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<title>Cattle Reference Manual: I/O Handling</title>
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<div class="refentry">
<a name="io-handling"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<h2><span class="refentrytitle">I/O Handling</span></h2>
<p>I/O Handling —
Explanation of the way Cattle performs I/O
</p>
</td>
<td class="gallery_image" valign="top" align="right"></td>
</tr></table></div>
<div class="refsect2">
<a name="id-1.5.2.3"></a><h3>I/O Basics</h3>
<p>
Cattle uses an I/O mechanism which allows for a huge degree
of flexibility while remaining relatively simple.
</p>
<p>
The I/O mechanism is based on callbacks: every time a
<a class="link" href="CattleInterpreter.html" title="CattleInterpreter">CattleInterpreter</a>
executes a Brainfuck instruction which requires some sort of
I/O, it invokes a user provided handler, and expects it to
perform the I/O operation.
</p>
<p>
Default I/O handlers are built into Cattle, so usually there
is no need to define a custom handler.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.5.2.4"></a><h3>Error reporting</h3>
<p>
Since I/O is not guaranteed to always be succesful, handlers
are provided a mean to report errors to the caller.
</p>
<p>
All handlers are passed a <a href="/usr/share/gtk-doc/html/glib/glib-Error-Reporting.html#GError">GError</a>
as last argument: if a handler fails, it is required to return
<a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS">FALSE</a> and to fill the
error with detailed information.
</p>
<p>
If the hanler returns <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#TRUE:CAPS">TRUE</a>, but
the error is set, the operation is not considered succesful;
this is because handlers written in languages other than C might
not be able to both return a value and fill the error at the
same time.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.5.2.5"></a><h3>Output handler</h3>
<p>
Implementing an output handler is pretty straightforward: the
handler is passed a single gint8
and has to display it to the user in some way.
</p>
<p>
As an example, here is an handler which shows the output of
a Brainfuck program on a
GtkTextView:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
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<td class="listing_code"><pre class="programlisting">gboolean
<span class="function">output_handler</span> <span class="gtkdoc opt">(</span>CattleInterpreter <span class="gtkdoc opt">*</span>interpreter<span class="gtkdoc opt">,</span>
gint8 output<span class="gtkdoc opt">,</span>
gpointer data<span class="gtkdoc opt">,</span>
GError <span class="gtkdoc opt">**</span>error<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
GtkTextView <span class="gtkdoc opt">*</span>view<span class="gtkdoc opt">;</span>
GtkTextBuffer <span class="gtkdoc opt">*</span>buffer<span class="gtkdoc opt">;</span>
GtkTextIter iter<span class="gtkdoc opt">;</span>
gchar text<span class="gtkdoc opt">[</span><span class="number">2</span><span class="gtkdoc opt">];</span>
view <span class="gtkdoc opt">=</span> <span class="function">GTK_TEXT_VIEW</span> <span class="gtkdoc opt">(</span>data<span class="gtkdoc opt">);</span>
<span class="comment">/* Get the buffer used by the GtkTextView */</span>
buffer <span class="gtkdoc opt">=</span> <span class="function">gtk_text_view_get_buffer</span> <span class="gtkdoc opt">(</span>view<span class="gtkdoc opt">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-ref">g_object_ref</a></span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">);</span>
<span class="comment">/* Get a reference to the end of the buffer */</span>
<span class="function">gtk_text_buffer_get_end_iter</span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">, &</span>iter<span class="gtkdoc opt">);</span>
<span class="comment">/* Create a string */</span>
text<span class="gtkdoc opt">[</span><span class="number">0</span><span class="gtkdoc opt">] = (</span>gchat<span class="gtkdoc opt">)</span> output<span class="gtkdoc opt">;</span>
text<span class="gtkdoc opt">[</span><span class="number">1</span><span class="gtkdoc opt">] =</span> <span class="string">'\0'</span><span class="gtkdoc opt">;</span>
<span class="comment">/* Insert the char at the end of the buffer */</span>
<span class="function">gtk_text_buffer_insert</span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">, &</span>iter<span class="gtkdoc opt">,</span> text<span class="gtkdoc opt">,</span> <span class="number">1</span><span class="gtkdoc opt">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> TRUE<span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
The code assumes a GtkTextView
has been provided when the signal handler has been assigned to
the interpreter, like in the following code:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
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9</pre></td>
<td class="listing_code"><pre class="programlisting">CattleInterpreter <span class="gtkdoc opt">*</span>interpreter<span class="gtkdoc opt">;</span>
GtkWidget <span class="gtkdoc opt">*</span>view<span class="gtkdoc opt">;</span>
interpreter <span class="gtkdoc opt">=</span> <span class="function"><a href="CattleInterpreter.html#cattle-interpreter-new">cattle_interpreter_new</a></span> <span class="gtkdoc opt">();</span>
view <span class="gtkdoc opt">=</span> <span class="function">gtk_text_view_new</span> <span class="gtkdoc opt">();</span>
<span class="function"><a href="CattleInterpreter.html#cattle-interpreter-set-output-handler">cattle_interpreter_set_output_handler</a></span> <span class="gtkdoc opt">(</span>interpreter<span class="gtkdoc opt">,</span>
output_handler<span class="gtkdoc opt">,</span>
<span class="gtkdoc opt">(</span>gpointer<span class="gtkdoc opt">)</span> view<span class="gtkdoc opt">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Depending on the case, it may make sense for the application
to buffer an entire line of output, or even the whole output,
before sending it to its intended destination.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.5.2.6"></a><h3>Input handler</h3>
<p>
Here is an input handler which uses readline to fetch input
from the user:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
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<td class="listing_code"><pre class="programlisting">gboolean
<span class="function">input_handler</span> <span class="gtkdoc opt">(</span>CattleInterpreter <span class="gtkdoc opt">*</span>interpreter<span class="gtkdoc opt">,</span>
gpointer data<span class="gtkdoc opt">,</span>
GError <span class="gtkdoc opt">**</span>error<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
CattleBuffer <span class="gtkdoc opt">*</span>buffer<span class="gtkdoc opt">;</span>
gchar <span class="gtkdoc opt">*</span>input<span class="gtkdoc opt">;</span>
gulong size<span class="gtkdoc opt">;</span>
<span class="comment">/* Read an input line using readline (no prompt) */</span>
input <span class="gtkdoc opt">=</span> <span class="function">readline</span> <span class="gtkdoc opt">(</span>NULL<span class="gtkdoc opt">);</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span>input <span class="gtkdoc opt">!=</span> NULL<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
<span class="comment">/* A line of input has been read successfully, but it</span>
<span class="comment"> * needs to be copied into a CattleBuffer before</span>
<span class="comment"> * feeding it to the interpreter. */</span>
<span class="comment">/* Calculate the size of the CattleBuffer. readline</span>
<span class="comment"> * returns a null-terminated string with the trailing</span>
<span class="comment"> * newline stripped; a CattleBuffer doesn't need to be</span>
<span class="comment"> * null-terminated, but stripping the newline would mean</span>
<span class="comment"> * losing part of the input. We can just create a</span>
<span class="comment"> * CattleBuffer as big as the string, and replace the</span>
<span class="comment"> * trailing null byte with a newline. */</span>
size <span class="gtkdoc opt">=</span> <span class="function">strlen</span> <span class="gtkdoc opt">(</span>input<span class="gtkdoc opt">) +</span> <span class="number">1</span><span class="gtkdoc opt">;</span>
buffer <span class="gtkdoc opt">=</span> <span class="function"><a href="CattleBuffer.html#cattle-buffer-new">cattle_buffer_new</a></span> <span class="gtkdoc opt">(</span>size<span class="gtkdoc opt">);</span>
<span class="function"><a href="CattleBuffer.html#cattle-buffer-set-contents">cattle_buffer_set_contents</a></span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">,</span> input<span class="gtkdoc opt">);</span>
<span class="function"><a href="CattleBuffer.html#cattle-buffer-set-value">cattle_buffer_set_value</a></span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">,</span> size <span class="gtkdoc opt">-</span> <span class="number">1</span><span class="gtkdoc opt">,</span> <span class="string">'</span><span class="gtkdoc esc">\n</span><span class="string">'</span><span class="gtkdoc opt">);</span>
<span class="comment">/* The CattleBuffer now contains a copy of the input, so</span>
<span class="comment"> * the string returned by readline can be safely freed */</span>
<span class="function">free</span> <span class="gtkdoc opt">(</span>input<span class="gtkdoc opt">);</span>
<span class="function"><a href="CattleInterpreter.html#cattle-interpreter-feed">cattle_interpreter_feed</a></span> <span class="gtkdoc opt">(</span>interpreter<span class="gtkdoc opt">,</span> buffer<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="keyword">else</span>
<span class="gtkdoc opt">{</span>
<span class="comment">/* readline returns NULL when the end of input has</span>
<span class="comment"> * been reached; to let the interpreter know no more</span>
<span class="comment"> * input is available, feed it an empty buffer */</span>
buffer <span class="gtkdoc opt">=</span> <span class="function"><a href="CattleBuffer.html#cattle-buffer-new">cattle_buffer_new</a></span> <span class="gtkdoc opt">(</span><span class="number">0</span><span class="gtkdoc opt">);</span>
<span class="function"><a href="CattleInterpreter.html#cattle-interpreter-feed">cattle_interpreter_feed</a></span> <span class="gtkdoc opt">(</span>interpreter<span class="gtkdoc opt">,</span> buffer<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span> <span class="gtkdoc opt">(</span>buffer<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> TRUE<span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Input works on a per-line basis: the handler must retrieve
a full line of input, including the trailing newline character,
and feed it to the interpreter.
</p>
<p>
If it is not possible to retrieve the whole line in a single
step, a part of it can be passed to the interpreter.
</p>
<p>
When the whole input has been consumed, the handler must feed
the interpreter an empty
<a class="link" href="CattleBuffer.html" title="CattleBuffer">CattleBuffer</a> to let it
know no more input is available.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.5.2.7"></a><h3>Debug</h3>
<p>
The debug handler is called whenever a <code class="code">#</code>
instruction is executed; the interpreter can be configured
to ignore this instruction, since it's not (strictly
speaking) part of the Brainfuck language.
</p>
<p>
The handler must display useful debugging information to
the developer; usually, this means dumping the contents of
the memory tape.
</p>
<p>
The following handler appends the contents of the tape to
a log file:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
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<td class="listing_code"><pre class="programlisting">gboolean
<span class="function">debug_handler</span> <span class="gtkdoc opt">(</span>CattleInterpreter <span class="gtkdoc opt">*</span>interpreter<span class="gtkdoc opt">,</span>
gpointer data<span class="gtkdoc opt">,</span>
GError <span class="gtkdoc opt">**</span>error<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
CattleTape <span class="gtkdoc opt">*</span>tape<span class="gtkdoc opt">;</span>
gint8 value<span class="gtkdoc opt">;</span>
<span class="gtkdoc kwb">FILE</span><span class="gtkdoc opt">*</span> fp<span class="gtkdoc opt">;</span>
tape <span class="gtkdoc opt">=</span> <span class="function"><a href="CattleInterpreter.html#cattle-interpreter-get-tape">cattle_interpreter_get_tape</a></span> <span class="gtkdoc opt">(</span>interpreter<span class="gtkdoc opt">);</span>
<span class="comment">/* Save the current tape position */</span>
<span class="function"><a href="CattleTape.html#cattle-tape-push-bookmark">cattle_tape_push_bookmark</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
fp <span class="gtkdoc opt">=</span> <span class="function">fopen</span> <span class="gtkdoc opt">(</span>LOG_FILE<span class="gtkdoc opt">,</span> <span class="string">"a"</span><span class="gtkdoc opt">);</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span>fp <span class="gtkdoc opt">==</span> NULL<span class="gtkdoc opt">) {</span>
<span class="comment">/* Set the error, release resources and return */</span>
<span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-Error-Reporting.html#g-set-error-literal">g_set_error_literal</a></span> <span class="gtkdoc opt">(</span>error<span class="gtkdoc opt">,</span>
CATTLE_INTERPRETER_ERROR<span class="gtkdoc opt">,</span>
CATTLE_INTERPRETER_ERROR_IO<span class="gtkdoc opt">,</span>
<span class="function">strerror</span> <span class="gtkdoc opt">(</span>errno<span class="gtkdoc opt">));</span>
<span class="function"><a href="CattleTape.html#cattle-tape-pop-bookmark">cattle_tape_pop_bookmark</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> FALSE<span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span>
<span class="comment">/* Rewind to the beginning of the tape */</span>
<span class="keyword">while</span> <span class="gtkdoc opt">(!</span><span class="function"><a href="CattleTape.html#cattle-tape-is-at-beginning">cattle_tape_is_at_beginning</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">)) {</span>
<span class="function"><a href="CattleTape.html#cattle-tape-move-left">cattle_tape_move_left</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function">fprintf</span> <span class="gtkdoc opt">(</span>fp<span class="gtkdoc opt">,</span> <span class="string">"[ "</span><span class="gtkdoc opt">);</span>
<span class="comment">/* Repeat until the end of the tape is reached */</span>
<span class="keyword">while</span> <span class="gtkdoc opt">(!</span><span class="function"><a href="CattleTape.html#cattle-tape-is-at-end">cattle_tape_is_at_end</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">)) {</span>
<span class="comment">/* Get the current value */</span>
value <span class="gtkdoc opt">=</span> <span class="function"><a href="CattleTape.html#cattle-tape-get-current-value">cattle_tape_get_current_value</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="comment">/* Show printable values directly and non-printable</span>
<span class="comment"> * values as their decimal ASCII value */</span>
<span class="keyword">if</span> <span class="gtkdoc opt">(</span>value <span class="gtkdoc opt">>=</span> <span class="number">33</span> <span class="gtkdoc opt">&&</span> value <span class="gtkdoc opt"><=</span> <span class="number">126</span><span class="gtkdoc opt">) {</span>
<span class="function">fprintf</span> <span class="gtkdoc opt">(</span>fp<span class="gtkdoc opt">,</span> <span class="string">"%c "</span><span class="gtkdoc opt">,</span> value<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="keyword">else</span> <span class="gtkdoc opt">{</span>
<span class="function">fprintf</span> <span class="gtkdoc opt">(</span>fp<span class="gtkdoc opt">,</span> <span class="string">"</span><span class="gtkdoc esc">\\</span><span class="string">%d "</span><span class="gtkdoc opt">, (</span>gint<span class="gtkdoc opt">)</span> value<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function"><a href="CattleTape.html#cattle-tape-move-right">cattle_tape_move_right</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span>
<span class="function">fprintf</span> <span class="gtkdoc opt">(</span>fp<span class="gtkdoc opt">,</span> <span class="string">"]</span><span class="gtkdoc esc">\n</span><span class="string">"</span><span class="gtkdoc opt">);</span>
<span class="function">fclose</span> <span class="gtkdoc opt">(</span>fp<span class="gtkdoc opt">);</span>
<span class="comment">/* Restore the original tape position */</span>
<span class="function"><a href="CattleTape.html#cattle-tape-pop-bookmark">cattle_tape_pop_bookmark</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span> <span class="gtkdoc opt">(</span>tape<span class="gtkdoc opt">);</span>
<span class="keyword">return</span> TRUE<span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
After the handler has run, the tape must be in the same exact
state it was before the signal emission, including the
position. The best way to ensure it is to use
<a class="link" href="CattleTape.html#cattle-tape-push-bookmark" title="cattle_tape_push_bookmark ()">cattle_tape_push_bookmark()</a>
at the beginning of the handler and
<a class="link" href="CattleTape.html#cattle-tape-pop-bookmark" title="cattle_tape_pop_bookmark ()">cattle_tape_pop_bookmark()</a>
at the end.
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