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‎reference/cstdio.html‎

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最終更新日時(UTC):
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<span itemprop="datePublished" content="2025-08-17T13:50:58">
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2025年08月17日 13時50分58秒
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<span itemprop="datePublished" content="2025-09-20T13:04:42">
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2025年09月20日 13時04分42秒
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<td><code><span href="https://cpprefjp.github.io/reference/cstdio/fopen_max.md.nolink">FOPEN_MAX</span></code></td>
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<td><code><a href="cstdio/fopen_max.html">FOPEN_MAX</a></code></td>
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<td>実装によって保証されるファイルを開ける最低限の数を表す整数定数</td>
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<td></td>
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‎reference/cstdio/fopen_max.html‎

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<span itemprop="datePublished" content="2025-09-20T13:04:42">
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2025年09月20日 13時04分42秒
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<span itemprop="name">rotarymars</span>
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<div class="identifier-type">macro</div><div class="header">&lt;cstdio&gt;</div><h1 itemprop="name"><span class="token">FOPEN_MAX</span></h1>
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<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="cp">#define FOPEN_MAX <i>unspecified</i></span>
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</code></pre></div>
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</p>
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<h2>概要</h2>
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<p>処理系によって保証されるファイルを同時に開ける最低限の数を表すマクロ。</p>
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<p>このマクロは、プログラムが同時にオープンできるファイルの最低限の数を定義する。値は実装依存であり、最低でも8以上であることがC標準によって保証されている。実際の実装では、より大きな値（例：16、256など）が使用されることが多い。</p>
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<p>また、<code>FOPEN_MAX</code>は<code><span href="https://cpprefjp.github.io/reference/cstdio/tmp_max.md.nolink">TMP_MAX</span></code>と等しいかそれより大きいことが保証される。</p>
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<h2>備考</h2>
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<ul>
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<li>この値は最低限の保証値であり、実際にはより多くのファイルを開ける場合がある</li>
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<li>システムリソースやOS固有の制限により、この値未満しかファイルを開けない場合もある</li>
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<li><code>FOPEN_MAX</code>で開けるファイルの数は、標準で開かれている3つのストリームも含むため、実際に新たに開けるファイルの数は<code>FOPEN_MAX - 3</code>個となる</li>
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</ul>
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<h2>例</h2>
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<p><div class="yata" id="e7c2b48c03c2e2ee128ec0d362ce758597bba745"><div class="codehilite"><pre><span></span><code><span class="cp">#include</span> <span class="cpf"><a href="../cstdio.html">&lt;cstdio&gt;</a></span><span class="cp"></span>
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<span class="cp">#include</span> <span class="cpf"><a href="../vector.html">&lt;vector&gt;</a></span><span class="cp"></span>
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<span class="cp">#include</span> <span class="cpf"><a href="../filesystem.html">&lt;filesystem&gt;</a></span><span class="cp"></span>
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<span class="cp">#include</span> <span class="cpf"><a href="../format.html">&lt;format&gt;</a></span><span class="cp"></span>
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<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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<span class="c1">// <span style="color:#ff0000">FOPEN_MAX</span>の値を表示</span>
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<span class="n"><a href="printf.html">std::printf</a></span><span class="p">(</span><span class="s">"<span style="color:#ff0000">FOPEN_MAX</span> = %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n"><span style="color:#ff0000">FOPEN_MAX</span></span><span class="p">);</span>
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<span class="c1">// 作業用ディレクトリを作成</span>
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<span class="n">std</span><span class="o">::</span><span class="n">filesystem</span><span class="o">::</span><span class="n">path</span> <span class="n">work_dir</span> <span class="o">=</span> <span class="s">"temp_fopen_test"</span><span class="p">;</span>
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<span class="n"><a href="../filesystem/create_directory.html">std::filesystem::create_directory</a></span><span class="p">(</span><span class="n">work_dir</span><span class="p">);</span>
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<span class="c1">// 複数のファイルを開いてみる（標準ストリーム3つを除く）</span>
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<span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="kt">FILE</span><span class="o">*&gt;</span> <span class="n">files</span><span class="p">;</span>
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<span class="k">const</span> <span class="kt">int</span> <span class="n">max_new_files</span> <span class="o">=</span> <span class="n"><span style="color:#ff0000">FOPEN_MAX</span></span> <span class="o">-</span> <span class="mi">3</span><span class="p">;</span> <span class="c1">// 標準ストリーム分を除く</span>
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<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="n">max_new_files</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
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<span class="k">auto</span> <span class="n">filepath</span> <span class="o">=</span> <span class="n">work_dir</span> <span class="o">/</span> <span class="n"><a href="../format/format.html">std::format</a></span><span class="p">(</span><span class="s">"temp{}.txt"</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span>
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<span class="n">std</span><span class="o">::</span><span class="kt">FILE</span><span class="o">*</span> <span class="n">file</span> <span class="o">=</span> <span class="n"><a href="fopen.html">std::fopen</a></span><span class="p">(</span><span class="n">filepath</span><span class="p">.</span><span class="n">string</span><span class="p">().</span><span class="n">c_str</span><span class="p">(),</span> <span class="s">"w"</span><span class="p">);</span>
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<span class="k">if</span> <span class="p">(</span><span class="n">file</span><span class="p">)</span> <span class="p">{</span>
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<span class="n">files</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="n">file</span><span class="p">);</span>
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<span class="n"><a href="fprintf.html">std::fprintf</a></span><span class="p">(</span><span class="n">file</span><span class="p">,</span> <span class="s">"File %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span>
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<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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<span class="n"><a href="printf.html">std::printf</a></span><span class="p">(</span><span class="s">"Failed to open file %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span>
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<span class="k">break</span><span class="p">;</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="n"><a href="printf.html">std::printf</a></span><span class="p">(</span><span class="s">"Successfully opened %zu files</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">files</span><span class="p">.</span><span class="n">size</span><span class="p">());</span>
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<span class="c1">// ファイルを閉じる</span>
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<span class="k">for</span> <span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="kt">FILE</span><span class="o">*</span> <span class="nl">file</span> <span class="p">:</span> <span class="n">files</span><span class="p">)</span> <span class="p">{</span>
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<span class="n"><a href="fclose.html">std::fclose</a></span><span class="p">(</span><span class="n">file</span><span class="p">);</span>
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<span class="p">}</span>
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<span class="c1">// 作業用ディレクトリごと削除</span>
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<span class="n"><a href="../filesystem/remove_all.html">std::filesystem::remove_all</a></span><span class="p">(</span><span class="n">work_dir</span><span class="p">);</span>
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<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
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<span class="p">}</span>
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</code></pre></div>
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</div></p>
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<h3>出力例</h3>
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<p><pre><code>FOPEN_MAX = 20
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Successfully opened 17 files
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</code></pre></p>
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<h2>処理系</h2>
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<ul>
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<li><a href="../../implementation.html#clang">Clang</a>: ??</li>
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<li><a href="../../implementation.html#gcc">GCC</a>: ??</li>
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<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
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</div>
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</div>
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</div>
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</main>
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<footer class="footer navbar navbar-default">
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<div class="container-fluid">
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<p><small>
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本サイトの情報は、
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<a href="https://creativecommons.org/licenses/by/3.0/deed.ja" rel="nofollow">クリエイティブ・コモンズ 表示 3.0 非移植 ライセンス(CC BY)</a>
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の下に提供されています。
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</small></p>
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</div>
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</footer>
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</body>
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</html>

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