/tree.png" alt="tree" />
<figcaption aria-hidden="true">tree</figcaption>
</figure>
<p>The following is the grammar rule extracted from
<code>turtle.y</code>. The rules there are based on the same idea above.
I don’t want to explain the whole rules below. Please look into each
line carefully so that you will understand all the rules and
actions.</p>
<pre class="bison"><code>program:
  statement { node_top = $$ = $1; }
| program statement {
        node_top = $$ = tree1 (N_program, $1, $2, NULL);
        }
;

statement:
  primary_procedure
| procedure_definition
;

primary_procedure:
  PU    { $$ = tree1 (N_PU, NULL, NULL, NULL); }
| PD    { $$ = tree1 (N_PD, NULL, NULL, NULL); }
| PW expression    { $$ = tree1 (N_PW, $2, NULL, NULL); }
| FD expression    { $$ = tree1 (N_FD, $2, NULL, NULL); }
| TR expression    { $$ = tree1 (N_TR, $2, NULL, NULL); }
| TL expression    { $$ = tree1 (N_TL, $2, NULL, NULL); } /* ver 0.5 */
| BC &#39;(&#39; expression &#39;,&#39; expression &#39;,&#39; expression &#39;)&#39; { $$ = tree1 (N_BC, $3, $5, $7); }
| FC &#39;(&#39; expression &#39;,&#39; expression &#39;,&#39; expression &#39;)&#39; { $$ = tree1 (N_FC, $3, $5, $7); }
 /* assignment */
| ID &#39;=&#39; expression   { $$ = tree1 (N_ASSIGN, tree3 (N_ID, $1), $3, NULL); }
 /* control flow */
| IF &#39;(&#39; expression &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39; { $$ = tree1 (N_IF, $3, $6, NULL); }
| RT    { $$ = tree1 (N_RT, NULL, NULL, NULL); }
| RS    { $$ = tree1 (N_RS, NULL, NULL, NULL); }
| RP &#39;(&#39; expression &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;    { $$ = tree1 (N_RP, $3, $6, NULL); }
 /* user defined procedure call */
| ID &#39;(&#39; &#39;)&#39;  { $$ = tree1 (N_procedure_call, tree3 (N_ID, $1), NULL, NULL); }
| ID &#39;(&#39; argument_list &#39;)&#39;  { $$ = tree1 (N_procedure_call, tree3 (N_ID, $1), $3, NULL); }
;

procedure_definition:
  DP ID &#39;(&#39;  &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;  {
         $$ = tree1 (N_procedure_definition, tree3 (N_ID, $2), NULL, $6);
        }
| DP ID &#39;(&#39; parameter_list &#39;)&#39; &#39;{&#39; primary_procedure_list &#39;}&#39;  {
         $$ = tree1 (N_procedure_definition, tree3 (N_ID, $2), $4, $7);
        }
;

parameter_list:
  ID { $$ = tree3 (N_ID, $1); }
| parameter_list &#39;,&#39; ID  { $$ = tree1 (N_parameter_list, $1, tree3 (N_ID, $3), NULL); }
;

argument_list:
  expression
| argument_list &#39;,&#39; expression { $$ = tree1 (N_argument_list, $1, $3, NULL); }
;

primary_procedure_list:
  primary_procedure
| primary_procedure_list primary_procedure {
         $$ = tree1 (N_primary_procedure_list, $1, $2, NULL);
        }
;

expression:
  expression &#39;=&#39; expression { $$ = tree1 (N_EQ, $1, $3, NULL); }
| expression &#39;&gt;&#39; expression { $$ = tree1 (N_GT, $1, $3, NULL); }
| expression &#39;&lt;&#39; expression { $$ = tree1 (N_LT, $1, $3, NULL); }
| expression &#39;+&#39; expression { $$ = tree1 (N_ADD, $1, $3, NULL); }
| expression &#39;-&#39; expression { $$ = tree1 (N_SUB, $1, $3, NULL); }
| expression &#39;*&#39; expression { $$ = tree1 (N_MUL, $1, $3, NULL); }
| expression &#39;/&#39; expression { $$ = tree1 (N_DIV, $1, $3, NULL); }
| &#39;-&#39; expression %prec UMINUS { $$ = tree1 (N_UMINUS, $2, NULL, NULL); }
| &#39;(&#39; expression &#39;)&#39; { $$ = $2; }
| ID    { $$ = tree3 (N_ID, $1); }
| NUM   { $$ = tree2 (N_NUM, $1); }
;</code></pre>
<h3 id="epilogue">Epilogue</h3>
<p>The epilogue is written in C language and copied to the parser
implementation file. Generally, you can put anything into the epilogue.
In the case of turtle interpreter, the runtime routine and some other
functions are in the epilogue.</p>
<h4 id="functions-to-create-tree-nodes">Functions to create tree
nodes</h4>
<p>There are three functions, <code>tree1</code>, <code>tree2</code> and
<code>tree3</code>.</p>
<ul>
<li><code>tree1</code> creates a node and sets the node type and
pointers to its three children (NULL is possible).</li>
<li><code>tree2</code> creates a node and sets the node type and a value
(double).</li>
<li><code>tree3</code> creates a node and sets the node type and a
pointer to a string.</li>
</ul>
<p>Each function gets memories first and build a node on them. The
memories are inserted to the list. They will be freed when runtime
routine finishes.</p>
<p>The three functions are called in the actions in the rules
section.</p>
<div class="sourceCode" id="cb30"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb30-1"><a href="#cb30-1" aria-hidden="true" tabindex="-1"></a><span class="co">/* Dynamically allocated memories are added to the single list. They will be freed in the finalize function. */</span></span>
<span id="cb30-2"><a href="#cb30-2" aria-hidden="true" tabindex="-1"></a>GSList <span class="op">*</span>list <span class="op">=</span> NULL<span class="op">;</span></span>
<span id="cb30-3"><a href="#cb30-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-4"><a href="#cb30-4" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span></span>
<span id="cb30-5"><a href="#cb30-5" aria-hidden="true" tabindex="-1"></a>tree1 <span class="op">(</span><span class="dt">int</span> type<span class="op">,</span> node_t <span class="op">*</span>child1<span class="op">,</span> node_t <span class="op">*</span>child2<span class="op">,</span> node_t <span class="op">*</span>child3<span class="op">)</span> <span class="op">{</span></span>
<span id="cb30-6"><a href="#cb30-6" aria-hidden="true" tabindex="-1"></a>  node_t <span class="op">*</span>new_node<span class="op">;</span></span>
<span id="cb30-7"><a href="#cb30-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-8"><a href="#cb30-8" aria-hidden="true" tabindex="-1"></a>  list <span class="op">=</span> g_slist_prepend <span class="op">(</span>list<span class="op">,</span> g_malloc <span class="op">(</span><span class="kw">sizeof</span> <span class="op">(</span>node_t<span class="op">)));</span></span>
<span id="cb30-9"><a href="#cb30-9" aria-hidden="true" tabindex="-1"></a>  new_node <span class="op">=</span> <span class="op">(</span>node_t <span class="op">*)</span> list<span class="op">-&gt;</span>data<span class="op">;</span></span>
<span id="cb30-10"><a href="#cb30-10" aria-hidden="true" tabindex="-1"></a>  new_node<span class="op">-&gt;</span>type <span class="op">=</span> type<span class="op">;</span></span>
<span id="cb30-11"><a href="#cb30-11" aria-hidden="true" tabindex="-1"></a>  child1<span class="op">(</span>new_node<span class="op">)</span> <span class="op">=</span> child1<span class="op">;</span></span>
<span id="cb30-12"><a href="#cb30-12" aria-hidden="true" tabindex="-1"></a>  child2<span class="op">(</span>new_node<span class="op">)</span> <span class="op">=</span> child2<span class="op">;</span></span>
<span id="cb30-13"><a href="#cb30-13" aria-hidden="true" tabindex="-1"></a>  child3<span class="op">(</span>new_node<span class="op">)</span> <span class="op">=</span> child3<span class="op">;</span></span>
<span id="cb30-14"><a href="#cb30-14" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> new_node<span class="op">;</span></span>
<span id="cb30-15"><a href="#cb30-15" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb30-16"><a href="#cb30-16" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-17"><a href="#cb30-17" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span></span>
<span id="cb30-18"><a href="#cb30-18" aria-hidden="true" tabindex="-1"></a>tree2 <span class="op">(</span><span class="dt">int</span> type<span class="op">,</span> <span class="dt">double</span> value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb30-19"><a href="#cb30-19" aria-hidden="true" tabindex="-1"></a>  node_t <span class="op">*</span>new_node<span class="op">;</span></span>
<span id="cb30-20"><a href="#cb30-20" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-21"><a href="#cb30-21" aria-hidden="true" tabindex="-1"></a>  list <span class="op">=</span> g_slist_prepend <span class="op">(</span>list<span class="op">,</span> g_malloc <span class="op">(</span><span class="kw">sizeof</span> <span class="op">(</span>node_t<span class="op">)));</span></span>
<span id="cb30-22"><a href="#cb30-22" aria-hidden="true" tabindex="-1"></a>  new_node <span class="op">=</span> <span class="op">(</span>node_t <span class="op">*)</span> list<span class="op">-&gt;</span>data<span class="op">;</span></span>
<span id="cb30-23"><a href="#cb30-23" aria-hidden="true" tabindex="-1"></a>  new_node<span class="op">-&gt;</span>type <span class="op">=</span> type<span class="op">;</span></span>
<span id="cb30-24"><a href="#cb30-24" aria-hidden="true" tabindex="-1"></a>  value<span class="op">(</span>new_node<span class="op">)</span> <span class="op">=</span> value<span class="op">;</span></span>
<span id="cb30-25"><a href="#cb30-25" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> new_node<span class="op">;</span></span>
<span id="cb30-26"><a href="#cb30-26" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb30-27"><a href="#cb30-27" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-28"><a href="#cb30-28" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span></span>
<span id="cb30-29"><a href="#cb30-29" aria-hidden="true" tabindex="-1"></a>tree3 <span class="op">(</span><span class="dt">int</span> type<span class="op">,</span> <span class="dt">char</span> <span class="op">*</span>name<span class="op">)</span> <span class="op">{</span></span>
<span id="cb30-30"><a href="#cb30-30" aria-hidden="true" tabindex="-1"></a>  node_t <span class="op">*</span>new_node<span class="op">;</span></span>
<span id="cb30-31"><a href="#cb30-31" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-32"><a href="#cb30-32" aria-hidden="true" tabindex="-1"></a>  list <span class="op">=</span> g_slist_prepend <span class="op">(</span>list<span class="op">,</span> g_malloc <span class="op">(</span><span class="kw">sizeof</span> <span class="op">(</span>node_t<span class="op">)));</span></span>
<span id="cb30-33"><a href="#cb30-33" aria-hidden="true" tabindex="-1"></a>  new_node <span class="op">=</span> <span class="op">(</span>node_t <span class="op">*)</span> list<span class="op">-&gt;</span>data<span class="op">;</span></span>
<span id="cb30-34"><a href="#cb30-34" aria-hidden="true" tabindex="-1"></a>  new_node<span class="op">-&gt;</span>type <span class="op">=</span> type<span class="op">;</span></span>
<span id="cb30-35"><a href="#cb30-35" aria-hidden="true" tabindex="-1"></a>  name<span class="op">(</span>new_node<span class="op">)</span> <span class="op">=</span> name<span class="op">;</span></span>
<span id="cb30-36"><a href="#cb30-36" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> new_node<span class="op">;</span></span>
<span id="cb30-37"><a href="#cb30-37" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h4 id="symbol-table">Symbol table</h4>
<p>Variables and user defined procedures are registered in a symbol
table. This table is a C array. It should be replaced by more
appropriate data structure with memory allocation in the future
version</p>
<ul>
<li>Variables are registered with its name and value.</li>
<li>Procedures are registered with its name and a pointer to the node of
the procedure.</li>
</ul>
<p>Therefore the table has the following fields.</p>
<ul>
<li>type to identify variable or procedure</li>
<li>name</li>
<li>value or pointer to a node</li>
</ul>
<div class="sourceCode" id="cb31"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb31-1"><a href="#cb31-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#define MAX_TABLE_SIZE 100</span></span>
<span id="cb31-2"><a href="#cb31-2" aria-hidden="true" tabindex="-1"></a><span class="kw">enum</span> <span class="op">{</span></span>
<span id="cb31-3"><a href="#cb31-3" aria-hidden="true" tabindex="-1"></a>  PROC<span class="op">,</span></span>
<span id="cb31-4"><a href="#cb31-4" aria-hidden="true" tabindex="-1"></a>  VAR</span>
<span id="cb31-5"><a href="#cb31-5" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb31-6"><a href="#cb31-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb31-7"><a href="#cb31-7" aria-hidden="true" tabindex="-1"></a><span class="kw">typedef</span> <span class="kw">union</span> _object_t object_t<span class="op">;</span></span>
<span id="cb31-8"><a href="#cb31-8" aria-hidden="true" tabindex="-1"></a><span class="kw">union</span> _object_t <span class="op">{</span></span>
<span id="cb31-9"><a href="#cb31-9" aria-hidden="true" tabindex="-1"></a>  node_t <span class="op">*</span>node<span class="op">;</span></span>
<span id="cb31-10"><a href="#cb31-10" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> value<span class="op">;</span></span>
<span id="cb31-11"><a href="#cb31-11" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb31-12"><a href="#cb31-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb31-13"><a href="#cb31-13" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> <span class="op">{</span></span>
<span id="cb31-14"><a href="#cb31-14" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> type<span class="op">;</span></span>
<span id="cb31-15"><a href="#cb31-15" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> <span class="op">*</span>name<span class="op">;</span></span>
<span id="cb31-16"><a href="#cb31-16" aria-hidden="true" tabindex="-1"></a>  object_t object<span class="op">;</span></span>
<span id="cb31-17"><a href="#cb31-17" aria-hidden="true" tabindex="-1"></a><span class="op">}</span> table<span class="op">[</span>MAX_TABLE_SIZE<span class="op">];</span></span>
<span id="cb31-18"><a href="#cb31-18" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> tp<span class="op">;</span></span>
<span id="cb31-19"><a href="#cb31-19" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb31-20"><a href="#cb31-20" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb31-21"><a href="#cb31-21" aria-hidden="true" tabindex="-1"></a>init_table <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb31-22"><a href="#cb31-22" aria-hidden="true" tabindex="-1"></a>  tp <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb31-23"><a href="#cb31-23" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p><code>init_table</code> initializes the table. This must be called
before registrations.</p>
<p>There are five functions to access the table,</p>
<ul>
<li><code>proc_install</code> installs a procedure.</li>
<li><code>var_install</code> installs a variable.</li>
<li><code>proc_lookup</code> looks up a procedure. If the procedure is
found, it returns a pointer to the node. Otherwise it returns NULL.</li>
<li><code>var_lookup</code> looks up a variable. If the variable is
found, it returns TRUE and sets the pointer (argument) to point the
value. Otherwise it returns FALSE.</li>
<li><code>var_replace</code> replaces the value of a variable. If the
variable hasn’t registered yet, it installs the variable.</li>
</ul>
<div class="sourceCode" id="cb32"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb32-1"><a href="#cb32-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span></span>
<span id="cb32-2"><a href="#cb32-2" aria-hidden="true" tabindex="-1"></a>tbl_lookup <span class="op">(</span><span class="dt">int</span> type<span class="op">,</span> <span class="dt">char</span> <span class="op">*</span>name<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-3"><a href="#cb32-3" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb32-4"><a href="#cb32-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-5"><a href="#cb32-5" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>tp <span class="op">==</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-6"><a href="#cb32-6" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb32-7"><a href="#cb32-7" aria-hidden="true" tabindex="-1"></a>  <span class="cf">for</span> <span class="op">(</span>i<span class="op">=</span><span class="dv">0</span><span class="op">;</span> i<span class="op">&lt;</span>tp<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span></span>
<span id="cb32-8"><a href="#cb32-8" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="op">(</span>type <span class="op">==</span> table<span class="op">[</span>i<span class="op">].</span>type <span class="op">&amp;&amp;</span> strcmp<span class="op">(</span>name<span class="op">,</span> table<span class="op">[</span>i<span class="op">].</span>name<span class="op">)</span> <span class="op">==</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-9"><a href="#cb32-9" aria-hidden="true" tabindex="-1"></a>      <span class="cf">return</span> i<span class="op">;</span></span>
<span id="cb32-10"><a href="#cb32-10" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb32-11"><a href="#cb32-11" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-12"><a href="#cb32-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-13"><a href="#cb32-13" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb32-14"><a href="#cb32-14" aria-hidden="true" tabindex="-1"></a>tbl_install <span class="op">(</span><span class="dt">int</span> type<span class="op">,</span> <span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> object_t object<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-15"><a href="#cb32-15" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>tp <span class="op">&gt;=</span> MAX_TABLE_SIZE<span class="op">)</span></span>
<span id="cb32-16"><a href="#cb32-16" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Symbol table overflow.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb32-17"><a href="#cb32-17" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>tbl_lookup <span class="op">(</span>type<span class="op">,</span> name<span class="op">)</span> <span class="op">&gt;=</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-18"><a href="#cb32-18" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Name %s is already registered.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">);</span></span>
<span id="cb32-19"><a href="#cb32-19" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb32-20"><a href="#cb32-20" aria-hidden="true" tabindex="-1"></a>    table<span class="op">[</span>tp<span class="op">].</span>type <span class="op">=</span> type<span class="op">;</span></span>
<span id="cb32-21"><a href="#cb32-21" aria-hidden="true" tabindex="-1"></a>    table<span class="op">[</span>tp<span class="op">].</span>name <span class="op">=</span> name<span class="op">;</span></span>
<span id="cb32-22"><a href="#cb32-22" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="op">(</span>type <span class="op">==</span> PROC<span class="op">)</span></span>
<span id="cb32-23"><a href="#cb32-23" aria-hidden="true" tabindex="-1"></a>      table<span class="op">[</span>tp<span class="op">++].</span>object<span class="op">.</span>node <span class="op">=</span> object<span class="op">.</span>node<span class="op">;</span></span>
<span id="cb32-24"><a href="#cb32-24" aria-hidden="true" tabindex="-1"></a>    <span class="cf">else</span></span>
<span id="cb32-25"><a href="#cb32-25" aria-hidden="true" tabindex="-1"></a>      table<span class="op">[</span>tp<span class="op">++].</span>object<span class="op">.</span>value <span class="op">=</span> object<span class="op">.</span>value<span class="op">;</span></span>
<span id="cb32-26"><a href="#cb32-26" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb32-27"><a href="#cb32-27" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-28"><a href="#cb32-28" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-29"><a href="#cb32-29" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb32-30"><a href="#cb32-30" aria-hidden="true" tabindex="-1"></a>proc_install <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> node_t <span class="op">*</span>node<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-31"><a href="#cb32-31" aria-hidden="true" tabindex="-1"></a>  object_t object<span class="op">;</span></span>
<span id="cb32-32"><a href="#cb32-32" aria-hidden="true" tabindex="-1"></a>  object<span class="op">.</span>node <span class="op">=</span> node<span class="op">;</span></span>
<span id="cb32-33"><a href="#cb32-33" aria-hidden="true" tabindex="-1"></a>  tbl_install <span class="op">(</span>PROC<span class="op">,</span> name<span class="op">,</span> object<span class="op">);</span></span>
<span id="cb32-34"><a href="#cb32-34" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-35"><a href="#cb32-35" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-36"><a href="#cb32-36" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb32-37"><a href="#cb32-37" aria-hidden="true" tabindex="-1"></a>var_install <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-38"><a href="#cb32-38" aria-hidden="true" tabindex="-1"></a>  object_t object<span class="op">;</span></span>
<span id="cb32-39"><a href="#cb32-39" aria-hidden="true" tabindex="-1"></a>  object<span class="op">.</span>value <span class="op">=</span> value<span class="op">;</span></span>
<span id="cb32-40"><a href="#cb32-40" aria-hidden="true" tabindex="-1"></a>  tbl_install <span class="op">(</span>VAR<span class="op">,</span> name<span class="op">,</span> object<span class="op">);</span></span>
<span id="cb32-41"><a href="#cb32-41" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-42"><a href="#cb32-42" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-43"><a href="#cb32-43" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb32-44"><a href="#cb32-44" aria-hidden="true" tabindex="-1"></a>var_replace <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-45"><a href="#cb32-45" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb32-46"><a href="#cb32-46" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">((</span>i <span class="op">=</span> tbl_lookup <span class="op">(</span>VAR<span class="op">,</span> name<span class="op">))</span> <span class="op">&gt;=</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-47"><a href="#cb32-47" aria-hidden="true" tabindex="-1"></a>    table<span class="op">[</span>i<span class="op">].</span>object<span class="op">.</span>value <span class="op">=</span> value<span class="op">;</span></span>
<span id="cb32-48"><a href="#cb32-48" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span></span>
<span id="cb32-49"><a href="#cb32-49" aria-hidden="true" tabindex="-1"></a>    var_install <span class="op">(</span>name<span class="op">,</span> value<span class="op">);</span></span>
<span id="cb32-50"><a href="#cb32-50" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-51"><a href="#cb32-51" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-52"><a href="#cb32-52" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span></span>
<span id="cb32-53"><a href="#cb32-53" aria-hidden="true" tabindex="-1"></a>proc_lookup <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-54"><a href="#cb32-54" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb32-55"><a href="#cb32-55" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">((</span>i <span class="op">=</span> tbl_lookup <span class="op">(</span>PROC<span class="op">,</span> name<span class="op">))</span> <span class="op">&lt;</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-56"><a href="#cb32-56" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> NULL<span class="op">;</span></span>
<span id="cb32-57"><a href="#cb32-57" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span></span>
<span id="cb32-58"><a href="#cb32-58" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> table<span class="op">[</span>i<span class="op">].</span>object<span class="op">.</span>node<span class="op">;</span></span>
<span id="cb32-59"><a href="#cb32-59" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb32-60"><a href="#cb32-60" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb32-61"><a href="#cb32-61" aria-hidden="true" tabindex="-1"></a>gboolean</span>
<span id="cb32-62"><a href="#cb32-62" aria-hidden="true" tabindex="-1"></a>var_lookup <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> <span class="op">*</span>value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-63"><a href="#cb32-63" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb32-64"><a href="#cb32-64" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">((</span>i <span class="op">=</span> tbl_lookup <span class="op">(</span>VAR<span class="op">,</span> name<span class="op">))</span> <span class="op">&lt;</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb32-65"><a href="#cb32-65" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> FALSE<span class="op">;</span></span>
<span id="cb32-66"><a href="#cb32-66" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb32-67"><a href="#cb32-67" aria-hidden="true" tabindex="-1"></a>    <span class="op">*</span>value <span class="op">=</span> table<span class="op">[</span>i<span class="op">].</span>object<span class="op">.</span>value<span class="op">;</span></span>
<span id="cb32-68"><a href="#cb32-68" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> TRUE<span class="op">;</span></span>
<span id="cb32-69"><a href="#cb32-69" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb32-70"><a href="#cb32-70" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h4 id="stack-for-parameters-and-arguments">Stack for parameters and
arguments</h4>
<p>Stack is a last-in first-out data structure. It is shortened to LIFO.
Turtle uses a stack to keep parameters and arguments. They are like
<code>auto</code> class variables in C language. They are pushed to the
stack whenever the procedure is called. LIFO structure is useful for
recursive calls.</p>
<p>Each element of the stack has name and value.</p>
<div class="sourceCode" id="cb33"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb33-1"><a href="#cb33-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#define MAX_STACK_SIZE 500</span></span>
<span id="cb33-2"><a href="#cb33-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> <span class="op">{</span></span>
<span id="cb33-3"><a href="#cb33-3" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> <span class="op">*</span>name<span class="op">;</span></span>
<span id="cb33-4"><a href="#cb33-4" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> value<span class="op">;</span></span>
<span id="cb33-5"><a href="#cb33-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span> stack<span class="op">[</span>MAX_STACK_SIZE<span class="op">];</span></span>
<span id="cb33-6"><a href="#cb33-6" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> sp<span class="op">,</span> sp_biggest<span class="op">;</span></span>
<span id="cb33-7"><a href="#cb33-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb33-8"><a href="#cb33-8" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb33-9"><a href="#cb33-9" aria-hidden="true" tabindex="-1"></a>init_stack <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb33-10"><a href="#cb33-10" aria-hidden="true" tabindex="-1"></a>  sp <span class="op">=</span> sp_biggest <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb33-11"><a href="#cb33-11" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p><code>sp</code> is a stack pointer. It is an index of the array
<code>stack</code> and it always points an element of the array to store
the next data. <code>sp_biggest</code> is the biggest number assigned to
<code>sp</code>. We can know the amount of elements used in the array
during the runtime. The purpose of the variable is to find appropriate
<code>MAX_STACK_SIZE</code>. It will be unnecessary in the future
version if the stack is implemented with better data structure and
memory allocation.</p>
<p>The runtime routine push data to the stack when it executes a node of
a procedure call. (The type of the node is
<code>N_procedure_call</code>.)</p>
<pre><code>dp drawline (angle, distance) { ... ... ... }
drawline (90, 100)</code></pre>
<ul>
<li>The first line defines a procedure <code>drawline</code>. The
runtime routine stores the name <code>drawline</code> and the node of
the procedure to the symbol table.</li>
<li>The second line calls the procedure. First, it looks for the
procedure in the symbol table and gets its node. Then it searches the
node for the parameters and gets <code>angle</code> and
<code>distance</code>.</li>
<li>It pushes (“angle”, 90.0) to the stack.</li>
<li>It pushes (“distance”, 100.0) to the stack.</li>
<li>It pushes (NULL, 2.0) to the stack. The number 2.0 is the number of
parameters (or arguments). It is used when the procedure returns.</li>
</ul>
<p>The following diagram shows the structure of the stack. First,
<code>procedure 1</code> is called. The procedure has two parameters. In
the <code>procedure 1</code>, another procedure <code>procedure 2</code>
is called. It has one parameter. In the <code>procedure 2</code>,
another procedure <code>procedure 3</code> is called. It has three
parameters. These three procedures are nested.</p>
<p>Programs push data to a stack from a low address memory to a high
address memory. In the following diagram, the lowest address is at the
top and the highest address is at the bottom. That is the order of the
address. However, “the top of the stack” is the last pushed data and
“the bottom of the stack” is the first pushed data. Therefore, “the top
of the stack” is the bottom of the rectangle in the diagram and “the
bottom of the stack” is the top of the rectangle.</p>
<figure>
<img src="image/stack.png" alt="Stack" />
<figcaption aria-hidden="true">Stack</figcaption>
</figure>
<p>There are four functions to access the stack.</p>
<ul>
<li><code>stack_push</code> pushes data to the stack.</li>
<li><code>stack_lookup</code> searches the stack for the variable given
its name as an argument. It searches only the parameters of the latest
procedure. It returns TRUE and sets the argument <code>value</code> to
point the value, if the variable has been found. Otherwise it returns
FALSE.</li>
<li><code>stack_replace</code> replaces the value of the variable in the
stack. If it succeeds, it returns TRUE. Otherwise returns FALSE.</li>
<li><code>stack_return</code> throws away the latest parameters. The
stack pointer goes back to the point before the latest procedure call so
that it points to parameters of the previous called procedure.</li>
</ul>
<div class="sourceCode" id="cb35"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb35-1"><a href="#cb35-1" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb35-2"><a href="#cb35-2" aria-hidden="true" tabindex="-1"></a>stack_push <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-3"><a href="#cb35-3" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>sp <span class="op">&gt;=</span> MAX_STACK_SIZE<span class="op">)</span></span>
<span id="cb35-4"><a href="#cb35-4" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Stack overflow.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb35-5"><a href="#cb35-5" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb35-6"><a href="#cb35-6" aria-hidden="true" tabindex="-1"></a>    stack<span class="op">[</span>sp<span class="op">].</span>name <span class="op">=</span> name<span class="op">;</span></span>
<span id="cb35-7"><a href="#cb35-7" aria-hidden="true" tabindex="-1"></a>    stack<span class="op">[</span>sp<span class="op">++].</span>value <span class="op">=</span> value<span class="op">;</span></span>
<span id="cb35-8"><a href="#cb35-8" aria-hidden="true" tabindex="-1"></a>    sp_biggest <span class="op">=</span> sp <span class="op">&gt;</span> sp_biggest <span class="op">?</span> sp <span class="op">:</span> sp_biggest<span class="op">;</span></span>
<span id="cb35-9"><a href="#cb35-9" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb35-10"><a href="#cb35-10" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb35-11"><a href="#cb35-11" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-12"><a href="#cb35-12" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span></span>
<span id="cb35-13"><a href="#cb35-13" aria-hidden="true" tabindex="-1"></a>stack_search <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-14"><a href="#cb35-14" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> depth<span class="op">,</span> i<span class="op">;</span></span>
<span id="cb35-15"><a href="#cb35-15" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-16"><a href="#cb35-16" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>sp <span class="op">==</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb35-17"><a href="#cb35-17" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb35-18"><a href="#cb35-18" aria-hidden="true" tabindex="-1"></a>  depth <span class="op">=</span> <span class="op">(</span><span class="dt">int</span><span class="op">)</span> stack<span class="op">[</span>sp<span class="op">-</span><span class="dv">1</span><span class="op">].</span>value<span class="op">;</span></span>
<span id="cb35-19"><a href="#cb35-19" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>depth <span class="op">+</span> <span class="dv">1</span> <span class="op">&gt;</span> sp<span class="op">)</span> <span class="co">/* something strange */</span></span>
<span id="cb35-20"><a href="#cb35-20" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Stack error.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb35-21"><a href="#cb35-21" aria-hidden="true" tabindex="-1"></a>  <span class="cf">for</span> <span class="op">(</span>i<span class="op">=</span><span class="dv">0</span><span class="op">;</span> i<span class="op">&lt;</span>depth<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span></span>
<span id="cb35-22"><a href="#cb35-22" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="op">(</span>strcmp<span class="op">(</span>name<span class="op">,</span> stack<span class="op">[</span>sp<span class="op">-(</span>i<span class="op">+</span><span class="dv">2</span><span class="op">)].</span>name<span class="op">)</span> <span class="op">==</span> <span class="dv">0</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-23"><a href="#cb35-23" aria-hidden="true" tabindex="-1"></a>      <span class="cf">return</span> sp<span class="op">-(</span>i<span class="op">+</span><span class="dv">2</span><span class="op">);</span></span>
<span id="cb35-24"><a href="#cb35-24" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb35-25"><a href="#cb35-25" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb35-26"><a href="#cb35-26" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb35-27"><a href="#cb35-27" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-28"><a href="#cb35-28" aria-hidden="true" tabindex="-1"></a>gboolean</span>
<span id="cb35-29"><a href="#cb35-29" aria-hidden="true" tabindex="-1"></a>stack_lookup <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> <span class="op">*</span>value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-30"><a href="#cb35-30" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb35-31"><a href="#cb35-31" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-32"><a href="#cb35-32" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">((</span>i <span class="op">=</span> stack_search <span class="op">(</span>name<span class="op">))</span> <span class="op">&lt;</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb35-33"><a href="#cb35-33" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> FALSE<span class="op">;</span></span>
<span id="cb35-34"><a href="#cb35-34" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb35-35"><a href="#cb35-35" aria-hidden="true" tabindex="-1"></a>    <span class="op">*</span>value <span class="op">=</span> stack<span class="op">[</span>i<span class="op">].</span>value<span class="op">;</span></span>
<span id="cb35-36"><a href="#cb35-36" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> TRUE<span class="op">;</span></span>
<span id="cb35-37"><a href="#cb35-37" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb35-38"><a href="#cb35-38" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb35-39"><a href="#cb35-39" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-40"><a href="#cb35-40" aria-hidden="true" tabindex="-1"></a>gboolean</span>
<span id="cb35-41"><a href="#cb35-41" aria-hidden="true" tabindex="-1"></a>stack_replace <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>name<span class="op">,</span> <span class="dt">double</span> value<span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-42"><a href="#cb35-42" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb35-43"><a href="#cb35-43" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-44"><a href="#cb35-44" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">((</span>i <span class="op">=</span> stack_search <span class="op">(</span>name<span class="op">))</span> <span class="op">&lt;</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb35-45"><a href="#cb35-45" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> FALSE<span class="op">;</span></span>
<span id="cb35-46"><a href="#cb35-46" aria-hidden="true" tabindex="-1"></a>  <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb35-47"><a href="#cb35-47" aria-hidden="true" tabindex="-1"></a>    stack<span class="op">[</span>i<span class="op">].</span>value <span class="op">=</span> value<span class="op">;</span></span>
<span id="cb35-48"><a href="#cb35-48" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> TRUE<span class="op">;</span></span>
<span id="cb35-49"><a href="#cb35-49" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb35-50"><a href="#cb35-50" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb35-51"><a href="#cb35-51" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-52"><a href="#cb35-52" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb35-53"><a href="#cb35-53" aria-hidden="true" tabindex="-1"></a>stack_return<span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-54"><a href="#cb35-54" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> depth<span class="op">;</span></span>
<span id="cb35-55"><a href="#cb35-55" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb35-56"><a href="#cb35-56" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>sp <span class="op">&lt;=</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb35-57"><a href="#cb35-57" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span><span class="op">;</span></span>
<span id="cb35-58"><a href="#cb35-58" aria-hidden="true" tabindex="-1"></a>  depth <span class="op">=</span> <span class="op">(</span><span class="dt">int</span><span class="op">)</span> stack<span class="op">[</span>sp<span class="op">-</span><span class="dv">1</span><span class="op">].</span>value<span class="op">;</span></span>
<span id="cb35-59"><a href="#cb35-59" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>depth <span class="op">+</span> <span class="dv">1</span> <span class="op">&gt;</span> sp<span class="op">)</span> <span class="co">/* something strange */</span></span>
<span id="cb35-60"><a href="#cb35-60" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Stack error.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb35-61"><a href="#cb35-61" aria-hidden="true" tabindex="-1"></a>  sp <span class="op">-=</span> depth <span class="op">+</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb35-62"><a href="#cb35-62" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h4 id="surface-and-cairo">Surface and cairo</h4>
<p>A global variable <code>surface</code> is shared by
<code>turtleapplication.c</code> and <code>turtle.y</code>. It is
initialized in <code>turtleapplication.c</code>.</p>
<p>The runtime routine has its own cairo context. This is different from
the cairo of GtkDrawingArea. Runtime routine draws a shape on the
<code>surface</code> with the cairo context. After runtime routine
returns to <code>run_cb</code>, <code>run_cb</code> adds the
GtkDrawingArea widget to the queue to redraw. When the widget is
redraw,the drawing function <code>draw_func</code> is called. It copies
the <code>surface</code> to the surface in the GtkDrawingArea
object.</p>
<p><code>turtle.y</code> has two functions <code>init_cairo</code> and
<code>destroy_cairo</code>.</p>
<ul>
<li><code>init_cairo</code> initializes static variables and cairo
context. The variables keep pen status (up or down), direction, initial
location, line width and color. The size of the <code>surface</code>
changes according to the size of the window. Whenever a user drags and
resizes the window, the <code>surface</code> is also resized.
<code>init_cairo</code> gets the size first and sets the initial
location of the turtle (center of the surface) and the transformation
matrix.</li>
<li><code>destroy_cairo</code> just destroys the cairo context.</li>
</ul>
<p>Turtle has its own coordinate. The origin is at the center of the
surface, and positive direction of x and y axes are right and up
respectively. But surfaces have its own coordinate. Its origin is at the
top-left corner of the surface and positive direction of x and y are
right and down respectively. A plane with the turtle’s coordinate is
called user space, which is the same as cairo’s user space. A plane with
the surface’s coordinate is called device space.</p>
<p>Cairo provides a transformation which is an affine transformation. It
transforms a user-space coordinate (x, y) into a device-space coordinate
(z, w).</p>
<figure>
<img src="image/transformation.png" alt="transformation" />
<figcaption aria-hidden="true">transformation</figcaption>
</figure>
<p><code>init_cairo</code> gets the width and height of the
<code>surface</code> (See the program below).</p>
<ul>
<li>The center of the surface is (0,0) with regard to the user-space
coordinate and (width/2, height/2) with regard to the device-space
coordinate.</li>
<li>The positive direction of x axis in the two spaces are the same. So,
(1,0) is transformed into (1+width/2,height/2).</li>
<li>The positive direction of y axis in the two spaces are opposite. So,
(0,1) is transformed into (width/2,-1+height/2).</li>
</ul>
<p>You can determine a, b, c, d, p and q by substituting the numbers
above for x, y, z and w in the equation above. The solution of the
simultaneous equations is:</p>
<pre><code>a = 1, b = 0, c = 0, d = -1, p = width/2, q = height/2</code></pre>
<p>Cairo provides a structure <code>cairo_matrix_t</code>.
<code>init_cairo</code> uses it and sets the cairo transformation (See
the program below). Once the matrix is set, the transformation always
performs whenever <code>cairo_stroke</code> function is invoked.</p>
<div class="sourceCode" id="cb37"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb37-1"><a href="#cb37-1" aria-hidden="true" tabindex="-1"></a><span class="co">/* status of the surface */</span></span>
<span id="cb37-2"><a href="#cb37-2" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> gboolean pen <span class="op">=</span> TRUE<span class="op">;</span></span>
<span id="cb37-3"><a href="#cb37-3" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">double</span> angle <span class="op">=</span> <span class="fl">90.0</span><span class="op">;</span> <span class="co">/* angle starts from x axis and measured counterclockwise */</span></span>
<span id="cb37-4"><a href="#cb37-4" aria-hidden="true" tabindex="-1"></a>                   <span class="co">/* Initially facing to the north */</span></span>
<span id="cb37-5"><a href="#cb37-5" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">double</span> cur_x <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb37-6"><a href="#cb37-6" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">double</span> cur_y <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb37-7"><a href="#cb37-7" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">double</span> line_width <span class="op">=</span> <span class="fl">2.0</span><span class="op">;</span></span>
<span id="cb37-8"><a href="#cb37-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-9"><a href="#cb37-9" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> color <span class="op">{</span></span>
<span id="cb37-10"><a href="#cb37-10" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> red<span class="op">;</span></span>
<span id="cb37-11"><a href="#cb37-11" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> green<span class="op">;</span></span>
<span id="cb37-12"><a href="#cb37-12" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> blue<span class="op">;</span></span>
<span id="cb37-13"><a href="#cb37-13" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb37-14"><a href="#cb37-14" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="kw">struct</span> color bc <span class="op">=</span> <span class="op">{</span><span class="fl">0.95</span><span class="op">,</span> <span class="fl">0.95</span><span class="op">,</span> <span class="fl">0.95</span><span class="op">};</span> <span class="co">/* white */</span></span>
<span id="cb37-15"><a href="#cb37-15" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="kw">struct</span> color fc <span class="op">=</span> <span class="op">{</span><span class="fl">0.0</span><span class="op">,</span> <span class="fl">0.0</span><span class="op">,</span> <span class="fl">0.0</span><span class="op">};</span> <span class="co">/* black */</span></span>
<span id="cb37-16"><a href="#cb37-16" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-17"><a href="#cb37-17" aria-hidden="true" tabindex="-1"></a><span class="co">/* cairo */</span></span>
<span id="cb37-18"><a href="#cb37-18" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> cairo_t <span class="op">*</span>cr<span class="op">;</span></span>
<span id="cb37-19"><a href="#cb37-19" aria-hidden="true" tabindex="-1"></a>gboolean</span>
<span id="cb37-20"><a href="#cb37-20" aria-hidden="true" tabindex="-1"></a>init_cairo <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb37-21"><a href="#cb37-21" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> width<span class="op">,</span> height<span class="op">;</span></span>
<span id="cb37-22"><a href="#cb37-22" aria-hidden="true" tabindex="-1"></a>  cairo_matrix_t matrix<span class="op">;</span></span>
<span id="cb37-23"><a href="#cb37-23" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-24"><a href="#cb37-24" aria-hidden="true" tabindex="-1"></a>  pen <span class="op">=</span> TRUE<span class="op">;</span></span>
<span id="cb37-25"><a href="#cb37-25" aria-hidden="true" tabindex="-1"></a>  angle <span class="op">=</span> <span class="fl">90.0</span><span class="op">;</span></span>
<span id="cb37-26"><a href="#cb37-26" aria-hidden="true" tabindex="-1"></a>  cur_x <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb37-27"><a href="#cb37-27" aria-hidden="true" tabindex="-1"></a>  cur_y <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb37-28"><a href="#cb37-28" aria-hidden="true" tabindex="-1"></a>  line_width <span class="op">=</span> <span class="fl">2.0</span><span class="op">;</span></span>
<span id="cb37-29"><a href="#cb37-29" aria-hidden="true" tabindex="-1"></a>  bc<span class="op">.</span>red <span class="op">=</span> <span class="fl">0.95</span><span class="op">;</span> bc<span class="op">.</span>green <span class="op">=</span> <span class="fl">0.95</span><span class="op">;</span> bc<span class="op">.</span>blue <span class="op">=</span> <span class="fl">0.95</span><span class="op">;</span></span>
<span id="cb37-30"><a href="#cb37-30" aria-hidden="true" tabindex="-1"></a>  fc<span class="op">.</span>red <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> fc<span class="op">.</span>green <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> fc<span class="op">.</span>blue <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb37-31"><a href="#cb37-31" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-32"><a href="#cb37-32" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>surface<span class="op">)</span> <span class="op">{</span></span>
<span id="cb37-33"><a href="#cb37-33" aria-hidden="true" tabindex="-1"></a>    width <span class="op">=</span> cairo_image_surface_get_width <span class="op">(</span>surface<span class="op">);</span></span>
<span id="cb37-34"><a href="#cb37-34" aria-hidden="true" tabindex="-1"></a>    height <span class="op">=</span> cairo_image_surface_get_height <span class="op">(</span>surface<span class="op">);</span></span>
<span id="cb37-35"><a href="#cb37-35" aria-hidden="true" tabindex="-1"></a>    matrix<span class="op">.</span>xx <span class="op">=</span> <span class="fl">1.0</span><span class="op">;</span> matrix<span class="op">.</span>xy <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> matrix<span class="op">.</span>x0 <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> width <span class="op">/</span> <span class="fl">2.0</span><span class="op">;</span></span>
<span id="cb37-36"><a href="#cb37-36" aria-hidden="true" tabindex="-1"></a>    matrix<span class="op">.</span>yx <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> matrix<span class="op">.</span>yy <span class="op">=</span> <span class="op">-</span><span class="fl">1.0</span><span class="op">;</span> matrix<span class="op">.</span>y0 <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> height <span class="op">/</span> <span class="fl">2.0</span><span class="op">;</span></span>
<span id="cb37-37"><a href="#cb37-37" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-38"><a href="#cb37-38" aria-hidden="true" tabindex="-1"></a>    cr <span class="op">=</span> cairo_create <span class="op">(</span>surface<span class="op">);</span></span>
<span id="cb37-39"><a href="#cb37-39" aria-hidden="true" tabindex="-1"></a>    cairo_transform <span class="op">(</span>cr<span class="op">,</span> <span class="op">&amp;</span>matrix<span class="op">);</span></span>
<span id="cb37-40"><a href="#cb37-40" aria-hidden="true" tabindex="-1"></a>    cairo_set_source_rgb <span class="op">(</span>cr<span class="op">,</span> bc<span class="op">.</span>red<span class="op">,</span> bc<span class="op">.</span>green<span class="op">,</span> bc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb37-41"><a href="#cb37-41" aria-hidden="true" tabindex="-1"></a>    cairo_paint <span class="op">(</span>cr<span class="op">);</span></span>
<span id="cb37-42"><a href="#cb37-42" aria-hidden="true" tabindex="-1"></a>    cairo_set_source_rgb <span class="op">(</span>cr<span class="op">,</span> fc<span class="op">.</span>red<span class="op">,</span> fc<span class="op">.</span>green<span class="op">,</span> fc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb37-43"><a href="#cb37-43" aria-hidden="true" tabindex="-1"></a>    cairo_move_to <span class="op">(</span>cr<span class="op">,</span> cur_x<span class="op">,</span> cur_y<span class="op">);</span></span>
<span id="cb37-44"><a href="#cb37-44" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> TRUE<span class="op">;</span></span>
<span id="cb37-45"><a href="#cb37-45" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span> <span class="cf">else</span></span>
<span id="cb37-46"><a href="#cb37-46" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> FALSE<span class="op">;</span></span>
<span id="cb37-47"><a href="#cb37-47" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb37-48"><a href="#cb37-48" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-49"><a href="#cb37-49" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb37-50"><a href="#cb37-50" aria-hidden="true" tabindex="-1"></a>destroy_cairo <span class="op">()</span> <span class="op">{</span></span>
<span id="cb37-51"><a href="#cb37-51" aria-hidden="true" tabindex="-1"></a>  cairo_destroy <span class="op">(</span>cr<span class="op">);</span></span>
<span id="cb37-52"><a href="#cb37-52" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h4 id="eval-function">Eval function</h4>
<p>A function <code>eval</code> evaluates an expression and returns the
value of the expression. It calls itself recursively. For example, if
the node is <code>N_ADD</code>, then:</p>
<ol type="1">
<li>Calls eval(child1(node)) and gets the value1.</li>
<li>Calls eval(child2(node)) and gets the value2.</li>
<li>Returns value1+value2.</li>
</ol>
<p>This is performed by a macro <code>calc</code> defined in the sixth
line in the following program.</p>
<div class="sourceCode" id="cb38"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb38-1"><a href="#cb38-1" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span></span>
<span id="cb38-2"><a href="#cb38-2" aria-hidden="true" tabindex="-1"></a>eval <span class="op">(</span>node_t <span class="op">*</span>node<span class="op">)</span> <span class="op">{</span></span>
<span id="cb38-3"><a href="#cb38-3" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> value <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb38-4"><a href="#cb38-4" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>node <span class="op">==</span> NULL<span class="op">)</span></span>
<span id="cb38-5"><a href="#cb38-5" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;No expression to evaluate.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb38-6"><a href="#cb38-6" aria-hidden="true" tabindex="-1"></a><span class="pp">#define calc(op) eval (child1(node)) op eval (child2(node))</span></span>
<span id="cb38-7"><a href="#cb38-7" aria-hidden="true" tabindex="-1"></a>  <span class="cf">switch</span> <span class="op">(</span>node<span class="op">-&gt;</span>type<span class="op">)</span> <span class="op">{</span></span>
<span id="cb38-8"><a href="#cb38-8" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_EQ<span class="op">:</span></span>
<span id="cb38-9"><a href="#cb38-9" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> calc<span class="op">(==);</span></span>
<span id="cb38-10"><a href="#cb38-10" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-11"><a href="#cb38-11" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_GT<span class="op">:</span></span>
<span id="cb38-12"><a href="#cb38-12" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> calc<span class="op">(&gt;);</span></span>
<span id="cb38-13"><a href="#cb38-13" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-14"><a href="#cb38-14" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_LT<span class="op">:</span></span>
<span id="cb38-15"><a href="#cb38-15" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> calc<span class="op">(&lt;);</span></span>
<span id="cb38-16"><a href="#cb38-16" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-17"><a href="#cb38-17" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_ADD<span class="op">:</span></span>
<span id="cb38-18"><a href="#cb38-18" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> calc<span class="op">(+);</span></span>
<span id="cb38-19"><a href="#cb38-19" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-20"><a href="#cb38-20" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_SUB<span class="op">:</span></span>
<span id="cb38-21"><a href="#cb38-21" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> calc<span class="op">(-);</span></span>
<span id="cb38-22"><a href="#cb38-22" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-23"><a href="#cb38-23" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_MUL<span class="op">:</span></span>
<span id="cb38-24"><a href="#cb38-24" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> calc<span class="op">(*);</span></span>
<span id="cb38-25"><a href="#cb38-25" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-26"><a href="#cb38-26" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_DIV<span class="op">:</span></span>
<span id="cb38-27"><a href="#cb38-27" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(</span>eval <span class="op">(</span>child2<span class="op">(</span>node<span class="op">))</span> <span class="op">==</span> <span class="fl">0.0</span><span class="op">)</span></span>
<span id="cb38-28"><a href="#cb38-28" aria-hidden="true" tabindex="-1"></a>        runtime_error <span class="op">(</span><span class="st">&quot;Division by zerp.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb38-29"><a href="#cb38-29" aria-hidden="true" tabindex="-1"></a>      <span class="cf">else</span></span>
<span id="cb38-30"><a href="#cb38-30" aria-hidden="true" tabindex="-1"></a>        value <span class="op">=</span> calc<span class="op">(/);</span></span>
<span id="cb38-31"><a href="#cb38-31" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-32"><a href="#cb38-32" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_UMINUS<span class="op">:</span></span>
<span id="cb38-33"><a href="#cb38-33" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> <span class="op">-(</span>eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">)));</span></span>
<span id="cb38-34"><a href="#cb38-34" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-35"><a href="#cb38-35" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_ID<span class="op">:</span></span>
<span id="cb38-36"><a href="#cb38-36" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(!</span> <span class="op">(</span>stack_lookup <span class="op">(</span>name<span class="op">(</span>node<span class="op">),</span> <span class="op">&amp;</span>value<span class="op">))</span> <span class="op">&amp;&amp;</span> <span class="op">!</span> var_lookup <span class="op">(</span>name<span class="op">(</span>node<span class="op">),</span> <span class="op">&amp;</span>value<span class="op">)</span> <span class="op">)</span></span>
<span id="cb38-37"><a href="#cb38-37" aria-hidden="true" tabindex="-1"></a>        runtime_error <span class="op">(</span><span class="st">&quot;Variable %s not defined.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb38-38"><a href="#cb38-38" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-39"><a href="#cb38-39" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_NUM<span class="op">:</span></span>
<span id="cb38-40"><a href="#cb38-40" aria-hidden="true" tabindex="-1"></a>      value <span class="op">=</span> value<span class="op">(</span>node<span class="op">);</span></span>
<span id="cb38-41"><a href="#cb38-41" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb38-42"><a href="#cb38-42" aria-hidden="true" tabindex="-1"></a>    <span class="cf">default</span><span class="op">:</span></span>
<span id="cb38-43"><a href="#cb38-43" aria-hidden="true" tabindex="-1"></a>      runtime_error <span class="op">(</span><span class="st">&quot;Illegal expression.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb38-44"><a href="#cb38-44" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb38-45"><a href="#cb38-45" aria-hidden="true" tabindex="-1"></a>  <span class="cf">return</span> value<span class="op">;</span></span>
<span id="cb38-46"><a href="#cb38-46" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h4 id="execute-function">Execute function</h4>
<p>Primary procedures and procedure definitions are analyzed and
executed by the function <code>execute</code>. It doesn’t return any
values. It calls itself recursively. The process of <code>N_RT</code>
and <code>N_procedure_call</code> is complicated. It will explained
after the following program. Other parts are not so difficult. Read the
program below carefully so that you will understand the process.</p>
<div class="sourceCode" id="cb39"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb39-1"><a href="#cb39-1" aria-hidden="true" tabindex="-1"></a><span class="co">/* procedure - return status */</span></span>
<span id="cb39-2"><a href="#cb39-2" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">int</span> proc_level <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb39-3"><a href="#cb39-3" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">int</span> ret_level <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb39-4"><a href="#cb39-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb39-5"><a href="#cb39-5" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb39-6"><a href="#cb39-6" aria-hidden="true" tabindex="-1"></a>execute <span class="op">(</span>node_t <span class="op">*</span>node<span class="op">)</span> <span class="op">{</span></span>
<span id="cb39-7"><a href="#cb39-7" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> d<span class="op">,</span> x<span class="op">,</span> y<span class="op">;</span></span>
<span id="cb39-8"><a href="#cb39-8" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> <span class="op">*</span>name<span class="op">;</span></span>
<span id="cb39-9"><a href="#cb39-9" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> n<span class="op">,</span> i<span class="op">;</span></span>
<span id="cb39-10"><a href="#cb39-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb39-11"><a href="#cb39-11" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>node <span class="op">==</span> NULL<span class="op">)</span></span>
<span id="cb39-12"><a href="#cb39-12" aria-hidden="true" tabindex="-1"></a>    runtime_error <span class="op">(</span><span class="st">&quot;Node is NULL.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb39-13"><a href="#cb39-13" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>proc_level <span class="op">&gt;</span> ret_level<span class="op">)</span></span>
<span id="cb39-14"><a href="#cb39-14" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span><span class="op">;</span></span>
<span id="cb39-15"><a href="#cb39-15" aria-hidden="true" tabindex="-1"></a>  <span class="cf">switch</span> <span class="op">(</span>node<span class="op">-&gt;</span>type<span class="op">)</span> <span class="op">{</span></span>
<span id="cb39-16"><a href="#cb39-16" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_program<span class="op">:</span></span>
<span id="cb39-17"><a href="#cb39-17" aria-hidden="true" tabindex="-1"></a>      execute <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-18"><a href="#cb39-18" aria-hidden="true" tabindex="-1"></a>      execute <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-19"><a href="#cb39-19" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-20"><a href="#cb39-20" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_PU<span class="op">:</span></span>
<span id="cb39-21"><a href="#cb39-21" aria-hidden="true" tabindex="-1"></a>      pen <span class="op">=</span> FALSE<span class="op">;</span></span>
<span id="cb39-22"><a href="#cb39-22" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-23"><a href="#cb39-23" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_PD<span class="op">:</span></span>
<span id="cb39-24"><a href="#cb39-24" aria-hidden="true" tabindex="-1"></a>      pen <span class="op">=</span> TRUE<span class="op">;</span></span>
<span id="cb39-25"><a href="#cb39-25" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-26"><a href="#cb39-26" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_PW<span class="op">:</span></span>
<span id="cb39-27"><a href="#cb39-27" aria-hidden="true" tabindex="-1"></a>      line_width <span class="op">=</span> eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span> <span class="co">/* line width */</span></span>
<span id="cb39-28"><a href="#cb39-28" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-29"><a href="#cb39-29" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_FD<span class="op">:</span></span>
<span id="cb39-30"><a href="#cb39-30" aria-hidden="true" tabindex="-1"></a>      d <span class="op">=</span> eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span> <span class="co">/* distance */</span></span>
<span id="cb39-31"><a href="#cb39-31" aria-hidden="true" tabindex="-1"></a>      x <span class="op">=</span> d <span class="op">*</span> cos <span class="op">(</span>angle<span class="op">*</span>M_PI<span class="op">/</span><span class="dv">180</span><span class="op">);</span></span>
<span id="cb39-32"><a href="#cb39-32" aria-hidden="true" tabindex="-1"></a>      y <span class="op">=</span> d <span class="op">*</span> sin <span class="op">(</span>angle<span class="op">*</span>M_PI<span class="op">/</span><span class="dv">180</span><span class="op">);</span></span>
<span id="cb39-33"><a href="#cb39-33" aria-hidden="true" tabindex="-1"></a>      <span class="co">/* initialize the current point = start point of the line */</span></span>
<span id="cb39-34"><a href="#cb39-34" aria-hidden="true" tabindex="-1"></a>      cairo_move_to <span class="op">(</span>cr<span class="op">,</span> cur_x<span class="op">,</span> cur_y<span class="op">);</span></span>
<span id="cb39-35"><a href="#cb39-35" aria-hidden="true" tabindex="-1"></a>      cur_x <span class="op">+=</span> x<span class="op">;</span></span>
<span id="cb39-36"><a href="#cb39-36" aria-hidden="true" tabindex="-1"></a>      cur_y <span class="op">+=</span> y<span class="op">;</span></span>
<span id="cb39-37"><a href="#cb39-37" aria-hidden="true" tabindex="-1"></a>      cairo_set_line_width <span class="op">(</span>cr<span class="op">,</span> line_width<span class="op">);</span></span>
<span id="cb39-38"><a href="#cb39-38" aria-hidden="true" tabindex="-1"></a>      cairo_set_source_rgb <span class="op">(</span>cr<span class="op">,</span> fc<span class="op">.</span>red<span class="op">,</span> fc<span class="op">.</span>green<span class="op">,</span> fc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb39-39"><a href="#cb39-39" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(</span>pen<span class="op">)</span></span>
<span id="cb39-40"><a href="#cb39-40" aria-hidden="true" tabindex="-1"></a>        cairo_line_to <span class="op">(</span>cr<span class="op">,</span> cur_x<span class="op">,</span> cur_y<span class="op">);</span></span>
<span id="cb39-41"><a href="#cb39-41" aria-hidden="true" tabindex="-1"></a>      <span class="cf">else</span></span>
<span id="cb39-42"><a href="#cb39-42" aria-hidden="true" tabindex="-1"></a>        cairo_move_to <span class="op">(</span>cr<span class="op">,</span> cur_x<span class="op">,</span> cur_y<span class="op">);</span></span>
<span id="cb39-43"><a href="#cb39-43" aria-hidden="true" tabindex="-1"></a>      cairo_stroke <span class="op">(</span>cr<span class="op">);</span></span>
<span id="cb39-44"><a href="#cb39-44" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-45"><a href="#cb39-45" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_TR<span class="op">:</span></span>
<span id="cb39-46"><a href="#cb39-46" aria-hidden="true" tabindex="-1"></a>      angle <span class="op">-=</span> eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-47"><a href="#cb39-47" aria-hidden="true" tabindex="-1"></a>      <span class="cf">for</span> <span class="op">(;</span> angle <span class="op">&lt;</span> <span class="dv">0</span><span class="op">;</span> angle <span class="op">+=</span> <span class="fl">360.0</span><span class="op">);</span></span>
<span id="cb39-48"><a href="#cb39-48" aria-hidden="true" tabindex="-1"></a>      <span class="cf">for</span> <span class="op">(;</span> angle<span class="op">&gt;</span><span class="dv">360</span><span class="op">;</span> angle <span class="op">-=</span> <span class="fl">360.0</span><span class="op">);</span></span>
<span id="cb39-49"><a href="#cb39-49" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-50"><a href="#cb39-50" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_BC<span class="op">:</span></span>
<span id="cb39-51"><a href="#cb39-51" aria-hidden="true" tabindex="-1"></a>      bc<span class="op">.</span>red <span class="op">=</span> eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-52"><a href="#cb39-52" aria-hidden="true" tabindex="-1"></a>      bc<span class="op">.</span>green <span class="op">=</span> eval <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-53"><a href="#cb39-53" aria-hidden="true" tabindex="-1"></a>      bc<span class="op">.</span>blue <span class="op">=</span> eval <span class="op">(</span>child3<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-54"><a href="#cb39-54" aria-hidden="true" tabindex="-1"></a><span class="pp">#define fixcolor(c)  c = c &lt; 0 ? 0 : (c &gt; 1 ? 1 : c)</span></span>
<span id="cb39-55"><a href="#cb39-55" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>bc<span class="op">.</span>red<span class="op">);</span></span>
<span id="cb39-56"><a href="#cb39-56" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>bc<span class="op">.</span>green<span class="op">);</span></span>
<span id="cb39-57"><a href="#cb39-57" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>bc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb39-58"><a href="#cb39-58" aria-hidden="true" tabindex="-1"></a>      <span class="co">/* clear the shapes and set the background color */</span></span>
<span id="cb39-59"><a href="#cb39-59" aria-hidden="true" tabindex="-1"></a>      cairo_set_source_rgb <span class="op">(</span>cr<span class="op">,</span> bc<span class="op">.</span>red<span class="op">,</span> bc<span class="op">.</span>green<span class="op">,</span> bc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb39-60"><a href="#cb39-60" aria-hidden="true" tabindex="-1"></a>      cairo_paint <span class="op">(</span>cr<span class="op">);</span></span>
<span id="cb39-61"><a href="#cb39-61" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-62"><a href="#cb39-62" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_FC<span class="op">:</span></span>
<span id="cb39-63"><a href="#cb39-63" aria-hidden="true" tabindex="-1"></a>      fc<span class="op">.</span>red <span class="op">=</span> eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-64"><a href="#cb39-64" aria-hidden="true" tabindex="-1"></a>      fc<span class="op">.</span>green <span class="op">=</span> eval <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-65"><a href="#cb39-65" aria-hidden="true" tabindex="-1"></a>      fc<span class="op">.</span>blue <span class="op">=</span> eval <span class="op">(</span>child3<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-66"><a href="#cb39-66" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>fc<span class="op">.</span>red<span class="op">);</span></span>
<span id="cb39-67"><a href="#cb39-67" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>fc<span class="op">.</span>green<span class="op">);</span></span>
<span id="cb39-68"><a href="#cb39-68" aria-hidden="true" tabindex="-1"></a>      fixcolor <span class="op">(</span>fc<span class="op">.</span>blue<span class="op">);</span></span>
<span id="cb39-69"><a href="#cb39-69" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-70"><a href="#cb39-70" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_ASSIGN<span class="op">:</span></span>
<span id="cb39-71"><a href="#cb39-71" aria-hidden="true" tabindex="-1"></a>      name <span class="op">=</span> name<span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-72"><a href="#cb39-72" aria-hidden="true" tabindex="-1"></a>      d <span class="op">=</span> eval <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-73"><a href="#cb39-73" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(!</span> stack_replace <span class="op">(</span>name<span class="op">,</span> d<span class="op">))</span> <span class="co">/* First, tries to replace the value in the stack (parameter).*/</span></span>
<span id="cb39-74"><a href="#cb39-74" aria-hidden="true" tabindex="-1"></a>        var_replace <span class="op">(</span>name<span class="op">,</span> d<span class="op">);</span> <span class="co">/* If the above fails, tries to replace the value in the table. If the variable isn&#39;t in the table, installs it, */</span></span>
<span id="cb39-75"><a href="#cb39-75" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-76"><a href="#cb39-76" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_IF<span class="op">:</span></span>
<span id="cb39-77"><a href="#cb39-77" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(</span>eval <span class="op">(</span>child1<span class="op">(</span>node<span class="op">)))</span></span>
<span id="cb39-78"><a href="#cb39-78" aria-hidden="true" tabindex="-1"></a>        execute <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-79"><a href="#cb39-79" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-80"><a href="#cb39-80" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_RT<span class="op">:</span></span>
<span id="cb39-81"><a href="#cb39-81" aria-hidden="true" tabindex="-1"></a>      ret_level<span class="op">--;</span></span>
<span id="cb39-82"><a href="#cb39-82" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-83"><a href="#cb39-83" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_RS<span class="op">:</span></span>
<span id="cb39-84"><a href="#cb39-84" aria-hidden="true" tabindex="-1"></a>      pen <span class="op">=</span> TRUE<span class="op">;</span></span>
<span id="cb39-85"><a href="#cb39-85" aria-hidden="true" tabindex="-1"></a>      angle <span class="op">=</span> <span class="fl">90.0</span><span class="op">;</span></span>
<span id="cb39-86"><a href="#cb39-86" aria-hidden="true" tabindex="-1"></a>      cur_x <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb39-87"><a href="#cb39-87" aria-hidden="true" tabindex="-1"></a>      cur_y <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb39-88"><a href="#cb39-88" aria-hidden="true" tabindex="-1"></a>      line_width <span class="op">=</span> <span class="fl">2.0</span><span class="op">;</span></span>
<span id="cb39-89"><a href="#cb39-89" aria-hidden="true" tabindex="-1"></a>      fc<span class="op">.</span>red <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> fc<span class="op">.</span>green <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span> fc<span class="op">.</span>blue <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb39-90"><a href="#cb39-90" aria-hidden="true" tabindex="-1"></a>      <span class="co">/* To change background color, use bc. */</span></span>
<span id="cb39-91"><a href="#cb39-91" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-92"><a href="#cb39-92" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_procedure_call<span class="op">:</span></span>
<span id="cb39-93"><a href="#cb39-93" aria-hidden="true" tabindex="-1"></a>      name <span class="op">=</span> name<span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-94"><a href="#cb39-94" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span>proc <span class="op">=</span> proc_lookup <span class="op">(</span>name<span class="op">);</span></span>
<span id="cb39-95"><a href="#cb39-95" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(!</span> proc<span class="op">)</span></span>
<span id="cb39-96"><a href="#cb39-96" aria-hidden="true" tabindex="-1"></a>        runtime_error <span class="op">(</span><span class="st">&quot;Procedure %s not defined.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">);</span></span>
<span id="cb39-97"><a href="#cb39-97" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(</span>strcmp <span class="op">(</span>name<span class="op">,</span> name<span class="op">(</span>child1<span class="op">(</span>proc<span class="op">)))</span> <span class="op">!=</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb39-98"><a href="#cb39-98" aria-hidden="true" tabindex="-1"></a>        runtime_error <span class="op">(</span><span class="st">&quot;Unexpected error. Procedure %s is called, but invoked procedure is %s.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">,</span> name<span class="op">(</span>child1<span class="op">(</span>proc<span class="op">)));</span></span>
<span id="cb39-99"><a href="#cb39-99" aria-hidden="true" tabindex="-1"></a><span class="co">/* make tuples (parameter (name), argument (value)) and push them to the stack */</span></span>
<span id="cb39-100"><a href="#cb39-100" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span>param_list<span class="op">;</span></span>
<span id="cb39-101"><a href="#cb39-101" aria-hidden="true" tabindex="-1"></a>node_t <span class="op">*</span>arg_list<span class="op">;</span></span>
<span id="cb39-102"><a href="#cb39-102" aria-hidden="true" tabindex="-1"></a>      param_list <span class="op">=</span> child2<span class="op">(</span>proc<span class="op">);</span></span>
<span id="cb39-103"><a href="#cb39-103" aria-hidden="true" tabindex="-1"></a>      arg_list <span class="op">=</span> child2<span class="op">(</span>node<span class="op">);</span></span>
<span id="cb39-104"><a href="#cb39-104" aria-hidden="true" tabindex="-1"></a>      <span class="cf">if</span> <span class="op">(</span>param_list <span class="op">==</span> NULL<span class="op">)</span> <span class="op">{</span></span>
<span id="cb39-105"><a href="#cb39-105" aria-hidden="true" tabindex="-1"></a>        <span class="cf">if</span> <span class="op">(</span>arg_list <span class="op">==</span> NULL<span class="op">)</span> <span class="op">{</span></span>
<span id="cb39-106"><a href="#cb39-106" aria-hidden="true" tabindex="-1"></a>          stack_push <span class="op">(</span>NULL<span class="op">,</span> <span class="fl">0.0</span><span class="op">);</span> <span class="co">/* number of argument == 0 */</span></span>
<span id="cb39-107"><a href="#cb39-107" aria-hidden="true" tabindex="-1"></a>        <span class="op">}</span> <span class="cf">else</span></span>
<span id="cb39-108"><a href="#cb39-108" aria-hidden="true" tabindex="-1"></a>          runtime_error <span class="op">(</span><span class="st">&quot;Procedure %s has different number of argument and parameter.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">);</span></span>
<span id="cb39-109"><a href="#cb39-109" aria-hidden="true" tabindex="-1"></a>      <span class="op">}</span><span class="cf">else</span> <span class="op">{</span></span>
<span id="cb39-110"><a href="#cb39-110" aria-hidden="true" tabindex="-1"></a><span class="co">/* Don&#39;t change the stack until finish evaluating the arguments. */</span></span>
<span id="cb39-111"><a href="#cb39-111" aria-hidden="true" tabindex="-1"></a><span class="pp">#define TEMP_STACK_SIZE 20</span></span>
<span id="cb39-112"><a href="#cb39-112" aria-hidden="true" tabindex="-1"></a>        <span class="dt">char</span> <span class="op">*</span>temp_param<span class="op">[</span>TEMP_STACK_SIZE<span class="op">];</span></span>
<span id="cb39-113"><a href="#cb39-113" aria-hidden="true" tabindex="-1"></a>        <span class="dt">double</span> temp_arg<span class="op">[</span>TEMP_STACK_SIZE<span class="op">];</span></span>
<span id="cb39-114"><a href="#cb39-114" aria-hidden="true" tabindex="-1"></a>        n <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb39-115"><a href="#cb39-115" aria-hidden="true" tabindex="-1"></a>        <span class="cf">for</span> <span class="op">(;</span> param_list<span class="op">-&gt;</span>type <span class="op">==</span> N_parameter_list<span class="op">;</span> param_list <span class="op">=</span> child1<span class="op">(</span>param_list<span class="op">))</span> <span class="op">{</span></span>
<span id="cb39-116"><a href="#cb39-116" aria-hidden="true" tabindex="-1"></a>          <span class="cf">if</span> <span class="op">(</span>arg_list<span class="op">-&gt;</span>type <span class="op">!=</span> N_argument_list<span class="op">)</span></span>
<span id="cb39-117"><a href="#cb39-117" aria-hidden="true" tabindex="-1"></a>            runtime_error <span class="op">(</span><span class="st">&quot;Procedure %s has different number of argument and parameter.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> name<span class="op">);</span></span>
<span id="cb39-118"><a href="#cb39-118" aria-hidden="true" tabindex="-1"></a>          <span class="cf">if</span> <span class="op">(</span>n <span class="op">&gt;=</span> TEMP_STACK_SIZE<span class="op">)</span></span>
<span id="cb39-119"><a href="#cb39-119" aria-hidden="true" tabindex="-1"></a>            runtime_error <span class="op">(</span><span class="st">&quot;Too many parameters. the number must be %d or less.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> TEMP_STACK_SIZE<span class="op">);</span></span>
<span id="cb39-120"><a href="#cb39-120" aria-hidden="true" tabindex="-1"></a>          temp_param<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> name<span class="op">(</span>child2<span class="op">(</span>param_list<span class="op">));</span></span>
<span id="cb39-121"><a href="#cb39-121" aria-hidden="true" tabindex="-1"></a>          temp_arg<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> eval <span class="op">(</span>child2<span class="op">(</span>arg_list<span class="op">));</span></span>
<span id="cb39-122"><a href="#cb39-122" aria-hidden="true" tabindex="-1"></a>          arg_list <span class="op">=</span> child1<span class="op">(</span>arg_list<span class="op">);</span></span>
<span id="cb39-123"><a href="#cb39-123" aria-hidden="true" tabindex="-1"></a>          <span class="op">++</span>n<span class="op">;</span></span>
<span id="cb39-124"><a href="#cb39-124" aria-hidden="true" tabindex="-1"></a>        <span class="op">}</span></span>
<span id="cb39-125"><a href="#cb39-125" aria-hidden="true" tabindex="-1"></a>        <span class="cf">if</span> <span class="op">(</span>param_list<span class="op">-&gt;</span>type <span class="op">==</span> N_ID <span class="op">&amp;&amp;</span> arg_list <span class="op">-&gt;</span> type <span class="op">!=</span> N_argument_list<span class="op">)</span> <span class="op">{</span></span>
<span id="cb39-126"><a href="#cb39-126" aria-hidden="true" tabindex="-1"></a>          temp_param<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> name<span class="op">(</span>param_list<span class="op">);</span></span>
<span id="cb39-127"><a href="#cb39-127" aria-hidden="true" tabindex="-1"></a>          temp_arg<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> eval <span class="op">(</span>arg_list<span class="op">);</span></span>
<span id="cb39-128"><a href="#cb39-128" aria-hidden="true" tabindex="-1"></a>          <span class="cf">if</span> <span class="op">(++</span>n <span class="op">&gt;=</span> TEMP_STACK_SIZE<span class="op">)</span></span>
<span id="cb39-129"><a href="#cb39-129" aria-hidden="true" tabindex="-1"></a>            runtime_error <span class="op">(</span><span class="st">&quot;Too many parameters. the number must be %d or less.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">,</span> TEMP_STACK_SIZE<span class="op">);</span></span>
<span id="cb39-130"><a href="#cb39-130" aria-hidden="true" tabindex="-1"></a>          temp_param<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> NULL<span class="op">;</span></span>
<span id="cb39-131"><a href="#cb39-131" aria-hidden="true" tabindex="-1"></a>          temp_arg<span class="op">[</span>n<span class="op">]</span> <span class="op">=</span> <span class="op">(</span><span class="dt">double</span><span class="op">)</span> n<span class="op">;</span></span>
<span id="cb39-132"><a href="#cb39-132" aria-hidden="true" tabindex="-1"></a>          <span class="op">++</span>n<span class="op">;</span></span>
<span id="cb39-133"><a href="#cb39-133" aria-hidden="true" tabindex="-1"></a>        <span class="op">}</span> <span class="cf">else</span></span>
<span id="cb39-134"><a href="#cb39-134" aria-hidden="true" tabindex="-1"></a>          runtime_error <span class="op">(</span><span class="st">&quot;Unexpected error.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb39-135"><a href="#cb39-135" aria-hidden="true" tabindex="-1"></a>        <span class="cf">for</span> <span class="op">(</span>i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op">&lt;</span> n<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span></span>
<span id="cb39-136"><a href="#cb39-136" aria-hidden="true" tabindex="-1"></a>          stack_push <span class="op">(</span>temp_param<span class="op">[</span>i<span class="op">],</span> temp_arg<span class="op">[</span>i<span class="op">]);</span></span>
<span id="cb39-137"><a href="#cb39-137" aria-hidden="true" tabindex="-1"></a>      <span class="op">}</span></span>
<span id="cb39-138"><a href="#cb39-138" aria-hidden="true" tabindex="-1"></a>      ret_level <span class="op">=</span> <span class="op">++</span>proc_level<span class="op">;</span></span>
<span id="cb39-139"><a href="#cb39-139" aria-hidden="true" tabindex="-1"></a>      execute <span class="op">(</span>child3<span class="op">(</span>proc<span class="op">));</span></span>
<span id="cb39-140"><a href="#cb39-140" aria-hidden="true" tabindex="-1"></a>      ret_level <span class="op">=</span> <span class="op">--</span>proc_level<span class="op">;</span></span>
<span id="cb39-141"><a href="#cb39-141" aria-hidden="true" tabindex="-1"></a>      stack_return <span class="op">();</span></span>
<span id="cb39-142"><a href="#cb39-142" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-143"><a href="#cb39-143" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_procedure_definition<span class="op">:</span></span>
<span id="cb39-144"><a href="#cb39-144" aria-hidden="true" tabindex="-1"></a>      name <span class="op">=</span> name<span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-145"><a href="#cb39-145" aria-hidden="true" tabindex="-1"></a>      proc_install <span class="op">(</span>name<span class="op">,</span> node<span class="op">);</span></span>
<span id="cb39-146"><a href="#cb39-146" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-147"><a href="#cb39-147" aria-hidden="true" tabindex="-1"></a>    <span class="cf">case</span> N_primary_procedure_list<span class="op">:</span></span>
<span id="cb39-148"><a href="#cb39-148" aria-hidden="true" tabindex="-1"></a>      execute <span class="op">(</span>child1<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-149"><a href="#cb39-149" aria-hidden="true" tabindex="-1"></a>      execute <span class="op">(</span>child2<span class="op">(</span>node<span class="op">));</span></span>
<span id="cb39-150"><a href="#cb39-150" aria-hidden="true" tabindex="-1"></a>      <span class="cf">break</span><span class="op">;</span></span>
<span id="cb39-151"><a href="#cb39-151" aria-hidden="true" tabindex="-1"></a>    <span class="cf">default</span><span class="op">:</span></span>
<span id="cb39-152"><a href="#cb39-152" aria-hidden="true" tabindex="-1"></a>      runtime_error <span class="op">(</span><span class="st">&quot;Unknown statement.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb39-153"><a href="#cb39-153" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb39-154"><a href="#cb39-154" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>A node <code>N_procedure_call</code> is created by the parser when it
has found a user defined procedure call. The procedure has been defined
in the prior statement. Suppose the parser reads the following example
code.</p>
<pre><code>dp drawline (angle, distance) {
  tr angle
  fd distance
}
drawline (90, 100)
drawline (90, 100)
drawline (90, 100)
drawline (90, 100)</code></pre>
<p>This example draws a square.</p>
<p>When The parser reads the lines from one to four, it creates nodes
like this:</p>
<figure>
<img src="image/tree2.png" alt="Nodes of drawline" />
<figcaption aria-hidden="true">Nodes of drawline</figcaption>
</figure>
<p>Runtime routine just stores the procedure to the symbol table with
its name and node.</p>
<figure>
<img src="image/table.png" alt="Symbol table" />
<figcaption aria-hidden="true">Symbol table</figcaption>
</figure>
<p>When the parser reads the fifth line in the example, it creates nodes
like this:</p>
<figure>
<img src="image/proc_call.png" alt="Nodes of procedure call" />
<figcaption aria-hidden="true">Nodes of procedure call</figcaption>
</figure>
<p>When the runtime routine meets <code>N_procedure_call</code> node, it
behaves like this:</p>
<ol type="1">
<li>Searches the symbol table for the procedure with the name.</li>
<li>Gets pointers to the node to parameters and the node to the
body.</li>
<li>Creates a temporary stack. Makes a tuple of each parameter name and
argument value. Pushes the tuples into the stack, and (NULL, number of
parameters) finally. If no error occurs, copies them from the temporary
stack to the parameter stack.</li>
<li>Increases <code>prc_level</code> by one. Sets <code>ret_level</code>
to the same value as <code>proc_level</code>. <code>proc_level</code> is
zero when runtime routine runs on the main routine. If it goes into a
procedure, <code>proc_level</code> increases by one. Therefore,
<code>proc_level</code> is the depth of the procedure call.
<code>ret_level</code> is the level to return. If it is the same as
<code>proc_level</code>, runtime routine executes commands in order of
the commands in the procedure. If it is smaller than
<code>proc_level</code>, runtime routine doesn’t execute commands until
it becomes the same level as <code>proc_level</code>.
<code>ret_level</code> is used to return the procedure.</li>
<li>Executes the node of the body of the procedure.</li>
<li>Decreases <code>proc_level</code> by one. Sets
<code>ret_level</code> to the same value as <code>proc_level</code>.
Calls <code>stack_return</code>.</li>
</ol>
<p>When the runtime routine meets <code>N_RT</code> node, it decreases
<code>ret_level</code> by one so that the following commands in the
procedure are ignored by the runtime routine.</p>
<h4 id="runtime-entry-and-error-functions">Runtime entry and error
functions</h4>
<p>A function <code>run</code> is the entry of the runtime routine. A
function <code>runtime_error</code> reports an error occurred during the
runtime routine runs. (Errors which occur during the parsing are called
syntax error and reported by <code>yyerror</code>.) After
<code>runtime_error</code> reports an error, it stops the command
execution and goes back to <code>run</code> to exit.</p>
<p>Setjmp and longjmp functions are used. They are declared in
<code>&lt;setjmp.h&gt;</code>. <code>setjmp (buf)</code> saves state
information in <code>buf</code> and returns zero.
<code>longjmp(buf, 1)</code> restores the state information from
<code>buf</code> and returns <code>1</code> (the second argument).
Because the information is the status at the time <code>setjmp</code> is
called, so longjmp resumes the execution at the next of setjmp function
call. In the following program, longjmp resumes at the assignment to the
variable <code>i</code>. When setjmp is called, 0 is assigned to
<code>i</code> and <code>execute(node_top)</code> is called. On the
other hand, when longjmp is called, 1 is assigned to <code>i</code> and
<code>execute(node_top)</code> is not called..</p>
<p><code>g_slist_free_full</code> frees all the allocated memories.</p>
<div class="sourceCode" id="cb41"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb41-1"><a href="#cb41-1" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> jmp_buf buf<span class="op">;</span></span>
<span id="cb41-2"><a href="#cb41-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb41-3"><a href="#cb41-3" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb41-4"><a href="#cb41-4" aria-hidden="true" tabindex="-1"></a>run <span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb41-5"><a href="#cb41-5" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb41-6"><a href="#cb41-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb41-7"><a href="#cb41-7" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(!</span> init_cairo<span class="op">())</span> <span class="op">{</span></span>
<span id="cb41-8"><a href="#cb41-8" aria-hidden="true" tabindex="-1"></a>    g_print <span class="op">(</span><span class="st">&quot;Cairo not initialized.</span><span class="sc">\n</span><span class="st">&quot;</span><span class="op">);</span></span>
<span id="cb41-9"><a href="#cb41-9" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span><span class="op">;</span></span>
<span id="cb41-10"><a href="#cb41-10" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb41-11"><a href="#cb41-11" aria-hidden="true" tabindex="-1"></a>  init_table<span class="op">();</span></span>
<span id="cb41-12"><a href="#cb41-12" aria-hidden="true" tabindex="-1"></a>  init_stack<span class="op">();</span></span>
<span id="cb41-13"><a href="#cb41-13" aria-hidden="true" tabindex="-1"></a>  ret_level <span class="op">=</span> proc_level <span class="op">=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb41-14"><a href="#cb41-14" aria-hidden="true" tabindex="-1"></a>  i <span class="op">=</span> setjmp <span class="op">(</span>buf<span class="op">);</span></span>
<span id="cb41-15"><a href="#cb41-15" aria-hidden="true" tabindex="-1"></a>  <span class="cf">if</span> <span class="op">(</span>i <span class="op">==</span> <span class="dv">0</span><span class="op">)</span></span>
<span id="cb41-16"><a href="#cb41-16" aria-hidden="true" tabindex="-1"></a>    execute<span class="op">(</span>node_top<span class="op">);</span></span>
<span id="cb41-17"><a href="#cb41-17" aria-hidden="true" tabindex="-1"></a>  <span class="co">/* else ... get here by calling longjmp */</span></span>
<span id="cb41-18"><a href="#cb41-18" aria-hidden="true" tabindex="-1"></a>  destroy_cairo <span class="op">();</span></span>
<span id="cb41-19"><a href="#cb41-19" aria-hidden="true" tabindex="-1"></a>  g_slist_free_full <span class="op">(</span>g_steal_pointer <span class="op">(&amp;</span>list<span class="op">),</span> g_free<span class="op">);</span></span>
<span id="cb41-20"><a href="#cb41-20" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb41-21"><a href="#cb41-21" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb41-22"><a href="#cb41-22" aria-hidden="true" tabindex="-1"></a><span class="co">/* format supports only %s, %f and %d */</span></span>
<span id="cb41-23"><a href="#cb41-23" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> <span class="dt">void</span></span>
<span id="cb41-24"><a href="#cb41-24" aria-hidden="true" tabindex="-1"></a>runtime_error <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>format<span class="op">,</span> <span class="op">...)</span> <span class="op">{</span></span>
<span id="cb41-25"><a href="#cb41-25" aria-hidden="true" tabindex="-1"></a>  <span class="dt">va_list</span> args<span class="op">;</span></span>
<span id="cb41-26"><a href="#cb41-26" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> <span class="op">*</span>f<span class="op">;</span></span>
<span id="cb41-27"><a href="#cb41-27" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> b<span class="op">[</span><span class="dv">3</span><span class="op">];</span></span>
<span id="cb41-28"><a href="#cb41-28" aria-hidden="true" tabindex="-1"></a>  <span class="dt">char</span> <span class="op">*</span>s<span class="op">;</span></span>
<span id="cb41-29"><a href="#cb41-29" aria-hidden="true" tabindex="-1"></a>  <span class="dt">double</span> v<span class="op">;</span></span>
<span id="cb41-30"><a href="#cb41-30" aria-hidden="true" tabindex="-1"></a>  <span class="dt">int</span> i<span class="op">;</span></span>
<span id="cb41-31"><a href="#cb41-31" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb41-32"><a href="#cb41-32" aria-hidden="true" tabindex="-1"></a>  va_start <span class="op">(</span>args<span class="op">,</span> format<span class="op">);</span></span>
<span id="cb41-33"><a href="#cb41-33" aria-hidden="true" tabindex="-1"></a>  <span class="cf">for</span> <span class="op">(</span>f <span class="op">=</span> format<span class="op">;</span> <span class="op">*</span>f<span class="op">;</span> f<span class="op">++)</span> <span class="op">{</span></span>
<span id="cb41-34"><a href="#cb41-34" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="op">(*</span>f <span class="op">!=</span> <span class="ch">&#39;%&#39;</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb41-35"><a href="#cb41-35" aria-hidden="true" tabindex="-1"></a>      b<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">=</span> <span class="op">*</span>f<span class="op">;</span></span>
<span id="cb41-36"><a href="#cb41-36" aria-hidden="true" tabindex="-1"></a>      b<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="ch">&#39;\0&#39;</span><span class="op">;</span></span>
<span id="cb41-37"><a href="#cb41-37" aria-hidden="true" tabindex="-1"></a>      g_print <span class="op">(</span><span class="st">&quot;%s&quot;</span><span class="op">,</span> b<span class="op">);</span></span>
<span id="cb41-38"><a href="#cb41-38" aria-hidden="true" tabindex="-1"></a>      <span class="cf">continue</span><span class="op">;</span></span>
<span id="cb41-39"><a href="#cb41-39" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb41-40"><a href="#cb41-40" aria-hidden="true" tabindex="-1"></a>    <span class="cf">switch</span> <span class="op">(*++</span>f<span class="op">)</span> <span class="op">{</span></span>
<span id="cb41-41"><a href="#cb41-41" aria-hidden="true" tabindex="-1"></a>      <span class="cf">case</span> <span class="ch">&#39;s&#39;</span><span class="op">:</span></span>
<span id="cb41-42"><a href="#cb41-42" aria-hidden="true" tabindex="-1"></a>        s <span class="op">=</span> va_arg<span class="op">(</span>args<span class="op">,</span> <span class="dt">char</span> <span class="op">*);</span></span>
<span id="cb41-43"><a href="#cb41-43" aria-hidden="true" tabindex="-1"></a>        g_print <span class="op">(</span><span class="st">&quot;%s&quot;</span><span class="op">,</span> s<span class="op">);</span></span>
<span id="cb41-44"><a href="#cb41-44" aria-hidden="true" tabindex="-1"></a>        <span class="cf">break</span><span class="op">;</span></span>
<span id="cb41-45"><a href="#cb41-45" aria-hidden="true" tabindex="-1"></a>      <span class="cf">case</span> <span class="ch">&#39;f&#39;</span><span class="op">:</span></span>
<span id="cb41-46"><a href="#cb41-46" aria-hidden="true" tabindex="-1"></a>        v <span class="op">=</span> va_arg<span class="op">(</span>args<span class="op">,</span> <span class="dt">double</span><span class="op">);</span></span>
<span id="cb41-47"><a href="#cb41-47" aria-hidden="true" tabindex="-1"></a>        g_print<span class="op">(</span><span class="st">&quot;%f&quot;</span><span class="op">,</span> v<span class="op">);</span></span>
<span id="cb41-48"><a href="#cb41-48" aria-hidden="true" tabindex="-1"></a>        <span class="cf">break</span><span class="op">;</span></span>
<span id="cb41-49"><a href="#cb41-49" aria-hidden="true" tabindex="-1"></a>      <span class="cf">case</span> <span class="ch">&#39;d&#39;</span><span class="op">:</span></span>
<span id="cb41-50"><a href="#cb41-50" aria-hidden="true" tabindex="-1"></a>        i <span class="op">=</span> va_arg<span class="op">(</span>args<span class="op">,</span> <span class="dt">int</span><span class="op">);</span></span>
<span id="cb41-51"><a href="#cb41-51" aria-hidden="true" tabindex="-1"></a>        g_print<span class="op">(</span><span class="st">&quot;%d&quot;</span><span class="op">,</span> i<span class="op">);</span></span>
<span id="cb41-52"><a href="#cb41-52" aria-hidden="true" tabindex="-1"></a>        <span class="cf">break</span><span class="op">;</span></span>
<span id="cb41-53"><a href="#cb41-53" aria-hidden="true" tabindex="-1"></a>      <span class="cf">default</span><span class="op">:</span></span>
<span id="cb41-54"><a href="#cb41-54" aria-hidden="true" tabindex="-1"></a>        b<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">=</span> <span class="ch">&#39;%&#39;</span><span class="op">;</span></span>
<span id="cb41-55"><a href="#cb41-55" aria-hidden="true" tabindex="-1"></a>        b<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="op">*</span>f<span class="op">;</span></span>
<span id="cb41-56"><a href="#cb41-56" aria-hidden="true" tabindex="-1"></a>        b<span class="op">[</span><span class="dv">2</span><span class="op">]</span> <span class="op">=</span> <span class="ch">&#39;\0&#39;</span><span class="op">;</span></span>
<span id="cb41-57"><a href="#cb41-57" aria-hidden="true" tabindex="-1"></a>        g_print <span class="op">(</span><span class="st">&quot;%s&quot;</span><span class="op">,</span> b<span class="op">);</span></span>
<span id="cb41-58"><a href="#cb41-58" aria-hidden="true" tabindex="-1"></a>        <span class="cf">break</span><span class="op">;</span></span>
<span id="cb41-59"><a href="#cb41-59" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb41-60"><a href="#cb41-60" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb41-61"><a href="#cb41-61" aria-hidden="true" tabindex="-1"></a>  va_end <span class="op">(</span>args<span class="op">);</span></span>
<span id="cb41-62"><a href="#cb41-62" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb41-63"><a href="#cb41-63" aria-hidden="true" tabindex="-1"></a>  longjmp <span class="op">(</span>buf<span class="op">,</span> <span class="dv">1</span><span class="op">);</span></span>
<span id="cb41-64"><a href="#cb41-64" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>A function <code>runtime_error</code> has a variable-length argument
list.</p>
<div class="sourceCode" id="cb42"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb42-1"><a href="#cb42-1" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> runtime_error <span class="op">(</span><span class="dt">char</span> <span class="op">*</span>format<span class="op">,</span> <span class="op">...)</span></span></code></pre></div>
<p>This is implemented with <code>&lt;stdarg.h&gt;</code> header file.
The <code>va_list</code> type variable <code>args</code> will refer to
each argument in turn. A function <code>va_start</code> initializes
<code>args</code>. A function <code>va_arg</code> returns an argument
and moves the reference of <code>args</code> to the next. A function
<code>va_end</code> cleans up everything necessary at the end.</p>
<p>The function <code>runtime_error</code> has a similar format of
printf standard function. But its format has only <code>%s</code>,
<code>%f</code> and <code>%d</code>.</p>
<p>The functions declared in <code>&lt;setjmp.h&gt;</code> and
<code>&lt;stdarg.h&gt;</code> are explained in the very famous book “The
C programming language” written by Brian Kernighan and Dennis Ritchie. I
referred to the book to write the program above.</p>
<p>The program <code>turtle</code> is unsophisticated and unpolished. If
you want to make your own language, you need to know more and more. I
don’t know any good textbook about compilers and interpreters. If you
know a good book, please let me know.</p>
<p>However, the following information is very useful (but old).</p>
<ul>
<li>Bison documentation</li>
<li>Flex documentation</li>
<li>Software tools written by Brian W. Kernighan &amp; P. J. Plauger
(1976)</li>
<li>Unix programming environment written by Brian W. Kernighan and Rob
Pike (1984)</li>
<li>Source code of a language, for example, ruby.</li>
</ul>
<p>Lately, lots of source codes are in the internet. Maybe reading
source codes is the most useful for programmers.</p>
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