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file_handle.html

file_handle.html 11 KB
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  3. <title>A file handle - Boost.Outcome documentation</title>
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    13. 

  13.   <div class="titlepage"><div><div><h1 style="clear: both">A file handle</h1></div></div></div>
    14. 

  14.   <p>Borrowing from <a href="https://ned14.github.io/llfio/classllfio__v2__xxx_1_1file__handle.html"><code>llfio::file_handle</code></a>
    15. 

  15. which uses this design pattern<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup>, here is a simplified <code>file_handle</code> implementation:</p>
    16. 

  16. 

  17. <div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">class</span><span class="err"> </span><span class="nc">file_handle</span>
    18. 

  18. <span class="p">{</span>
    19. 

  19.   <span class="kt">int</span> <span class="n">_fd</span><span class="p">{</span><span class="o">-</span><span class="mi">1</span><span class="p">};</span>  <span class="c1">// file descriptor
    20. 

  20. </span><span class="c1"></span>  <span class="k">struct</span> <span class="n">stat</span> <span class="n">_stat</span>
    21. 

  21.   <span class="p">{</span>
    22. 

  22.     <span class="mi">0</span>
    23. 

  23.   <span class="p">};</span>  <span class="c1">// stat of the fd at open
    24. 

  24. </span><span class="c1"></span>
    25. 

  25.   <span class="c1">// Phase 1 private constexpr constructor
    26. 

  26. </span><span class="c1"></span>  <span class="k">constexpr</span> <span class="nf">file_handle</span><span class="p">()</span> <span class="p">{}</span>
    27. 

  27. 

  28. <span class="k">public</span><span class="o">:</span>
    29. 

  29.   <span class="k">using</span> <span class="n">path_type</span> <span class="o">=</span> <span class="n">filesystem</span><span class="o">::</span><span class="n">path</span><span class="p">;</span>
    30. 

  30. 

  31.   <span class="c1">//! The behaviour of the handle: does it read, read and write, or atomic append?
    32. 

  32. </span><span class="c1"></span>  <span class="k">enum</span> <span class="k">class</span><span class="err"> </span><span class="nc">mode</span> <span class="o">:</span> <span class="kt">unsigned</span> <span class="kt">char</span>  <span class="c1">// bit 0 set means writable
    33. 

  33. </span><span class="c1"></span>  <span class="p">{</span>
    34. 

  34.     <span class="n">unchanged</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span>
    35. 

  35.     <span class="n">none</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span>        <span class="c1">//!&lt; No ability to read or write anything, but can synchronise (SYNCHRONIZE or 0)
    36. 

  36. </span><span class="c1"></span>    <span class="n">attr_read</span> <span class="o">=</span> <span class="mi">4</span><span class="p">,</span>   <span class="c1">//!&lt; Ability to read attributes (FILE_READ_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)
    37. 

  37. </span><span class="c1"></span>    <span class="n">attr_write</span> <span class="o">=</span> <span class="mi">5</span><span class="p">,</span>  <span class="c1">//!&lt; Ability to read and write attributes (FILE_READ_ATTRIBUTES|FILE_WRITE_ATTRIBUTES|SYNCHRONIZE or O_RDONLY)
    38. 

  38. </span><span class="c1"></span>    <span class="n">read</span> <span class="o">=</span> <span class="mi">6</span><span class="p">,</span>        <span class="c1">//!&lt; Ability to read (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|SYNCHRONISE or O_RDONLY)
    39. 

  39. </span><span class="c1"></span>    <span class="n">write</span> <span class="o">=</span> <span class="mi">7</span><span class="p">,</span>       <span class="c1">//!&lt; Ability to read and write (READ_CONTROL|FILE_READ_DATA|FILE_READ_ATTRIBUTES|FILE_READ_EA|FILE_WRITE_DATA|FILE_WRITE_ATTRIBUTES|FILE_WRITE_EA|FILE_APPEND_DATA|SYNCHRONISE or O_RDWR)
    40. 

  40. </span><span class="c1"></span>    <span class="n">append</span> <span class="o">=</span> <span class="mi">9</span>       <span class="c1">//!&lt; All mainstream OSs and CIFS guarantee this is atomic with respect to all other appenders (FILE_APPEND_DATA|SYNCHRONISE or O_APPEND)
    41. 

  41. </span><span class="c1"></span>  <span class="p">};</span>
    42. 

  42. 

  43.   <span class="c1">// Moves but not copies permitted
    44. 

  44. </span><span class="c1"></span>  <span class="n">file_handle</span><span class="p">(</span><span class="k">const</span> <span class="n">file_handle</span> <span class="o">&amp;</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
    45. 

  45.   <span class="n">file_handle</span><span class="p">(</span><span class="n">file_handle</span> <span class="o">&amp;&amp;</span><span class="n">o</span><span class="p">)</span> <span class="k">noexcept</span> <span class="o">:</span> <span class="n">_fd</span><span class="p">(</span><span class="n">o</span><span class="p">.</span><span class="n">_fd</span><span class="p">)</span> <span class="p">{</span> <span class="n">o</span><span class="p">.</span><span class="n">_fd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span> <span class="p">}</span>
    46. 

  46.   <span class="n">file_handle</span> <span class="o">&amp;</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">file_handle</span> <span class="o">&amp;</span><span class="p">)</span> <span class="o">=</span> <span class="k">delete</span><span class="p">;</span>
    47. 

  47.   <span class="n">file_handle</span> <span class="o">&amp;</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">file_handle</span> <span class="o">&amp;&amp;</span><span class="n">o</span><span class="p">)</span> <span class="k">noexcept</span>
    48. 

  48.   <span class="p">{</span>
    49. 

  49.     <span class="k">this</span><span class="o">-&gt;~</span><span class="n">file_handle</span><span class="p">();</span>
    50. 

  50.     <span class="k">new</span><span class="p">(</span><span class="k">this</span><span class="p">)</span> <span class="n">file_handle</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">o</span><span class="p">));</span>
    51. 

  51.     <span class="k">return</span> <span class="o">*</span><span class="k">this</span><span class="p">;</span>
    52. 

  52.   <span class="p">}</span>
    53. 

  53.   <span class="c1">// Destruction closes the handle
    54. 

  54. </span><span class="c1"></span>  <span class="o">~</span><span class="n">file_handle</span><span class="p">()</span>
    55. 

  55.   <span class="p">{</span>
    56. 

  56.     <span class="k">if</span><span class="p">(</span><span class="n">_fd</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
    57. 

  57.     <span class="p">{</span>
    58. 

  58.       <span class="k">if</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span> <span class="o">==</span> <span class="o">::</span><span class="n">close</span><span class="p">(</span><span class="n">_fd</span><span class="p">))</span>
    59. 

  59.       <span class="p">{</span>
    60. 

  60.         <span class="kt">int</span> <span class="n">e</span> <span class="o">=</span> <span class="n">errno</span><span class="p">;</span>
    61. 

  61.         <span class="n">std</span><span class="o">::</span><span class="n">cerr</span> <span class="o">&lt;&lt;</span> <span class="s">&#34;FATAL: Closing the fd during destruction failed due to &#34;</span> <span class="o">&lt;&lt;</span> <span class="n">strerror</span><span class="p">(</span><span class="n">e</span><span class="p">)</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    62. 

  62.         <span class="n">std</span><span class="o">::</span><span class="n">terminate</span><span class="p">();</span>
    63. 

  63.       <span class="p">}</span>
    64. 

  64.       <span class="n">_fd</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span>
    65. 

  65.     <span class="p">}</span>
    66. 

  66.   <span class="p">}</span>
    67. 

  67. 

  68.   <span class="c1">// Phase 2 static member constructor function, which cannot throw
    69. 

  69. </span><span class="c1"></span>  <span class="k">static</span> <span class="kr">inline</span> <span class="n">outcome</span><span class="o">::</span><span class="n">result</span><span class="o">&lt;</span><span class="n">file_handle</span><span class="o">&gt;</span> <span class="n">file</span><span class="p">(</span><span class="n">path_type</span> <span class="n">path</span><span class="p">,</span> <span class="n">mode</span> <span class="n">mode</span> <span class="o">=</span> <span class="n">mode</span><span class="o">::</span><span class="n">read</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span>
    70. 

  70. <span class="p">};</span>
    71. 

  71. </code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/constructors.cpp#L52" class="code-snippet-url" target="_blank">View this code on Github</a></div>
    72. 

  72. 

  73. 

  74. <p>Note the default member initialisers, these are particularly convenient for
    75. 

  75. implementing phase 1 of construction. Note also the <code>constexpr</code> constructor,
    76. 

  76. which thanks to the default member initialisers is otherwise empty.</p>
    77. 

  77. 

  78. <p>File handles are very expensive to copy as they involve a syscall to duplicate
    79. 

  79. the file descriptor, so we enable moves only.</p>
    80. 

  80. 

  81. <p>The destructor closes the file descriptor if it is not -1, and if the close
    82. 

  82. fails, seeing as there is nothing else we can do without leaking the file
    83. 

  83. descriptor, we fatal exit the process.</p>
    84. 

  84. 

  85. <p>Finally we declare the phase 2 constructor which is a static member function.</p>
    86. 

  86. <div class="footnotes">
    87. 

  87. 

  88. <hr />
    89. 

  89. 

  90. <ol>
    91. 

  91. <li id="fn:1">LLFIO uses Outcome &ldquo;in anger&rdquo;, both in Standard and Experimental configurations. If you would like a real world user of Outcome to study the source code of, it can be studied at <a href="https://github.com/ned14/llfio/">https://github.com/ned14/llfio/</a>.
    92. 

  92.  <a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li>
    93. 

  93. </ol>
    94. 

  94. </div>
    95. 

  95. 

  96. 

  97.         </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
    98. 

  98. <hr>
    99. 

  99. <div class="spirit-nav">
    100. 

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    103. 

  103. </html>
    104.