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@@ -129,9 +129,9 @@
|
||||
|
||||
<h2 id="author"><a name="author"></a>Author:</h2>
|
||||
<ul>
|
||||
<li>Name: <a href="../../authors.html#Nicholas_Carlini">Nicholas
|
||||
Carlini</a><br> Location: <a href="../../location.html#US">US</a> -
|
||||
<em>United States of America</em> (<em>United States</em>) <br><br></li>
|
||||
<li>Name: <a href="../../authors.html#Nicholas_Carlini">Nicholas Carlini</a><br>
|
||||
Location: <a href="../../location.html#US">US</a> - <em>United States of America</em> (<em>United States</em>)
|
||||
<br><br></li>
|
||||
</ul>
|
||||
|
||||
<!-- END: next line ends content generated by: bin/output-index-author.sh -->
|
||||
@@ -144,8 +144,7 @@ Carlini</a><br> Location: <a href="../../location.html#US">US</a> -
|
||||
<h3 id="bugs-and-misfeatures">Bugs and (Mis)features:</h3>
|
||||
<p>The current status of this entry is:</p>
|
||||
<pre><code>STATUS: INABIAF - please **DO NOT** fix</code></pre>
|
||||
<p>For more detailed information see <a
|
||||
href="../../bugs.html#2020_carlini">2020 carlini bugs</a>.</p>
|
||||
<p>For more detailed information see <a href="../../bugs.html#2020_carlini">2020 carlini bugs</a>.</p>
|
||||
<h2 id="to-use">To use:</h2>
|
||||
<div class="sourceCode" id="cb3"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span></span></code></pre></div>
|
||||
<h2 id="try">Try:</h2>
|
||||
@@ -154,11 +153,10 @@ href="../../bugs.html#2020_carlini">2020 carlini bugs</a>.</p>
|
||||
<div class="sourceCode" id="cb5"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="co"># Play with a friend, real or imagined, P1 and P2, squares are numbered 1..9</span></span>
|
||||
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span></span></code></pre></div>
|
||||
<h2 id="judges-remarks">Judges’ remarks:</h2>
|
||||
<p>This program consists of a single <code>printf(3)</code> statement
|
||||
wrapped in a <code>while</code> loop. You would not think that this
|
||||
would amount to much, but you would be very, very wrong. A clue to what
|
||||
is happening and how this works is encoded in the ASCII art of the
|
||||
program source.</p>
|
||||
<p>This program consists of a single <code>printf(3)</code> statement wrapped in a <code>while</code>
|
||||
loop. You would not think that this would amount to much, but you would be very,
|
||||
very wrong. A clue to what is happening and how this works is encoded in the
|
||||
ASCII art of the program source.</p>
|
||||
<h2 id="authors-remarks">Author’s remarks:</h2>
|
||||
<p>HOW ABOUT A NICE GAME OF <strike>CHESS</strike>TIC-TAC-TOE?</p>
|
||||
<h3 id="usage">USAGE</h3>
|
||||
@@ -177,145 +175,116 @@ program source.</p>
|
||||
<li>A player makes an illegal move; their opponent wins.</li>
|
||||
</ul>
|
||||
<h3 id="limitations">LIMITATIONS</h3>
|
||||
<p>Bad things may happen if the entered move is outside of the range
|
||||
[1..9]</p>
|
||||
<p>While the game will stop if one player tries to overwrite another
|
||||
player’s move, it will happily allow a player to move on top of
|
||||
themself. However, given that this is an effective no-op and guarantees
|
||||
a loss to a competent opponent, if you do this you probably deserve to
|
||||
lose.</p>
|
||||
<p>Your terminal may beep at you every turn. We’ll call this a feature
|
||||
that … the program … is hard at work processing your move! That is!
|
||||
Please be patient.</p>
|
||||
<p>After macro expansion the format string is roughly 100K. This makes
|
||||
some compilers unhappy, and others slow. (For example, passing GCC the
|
||||
flags <code>-Wall -Wextra -pedantic</code> will result in a warning
|
||||
about oversized strings.)</p>
|
||||
<p>Under some terminals the program may exit early. The following should
|
||||
always work:</p>
|
||||
<p>Bad things may happen if the entered move is outside of the range [1..9]</p>
|
||||
<p>While the game will stop if one player tries to overwrite another player’s move,
|
||||
it will happily allow a player to move on top of themself. However, given that
|
||||
this is an effective no-op and guarantees a loss to a competent opponent, if you
|
||||
do this you probably deserve to lose.</p>
|
||||
<p>Your terminal may beep at you every turn. We’ll call this a feature that …
|
||||
the program … is hard at work processing your move! That is! Please be
|
||||
patient.</p>
|
||||
<p>After macro expansion the format string is roughly 100K. This makes some
|
||||
compilers unhappy, and others slow. (For example, passing GCC the flags <code>-Wall -Wextra -pedantic</code> will result in a warning about oversized strings.)</p>
|
||||
<p>Under some terminals the program may exit early. The following should always work:</p>
|
||||
<div class="sourceCode" id="cb8"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="bu">echo</span> <span class="st">"1 2 3 4 5 6 7"</span> <span class="kw">|</span> <span class="ex">./prog</span></span></code></pre></div>
|
||||
<h3 id="obfuscation">OBFUSCATION</h3>
|
||||
<p>The entirety of the program consists of a single iterated call to
|
||||
<code>printf(3)</code>.</p>
|
||||
<p>The entirety of the program consists of a single iterated call to <code>printf(3)</code>.</p>
|
||||
<div class="sourceCode" id="cb9"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
|
||||
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a> <span class="cf">while</span><span class="op">(*</span>d<span class="op">)</span> printf<span class="op">(</span>fmt<span class="op">,</span> arg<span class="op">);</span></span>
|
||||
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
|
||||
<p>Here, <code>fmt</code> is a single string, and <code>arg</code> is a
|
||||
series of arguments to <code>printf(3)</code>.</p>
|
||||
<p>While its primary purpose is to serve as The One True Debugger,
|
||||
<code>printf(3)</code> also happens to be Turing complete. (See
|
||||
“Control-Flow Bending: On the Effectiveness of Control-Flow Integrity”
|
||||
where we introduced this in an actual, published, academic paper. The
|
||||
<p>Here, <code>fmt</code> is a single string, and <code>arg</code> is a series of arguments to
|
||||
<code>printf(3)</code>.</p>
|
||||
<p>While its primary purpose is to serve as The One True Debugger, <code>printf(3)</code> also happens
|
||||
to be Turing complete. (See “Control-Flow Bending: On the Effectiveness of Control-Flow
|
||||
Integrity” where we introduced this in an actual, published, academic paper. The
|
||||
things you can get away with sometimes.)</p>
|
||||
<p>We <strike>ab</strike>use this fact to implement the logic of
|
||||
tic-tac-toe entirely within this one <code>printf(3)</code> call (and a
|
||||
call to <code>scanf(3)</code> to read user input).</p>
|
||||
<p>We <strike>ab</strike>use this fact to implement the logic of tic-tac-toe entirely within
|
||||
this one <code>printf(3)</code> call (and a call to <code>scanf(3)</code> to read user input).</p>
|
||||
<p>Here’s (briefly) how it works.</p>
|
||||
<h4 id="preliminaries">Preliminaries</h4>
|
||||
<p>This program uses three <code>printf(3)</code> format specifiers.</p>
|
||||
<ul>
|
||||
<li><p><code>%d</code> takes an integer argument and prints it.</p></li>
|
||||
<li><p><code>%s</code> takes a string argument and prints it.</p></li>
|
||||
<li><p><code>%n</code> takes a pointer and writes (!!) the number of
|
||||
bytes printed so far.</p></li>
|
||||
<li><p><code>%n</code> takes a pointer and writes (!!) the number of bytes printed so far.</p></li>
|
||||
</ul>
|
||||
<p>Okay, everyone probably knows this. Let’s get a bit more
|
||||
advanced.</p>
|
||||
<p>Okay, everyone probably knows this. Let’s get a bit more advanced.</p>
|
||||
<p>Format specifiers can take extra “arguments”.</p>
|
||||
<ul>
|
||||
<li><p><code>"%hhn"</code>: store the number of bytes written mod 256 to
|
||||
the <code>char</code> pointer.</p></li>
|
||||
<li><p><code>"%2$d"</code>: print argument 2 to <code>printf(3)</code>
|
||||
(and not the sequentially next argument).</p></li>
|
||||
<li><p><code>"%8d"</code>: pad the printed integer to 8
|
||||
characters.</p></li>
|
||||
<li><p><code>"%3$.*4$d"</code>: print argument 3 to
|
||||
<code>printf(3)</code> with as many zeros as in argument 4.</p></li>
|
||||
<li><p><code>"%hhn"</code>: store the number of bytes written mod 256 to the <code>char</code> pointer.</p></li>
|
||||
<li><p><code>"%2$d"</code>: print argument 2 to <code>printf(3)</code> (and not the sequentially next argument).</p></li>
|
||||
<li><p><code>"%8d"</code>: pad the printed integer to 8 characters.</p></li>
|
||||
<li><p><code>"%3$.*4$d"</code>: print argument 3 to <code>printf(3)</code> with as many zeros as in argument 4.</p></li>
|
||||
</ul>
|
||||
<p>For example, the following expression</p>
|
||||
<div class="sourceCode" id="cb10"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>printf<span class="op">(</span><span class="st">"%1$.*2$d%3$hhn"</span><span class="op">,</span> <span class="dv">5</span><span class="op">,</span> <span class="dv">10</span><span class="op">,</span> <span class="op">&</span>x<span class="op">)</span></span></code></pre></div>
|
||||
<p>will have the same effect as if we had written</p>
|
||||
<div class="sourceCode" id="cb11"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="dv">10</span><span class="op">;</span></span></code></pre></div>
|
||||
<p>because it will print out <code>0000000005</code> (5 padded to size
|
||||
10) and then write the number of bytes written to <code>x</code>.</p>
|
||||
<h4 id="printf3-oriented-programming"><code>printf(3)</code> Oriented
|
||||
Programming</h4>
|
||||
<p>because it will print out <code>0000000005</code> (5 padded to size 10) and then write the
|
||||
number of bytes written to <code>x</code>.</p>
|
||||
<h4 id="printf3-oriented-programming"><code>printf(3)</code> Oriented Programming</h4>
|
||||
<p>Alright, now we can get to the real fun.</p>
|
||||
<p>We perform arbitrary computation with <code>printf(3)</code> treating
|
||||
memory as a binary array–one bit per pair of bytes:</p>
|
||||
<p>We perform arbitrary computation with <code>printf(3)</code> treating memory as a binary
|
||||
array–one bit per pair of bytes:</p>
|
||||
<ul>
|
||||
<li><p>The zero bit is represented by the sequence
|
||||
<code>00 00</code>.</p></li>
|
||||
<li><p>The one-bit is represented by the sequence <code>xx 00</code>
|
||||
where <code>xx</code> is any non-zero byte.</p></li>
|
||||
<li><p>The zero bit is represented by the sequence <code>00 00</code>.</p></li>
|
||||
<li><p>The one-bit is represented by the sequence <code>xx 00</code> where <code>xx</code> is any non-zero byte.</p></li>
|
||||
</ul>
|
||||
<p>We can use format strings to compute the OR/NOT of arbitrary
|
||||
“bits”.</p>
|
||||
<p>We can use format strings to compute the OR/NOT of arbitrary “bits”.</p>
|
||||
<p>We’ll start with the simplest, OR:</p>
|
||||
<div class="sourceCode" id="cb12"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a>printf<span class="op">(</span><span class="st">"%1$s%2$s%3$hhn"</span><span class="op">,</span> a<span class="op">,</span> b<span class="op">,</span> c<span class="op">)</span></span></code></pre></div>
|
||||
<p>will compute</p>
|
||||
<div class="sourceCode" id="cb13"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>c <span class="op">=</span> strlen<span class="op">(</span>a<span class="op">)</span> <span class="op">+</span> strlen<span class="op">(</span>b<span class="op">)</span></span></code></pre></div>
|
||||
<p>but given that <code>strlen(x)</code> is <code>1</code> for a 1-bit
|
||||
and <code>0</code> for a 0-bit, we have</p>
|
||||
<p>but given that <code>strlen(x)</code> is <code>1</code> for a 1-bit and <code>0</code> for a 0-bit, we have</p>
|
||||
<div class="sourceCode" id="cb14"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>c <span class="op">=</span> a <span class="op">|</span> b</span></code></pre></div>
|
||||
<p>Computing the NOT of a single value is also easy:</p>
|
||||
<div class="sourceCode" id="cb15"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>printf<span class="op">(</span><span class="st">"%1$255d%1$s</span><span class="sc">%hhn</span><span class="st">"</span><span class="op">,</span> a<span class="op">,</span> b<span class="op">)</span></span></code></pre></div>
|
||||
<p>will compute</p>
|
||||
<div class="sourceCode" id="cb16"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>b <span class="op">=</span> <span class="op">(</span>strlen<span class="op">(</span>a<span class="op">)+</span><span class="dv">255</span><span class="op">)%</span><span class="dv">256</span> <span class="op">=</span> strlen<span class="op">(</span>a<span class="op">)-</span><span class="dv">1</span></span></code></pre></div>
|
||||
<p>and again, because <code>strlen(x)</code> is either <code>1</code> or
|
||||
<code>0</code> we have</p>
|
||||
<p>and again, because <code>strlen(x)</code> is either <code>1</code> or <code>0</code> we have</p>
|
||||
<div class="sourceCode" id="cb17"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>c <span class="op">=</span> <span class="op">!</span>b</span></code></pre></div>
|
||||
<p>From here we can compute any binary circuit. Doing something
|
||||
efficient, though, still takes work.</p>
|
||||
<p>From here we can compute any binary circuit. Doing something efficient,
|
||||
though, still takes work.</p>
|
||||
<h4 id="tic-tac-toe">Tic-Tac-Toe</h4>
|
||||
<p>The game itself is represented as a board of 18 bits, 9 bits per
|
||||
player, along with a turn counter that alternates between player 1 and
|
||||
player 2.</p>
|
||||
<p>To detect who has won, we implement the following logic. Let
|
||||
<code>A</code>, <code>B</code>, and <code>C</code> be pointers to three
|
||||
squares in a row to test, and <code>D</code> be where to save if there
|
||||
is a win or not.</p>
|
||||
<p>The game itself is represented as a board of 18 bits, 9 bits per player, along
|
||||
with a turn counter that alternates between player 1 and player 2.</p>
|
||||
<p>To detect who has won, we implement the following logic. Let <code>A</code>, <code>B</code>, and <code>C</code> be
|
||||
pointers to three squares in a row to test, and <code>D</code> be where to save if there is a
|
||||
win or not.</p>
|
||||
<div class="sourceCode" id="cb18"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a><span class="st">"</span><span class="sc">%A</span><span class="st">$s%B$s%C$s%1$253d%11$hhn"</span> <span class="co">// r11 = !(*A & *B & *C)</span></span>
|
||||
<span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a>ZERO</span>
|
||||
<span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a><span class="st">"%11$s%1$255d</span><span class="sc">%11hhn</span><span class="st">"</span> <span class="co">// r11 = !r11</span></span>
|
||||
<span id="cb18-4"><a href="#cb18-4" aria-hidden="true" tabindex="-1"></a>ZERO</span>
|
||||
<span id="cb18-5"><a href="#cb18-5" aria-hidden="true" tabindex="-1"></a><span class="st">"%11$s%D$s%D$hhn"</span> <span class="co">// *D = *D | r11</span></span></code></pre></div>
|
||||
<p>That is, we set <code>*D</code> to <code>1</code> if there is a
|
||||
three-in-a-row. We repeat this for all possible three-in-a-row
|
||||
configurations, for both players.</p>
|
||||
<p>The ZERO macro ensures that the number of bytes written out is 0 mod
|
||||
256 with the following expression</p>
|
||||
<p>That is, we set <code>*D</code> to <code>1</code> if there is a three-in-a-row. We repeat this for all
|
||||
possible three-in-a-row configurations, for both players.</p>
|
||||
<p>The ZERO macro ensures that the number of bytes written out is 0 mod 256 with
|
||||
the following expression</p>
|
||||
<div class="sourceCode" id="cb19"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a><span class="st">"%1$hhn%1$s"</span> <span class="op">(</span>repeated <span class="dv">256</span> times<span class="op">)</span></span></code></pre></div>
|
||||
<p>where argument 1 is a pointer to a temporary variable followed by a
|
||||
NUL byte.</p>
|
||||
<p>This works because if the current count is 0 mod 256, then
|
||||
<code>"%1$hhn"</code> will write zero to argument 1 and then
|
||||
<code>"%1$s"</code> will never emit any text. If, on the other hand, the
|
||||
count is not 0 mod 256, a length-1 string will be written to argument 1,
|
||||
and then <code>"%1$s"</code> will increment the count by one. By
|
||||
repeating this 256 times we’re eventually going to reach 0 mod 256.</p>
|
||||
<p>where argument 1 is a pointer to a temporary variable followed by a NUL byte.</p>
|
||||
<p>This works because if the current count is 0 mod 256, then <code>"%1$hhn"</code> will write
|
||||
zero to argument 1 and then <code>"%1$s"</code> will never emit any text. If, on the other
|
||||
hand, the count is not 0 mod 256, a length-1 string will be written to argument
|
||||
1, and then <code>"%1$s"</code> will increment the count by one. By repeating this 256
|
||||
times we’re eventually going to reach 0 mod 256.</p>
|
||||
<p>Checking if there has been an invalid move is achieved similarly.</p>
|
||||
<p>In order to decide what to print out, we have to cast the “in-memory”
|
||||
array of bits to Xs and Os to print out. This is actually rather
|
||||
straightforward. Given in <code>1$</code> the pointer to player 1’s
|
||||
square, and <code>2$</code> the pointer to player 2’s, and in
|
||||
<code>3$</code> the pointer to the board string, we can compute</p>
|
||||
<p>In order to decide what to print out, we have to cast the “in-memory” array of
|
||||
bits to Xs and Os to print out. This is actually rather straightforward. Given
|
||||
in <code>1$</code> the pointer to player 1’s square, and <code>2$</code> the pointer to player 2’s, and
|
||||
in <code>3$</code> the pointer to the board string, we can compute</p>
|
||||
<div class="sourceCode" id="cb20"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a><span class="st">"%1$s"</span> <span class="op">(</span>repeated <span class="dv">47</span> times<span class="op">)</span> <span class="st">"%2$s"</span> <span class="op">(</span>repeated <span class="dv">56</span> times<span class="op">)</span> <span class="op">%</span><span class="dv">1</span>$<span class="dv">32</span><span class="er">d</span><span class="op">%</span><span class="dv">3</span>$hhn<span class="st">"</span></span></code></pre></div>
|
||||
<p>which will, in effect, compute</p>
|
||||
<div class="sourceCode" id="cb21"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>r3 <span class="op">=</span> <span class="op">(*</span>r1<span class="op">)</span> <span class="op">*</span> <span class="dv">47</span> <span class="op">+</span> <span class="op">(*</span>r2<span class="op">)</span> <span class="op">*</span> <span class="dv">56</span> <span class="op">+</span> <span class="dv">32</span></span></code></pre></div>
|
||||
<p>which will output <code>' '</code> if neither are true,
|
||||
<code>'X'</code> if <code>r1</code> is true, or <code>'O'</code> if
|
||||
<p>which will output <code>' '</code> if neither are true, <code>'X'</code> if <code>r1</code> is true, or <code>'O'</code> if
|
||||
<code>r2</code> is true.</p>
|
||||
<h4 id="further-obfuscations">Further Obfuscations</h4>
|
||||
<p>In order to be able to finally display the board, while still only
|
||||
using one <code>printf(3)</code> statement, we finish the statement
|
||||
with</p>
|
||||
<p>In order to be able to finally display the board, while still only using one
|
||||
<code>printf(3)</code> statement, we finish the statement with</p>
|
||||
<div class="sourceCode" id="cb22"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb22-1"><a href="#cb22-1" aria-hidden="true" tabindex="-1"></a><span class="st">"</span><span class="sc">\n\033</span><span class="st">[2J</span><span class="sc">\n</span><span class="st">%26$s"</span></span></code></pre></div>
|
||||
<p>which is the escape sequence to clear the screen, and then prints
|
||||
argument 26. Argument 26 is a pointer to a <code>char*</code> in memory,
|
||||
that initially is undefined, but within the <code>printf(3)</code>
|
||||
statement we will construct this string to look like a tic-tac-toe
|
||||
board.</p>
|
||||
<p>which is the escape sequence to clear the screen, and then prints argument 26.
|
||||
Argument 26 is a pointer to a <code>char*</code> in memory, that initially is undefined,
|
||||
but within the <code>printf(3)</code> statement we will construct this string to look like a
|
||||
tic-tac-toe board.</p>
|
||||
<p>After the board, we need to print one of the following strings:</p>
|
||||
<pre><code>P1>_
|
||||
P2>_
|
||||
@@ -323,86 +292,63 @@ P1 WINS
|
||||
P2 WINS
|
||||
P1 TIES
|
||||
P2 TIES</code></pre>
|
||||
<p>Depending on if it’s P1 or P2’s turn to move, the game is over and
|
||||
someone won, or the game is over and it is a draw.</p>
|
||||
<p>This turns out not to be as hard as it might look. Using the same
|
||||
trick as before, we set byte four to be</p>
|
||||
<p>Depending on if it’s P1 or P2’s turn to move, the game is over and someone
|
||||
won, or the game is over and it is a draw.</p>
|
||||
<p>This turns out not to be as hard as it might look. Using the same trick as
|
||||
before, we set byte four to be</p>
|
||||
<div class="sourceCode" id="cb24"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb24-1"><a href="#cb24-1" aria-hidden="true" tabindex="-1"></a><span class="op">*</span>byte4 <span class="op">=</span> is_win <span class="op">*</span> <span class="ch">'W'</span> <span class="op">+</span> is_tie <span class="op">*</span> <span class="ch">'T'</span></span></code></pre></div>
|
||||
<p>The byte <code>'I'</code> and <code>'S'</code> can always be the
|
||||
same, and we do the same for <code>'E'</code>/<code>'N'</code>.</p>
|
||||
<p>We do this same on-the-fly creation of the <code>scanf(3)</code>
|
||||
format string, but for a different reason. We first want to run
|
||||
<code>printf(3)</code> to show the first board, and then alternate
|
||||
between runs to <code>scanf(3)</code> and <code>printf(3)</code> reading
|
||||
and then displaying moves. Importantly, we <em>do not</em> want a final
|
||||
<code>scanf(3)</code> when the game ends. It should just exit.</p>
|
||||
<p>The byte <code>'I'</code> and <code>'S'</code> can always be the same, and we do the same for <code>'E'</code>/<code>'N'</code>.</p>
|
||||
<p>We do this same on-the-fly creation of the <code>scanf(3)</code> format string, but for a
|
||||
different reason. We first want to run <code>printf(3)</code> to show the first board, and
|
||||
then alternate between runs to <code>scanf(3)</code> and <code>printf(3)</code> reading and then
|
||||
displaying moves. Importantly, we <em>do not</em> want a final <code>scanf(3)</code> when the game
|
||||
ends. It should just exit.</p>
|
||||
<p>One option would be to implement the logic as</p>
|
||||
<div class="sourceCode" id="cb25"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb25-1"><a href="#cb25-1" aria-hidden="true" tabindex="-1"></a>printf<span class="op">()</span></span>
|
||||
<span id="cb25-2"><a href="#cb25-2" aria-hidden="true" tabindex="-1"></a><span class="cf">while</span> <span class="op">(*</span>ok<span class="op">)</span> <span class="op">{</span></span>
|
||||
<span id="cb25-3"><a href="#cb25-3" aria-hidden="true" tabindex="-1"></a> scanf<span class="op">();</span></span>
|
||||
<span id="cb25-4"><a href="#cb25-4" aria-hidden="true" tabindex="-1"></a> printf<span class="op">();</span></span>
|
||||
<span id="cb25-5"><a href="#cb25-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
|
||||
<p>but this would DOUBLE the number of calls to <code>printf(3)</code>
|
||||
we require. So instead we implement it like this</p>
|
||||
<p>but this would DOUBLE the number of calls to <code>printf(3)</code> we require. So instead we
|
||||
implement it like this</p>
|
||||
<div class="sourceCode" id="cb26"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb26-1"><a href="#cb26-1" aria-hidden="true" tabindex="-1"></a><span class="cf">while</span> <span class="op">(*</span>ok<span class="op">)</span> <span class="op">{</span></span>
|
||||
<span id="cb26-2"><a href="#cb26-2" aria-hidden="true" tabindex="-1"></a> scanf<span class="op">();</span></span>
|
||||
<span id="cb26-3"><a href="#cb26-3" aria-hidden="true" tabindex="-1"></a> printf<span class="op">();</span></span>
|
||||
<span id="cb26-4"><a href="#cb26-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
|
||||
<p>(In reality we actually pass <code>scanf(3)</code> as an argument to
|
||||
avoid the extra statement, but it has the same effect.)</p>
|
||||
<p>Notice there is now no initial <code>printf(3)</code>. In order make
|
||||
sure the program doesn’t block before the first <code>printf(3)</code>,
|
||||
we initialize the <code>scanf(3)</code> format to the NUL string so that
|
||||
it returns right away without blocking. The first time the
|
||||
<code>printf(3)</code> call runs, it writes out <code>"%hhd"</code> to
|
||||
create the <code>scanf(3)</code> format string.</p>
|
||||
<h2 id="copyright-and-cc-by-sa-4.0-license">Copyright and CC BY-SA 4.0
|
||||
License:</h2>
|
||||
<p>This file is Copyright (c) 2023 by Landon Curt Noll. All Rights
|
||||
Reserved. You are free to share and adapt this file under the terms of
|
||||
this license:</p>
|
||||
<p>(In reality we actually pass <code>scanf(3)</code> as an argument to avoid the extra
|
||||
statement, but it has the same effect.)</p>
|
||||
<p>Notice there is now no initial <code>printf(3)</code>. In order make sure the program doesn’t
|
||||
block before the first <code>printf(3)</code>, we initialize the <code>scanf(3)</code> format to the NUL
|
||||
string so that it returns right away without blocking. The first time the
|
||||
<code>printf(3)</code> call runs, it writes out <code>"%hhd"</code> to create the <code>scanf(3)</code> format string.</p>
|
||||
<h2 id="copyright-and-cc-by-sa-4.0-license">Copyright and CC BY-SA 4.0 License:</h2>
|
||||
<p>This file is Copyright (c) 2023 by Landon Curt Noll. All Rights Reserved.
|
||||
You are free to share and adapt this file under the terms of this license:</p>
|
||||
<pre><code>Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)</code></pre>
|
||||
<p>For more information, see:
|
||||
https://creativecommons.org/licenses/by-sa/4.0/</p>
|
||||
<p>For more information, see: https://creativecommons.org/licenses/by-sa/4.0/</p>
|
||||
|
||||
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|
||||
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|
||||
|
||||
<!-- START: this line starts content generated by: bin/output-index-inventory.sh -->
|
||||
|
||||
<h1 id="inventory-for-2020carlini"><a name="inventory"></a>Inventory for
|
||||
2020/carlini</h1>
|
||||
<h1 id="inventory-for-2020carlini"><a name="inventory"></a>Inventory for 2020/carlini</h1>
|
||||
<p><a href="#top">Jump to top</a></p>
|
||||
<h2 id="primary-files">Primary files</h2>
|
||||
<ul>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/prog.c">prog.c</a>
|
||||
- entry source</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/Makefile">Makefile</a>
|
||||
- entry Makefile</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/prog.orig.c">prog.orig.c</a>
|
||||
- original source</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/try.sh">try.sh</a>
|
||||
- how to run</li>
|
||||
<li><a href="2020_carlini.tar.bz2">2020_carlini.tar.bz2</a> - download
|
||||
entry source</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/prog.c">prog.c</a> - entry source</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/Makefile">Makefile</a> - entry Makefile</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/prog.orig.c">prog.orig.c</a> - original source</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/try.sh">try.sh</a> - how to run</li>
|
||||
<li><a href="2020_carlini.tar.bz2">2020_carlini.tar.bz2</a> - download entry source</li>
|
||||
</ul>
|
||||
<h2 id="secondary-files">Secondary files</h2>
|
||||
<ul>
|
||||
<li><a href="index.html">index.html</a> - entry home page</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/.entry.json">.entry.json</a>
|
||||
- description of entry in JSON</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/.gitignore">.gitignore</a>
|
||||
- list of files that should not be committed under git</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/.entry.json">.entry.json</a> - description of entry in JSON</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/.gitignore">.gitignore</a> - list of files that should not be committed under git</li>
|
||||
<li><a href=".path">.path</a> - directory path from top directory</li>
|
||||
<li><a
|
||||
href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/README.md">README.md</a>
|
||||
- markdown source for index.html</li>
|
||||
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/carlini/README.md">README.md</a> - markdown source for index.html</li>
|
||||
</ul>
|
||||
|
||||
<!-- END: next line ends content generated by: bin/output-index-inventory.sh -->
|
||||
|
||||
Reference in new issue
Block a user