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| <div class="section" id="the-arrow-c-stream-interface"> |
| <span id="c-stream-interface"></span><h1>The Arrow C stream interface<a class="headerlink" href="#the-arrow-c-stream-interface" title="Permalink to this headline">¶</a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>This interface is experimental and may evolve based on feedback from |
| early users. ABI stability is not guaranteed yet. Feel free to |
| <a class="reference external" href="https://arrow.apache.org/community/">contact us</a>.</p> |
| </div> |
| <p>The C stream interface builds on the structures defined in the |
| <a class="reference internal" href="CDataInterface.html#c-data-interface"><span class="std std-ref">C data interface</span></a> and combines them into a higher-level |
| specification so as to ease the communication of streaming data within a single |
| process.</p> |
| <div class="section" id="semantics"> |
| <h2>Semantics<a class="headerlink" href="#semantics" title="Permalink to this headline">¶</a></h2> |
| <p>An Arrow C stream exposes a streaming source of data chunks, each with the |
| same schema. Chunks are obtained by calling a blocking pull-style iteration |
| function.</p> |
| </div> |
| <div class="section" id="structure-definition"> |
| <h2>Structure definition<a class="headerlink" href="#structure-definition" title="Permalink to this headline">¶</a></h2> |
| <p>The C stream interface is defined by a single <code class="docutils literal notranslate"><span class="pre">struct</span></code> definition:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">ArrowArrayStream</span> <span class="p">{</span> |
| <span class="c1">// Callbacks providing stream functionality</span> |
| <span class="kt">int</span> <span class="p">(</span><span class="o">*</span><span class="n">get_schema</span><span class="p">)(</span><span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span><span class="p">,</span> <span class="k">struct</span> <span class="n">ArrowSchema</span><span class="o">*</span> <span class="n">out</span><span class="p">);</span> |
| <span class="kt">int</span> <span class="p">(</span><span class="o">*</span><span class="n">get_next</span><span class="p">)(</span><span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span><span class="p">,</span> <span class="k">struct</span> <span class="n">ArrowArray</span><span class="o">*</span> <span class="n">out</span><span class="p">);</span> |
| <span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="p">(</span><span class="o">*</span><span class="n">get_last_error</span><span class="p">)(</span><span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span><span class="p">);</span> |
| |
| <span class="c1">// Release callback</span> |
| <span class="kt">void</span> <span class="p">(</span><span class="o">*</span><span class="n">release</span><span class="p">)(</span><span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span><span class="p">);</span> |
| |
| <span class="c1">// Opaque producer-specific data</span> |
| <span class="kt">void</span><span class="o">*</span> <span class="n">private_data</span><span class="p">;</span> |
| <span class="p">};</span> |
| </pre></div> |
| </div> |
| <div class="section" id="the-arrowarraystream-structure"> |
| <h3>The ArrowArrayStream structure<a class="headerlink" href="#the-arrowarraystream-structure" title="Permalink to this headline">¶</a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code> provides the required callbacks to interact with a |
| streaming source of Arrow arrays. It has the following fields:</p> |
| <dl class="c member"> |
| <dt id="c.ArrowArrayStream.get_schema"> |
| int (*<code class="sig-prename descclassname">ArrowArrayStream<code class="sig-prename descclassname">.</code></code><code class="sig-name descname">get_schema</code>)<span class="sig-paren">(</span><em class="property">struct</em> ArrowArrayStream*, <em class="property">struct</em> ArrowSchema *out<span class="sig-paren">)</span><a class="headerlink" href="#c.ArrowArrayStream.get_schema" title="Permalink to this definition">¶</a><br /></dt> |
| <dd><p><em>Mandatory.</em> This callback allows the consumer to query the schema of |
| the chunks of data in the stream. The schema is the same for all |
| data chunks.</p> |
| <p>This callback must NOT be called on a released <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code>.</p> |
| <p><em>Return value:</em> 0 on success, a non-zero |
| <a class="reference internal" href="#c-stream-interface-error-codes"><span class="std std-ref">error code</span></a> otherwise.</p> |
| </dd></dl> |
| |
| <dl class="c member"> |
| <dt id="c.ArrowArrayStream.get_next"> |
| int (*<code class="sig-prename descclassname">ArrowArrayStream<code class="sig-prename descclassname">.</code></code><code class="sig-name descname">get_next</code>)<span class="sig-paren">(</span><em class="property">struct</em> ArrowArrayStream*, <em class="property">struct</em> ArrowArray *out<span class="sig-paren">)</span><a class="headerlink" href="#c.ArrowArrayStream.get_next" title="Permalink to this definition">¶</a><br /></dt> |
| <dd><p><em>Mandatory.</em> This callback allows the consumer to get the next chunk |
| of data in the stream.</p> |
| <p>This callback must NOT be called on a released <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code>.</p> |
| <p><em>Return value:</em> 0 on success, a non-zero |
| <a class="reference internal" href="#c-stream-interface-error-codes"><span class="std std-ref">error code</span></a> otherwise.</p> |
| <p>On success, the consumer must check whether the <code class="docutils literal notranslate"><span class="pre">ArrowArray</span></code> is |
| marked <a class="reference internal" href="CDataInterface.html#c-data-interface-released"><span class="std std-ref">released</span></a>. If the |
| <code class="docutils literal notranslate"><span class="pre">ArrowArray</span></code> is released, then the end of stream has been reached. |
| Otherwise, the <code class="docutils literal notranslate"><span class="pre">ArrowArray</span></code> contains a valid data chunk.</p> |
| </dd></dl> |
| |
| <dl class="c member"> |
| <dt id="c.ArrowArrayStream.get_last_error"> |
| <em class="property">const</em> char *(*<code class="sig-prename descclassname">ArrowArrayStream<code class="sig-prename descclassname">.</code></code><code class="sig-name descname">get_last_error</code>)<span class="sig-paren">(</span><em class="property">struct</em> ArrowArrayStream*<span class="sig-paren">)</span><a class="headerlink" href="#c.ArrowArrayStream.get_last_error" title="Permalink to this definition">¶</a><br /></dt> |
| <dd><p><em>Mandatory.</em> This callback allows the consumer to get a textual description |
| of the last error.</p> |
| <p>This callback must ONLY be called if the last operation on the |
| <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code> returned an error. It must NOT be called on a |
| released <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code>.</p> |
| <p><em>Return value:</em> a pointer to a NULL-terminated character string (UTF8-encoded). |
| NULL can also be returned if no detailed description is available.</p> |
| <p>The returned pointer is only guaranteed to be valid until the next call of |
| one of the stream’s callbacks. The character string it points to should |
| be copied to consumer-managed storage if it is intended to survive longer.</p> |
| </dd></dl> |
| |
| <dl class="c member"> |
| <dt id="c.ArrowArrayStream.release"> |
| void (*<code class="sig-prename descclassname">ArrowArrayStream<code class="sig-prename descclassname">.</code></code><code class="sig-name descname">release</code>)<span class="sig-paren">(</span><em class="property">struct</em> ArrowArrayStream*<span class="sig-paren">)</span><a class="headerlink" href="#c.ArrowArrayStream.release" title="Permalink to this definition">¶</a><br /></dt> |
| <dd><p><em>Mandatory.</em> A pointer to a producer-provided release callback.</p> |
| </dd></dl> |
| |
| <dl class="c member"> |
| <dt id="c.ArrowArrayStream.private_data"> |
| void *<code class="sig-prename descclassname">ArrowArrayStream<code class="sig-prename descclassname">.</code></code><code class="sig-name descname">private_data</code><a class="headerlink" href="#c.ArrowArrayStream.private_data" title="Permalink to this definition">¶</a><br /></dt> |
| <dd><p><em>Optional.</em> An opaque pointer to producer-provided private data.</p> |
| <p>Consumers MUST not process this member. Lifetime of this member |
| is handled by the producer, and especially by the release callback.</p> |
| </dd></dl> |
| |
| </div> |
| <div class="section" id="error-codes"> |
| <span id="c-stream-interface-error-codes"></span><h3>Error codes<a class="headerlink" href="#error-codes" title="Permalink to this headline">¶</a></h3> |
| <p>The <code class="docutils literal notranslate"><span class="pre">get_schema</span></code> and <code class="docutils literal notranslate"><span class="pre">get_next</span></code> callbacks may return an error under the form |
| of a non-zero integer code. Such error codes should be interpreted like |
| <code class="docutils literal notranslate"><span class="pre">errno</span></code> numbers (as defined by the local platform). Note that the symbolic |
| forms of these constants are stable from platform to platform, but their numeric |
| values are platform-specific.</p> |
| <p>In particular, it is recommended to recognize the following values:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>: for a parameter or input validation error</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">ENOMEM</span></code>: for a memory allocation failure (out of memory)</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EIO</span></code>: for a generic input/output error</p></li> |
| </ul> |
| <div class="admonition seealso"> |
| <p class="admonition-title">See also</p> |
| <p><a class="reference external" href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html">Standard POSIX error codes</a>.</p> |
| <p><a class="reference external" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/errno-doserrno-sys-errlist-and-sys-nerr">Error codes recognized by the Windows C runtime library</a>.</p> |
| </div> |
| </div> |
| <div class="section" id="result-lifetimes"> |
| <h3>Result lifetimes<a class="headerlink" href="#result-lifetimes" title="Permalink to this headline">¶</a></h3> |
| <p>The data returned by the <code class="docutils literal notranslate"><span class="pre">get_schema</span></code> and <code class="docutils literal notranslate"><span class="pre">get_next</span></code> callbacks must be |
| released independently. Their lifetimes are not tied to that of the |
| <code class="docutils literal notranslate"><span class="pre">ArrowArrayStream</span></code>.</p> |
| </div> |
| <div class="section" id="stream-lifetime"> |
| <h3>Stream lifetime<a class="headerlink" href="#stream-lifetime" title="Permalink to this headline">¶</a></h3> |
| <p>Lifetime of the C stream is managed using a release callback with similar |
| usage as in the <a class="reference internal" href="CDataInterface.html#c-data-interface-released"><span class="std std-ref">C data interface</span></a>.</p> |
| </div> |
| </div> |
| <div class="section" id="c-consumer-example"> |
| <h2>C consumer example<a class="headerlink" href="#c-consumer-example" title="Permalink to this headline">¶</a></h2> |
| <p>Let’s say a particular database provides the following C API to execute |
| a SQL query and return the result set as a Arrow C stream:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">MyDB_Query</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">query</span><span class="p">,</span> <span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span> <span class="n">result_set</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Then a consumer could use the following code to iterate over the results:</p> |
| <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span> <span class="kt">void</span> <span class="nf">handle_error</span><span class="p">(</span><span class="kt">int</span> <span class="n">errcode</span><span class="p">,</span> <span class="k">struct</span> <span class="n">ArrowArrayStream</span><span class="o">*</span> <span class="n">stream</span><span class="p">)</span> <span class="p">{</span> |
| <span class="c1">// Print stream error</span> |
| <span class="k">const</span> <span class="kt">char</span><span class="o">*</span> <span class="n">errdesc</span> <span class="o">=</span> <span class="n">stream</span><span class="o">-></span><span class="n">get_last_error</span><span class="p">(</span><span class="n">stream</span><span class="p">);</span> |
| <span class="k">if</span> <span class="p">(</span><span class="n">errdesc</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> |
| <span class="n">fputs</span><span class="p">(</span><span class="n">errdesc</span><span class="p">,</span> <span class="n">stderr</span><span class="p">);</span> |
| <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> |
| <span class="n">fputs</span><span class="p">(</span><span class="n">strerror</span><span class="p">(</span><span class="n">errcode</span><span class="p">),</span> <span class="n">stderr</span><span class="p">);</span> |
| <span class="p">}</span> |
| <span class="c1">// Release stream and abort</span> |
| <span class="n">stream</span><span class="o">-></span><span class="n">release</span><span class="p">(</span><span class="n">stream</span><span class="p">),</span> |
| <span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span> |
| <span class="p">}</span> |
| |
| <span class="kt">void</span> <span class="nf">run_query</span><span class="p">()</span> <span class="p">{</span> |
| <span class="k">struct</span> <span class="n">ArrowArrayStream</span> <span class="n">stream</span><span class="p">;</span> |
| <span class="k">struct</span> <span class="n">ArrowSchema</span> <span class="n">schema</span><span class="p">;</span> |
| <span class="k">struct</span> <span class="n">ArrowArray</span> <span class="n">chunk</span><span class="p">;</span> |
| <span class="kt">int</span> <span class="n">errcode</span><span class="p">;</span> |
| |
| <span class="n">MyDB_Query</span><span class="p">(</span><span class="s">"SELECT * FROM my_table"</span><span class="p">,</span> <span class="o">&</span><span class="n">stream</span><span class="p">);</span> |
| |
| <span class="c1">// Query result set schema</span> |
| <span class="n">errcode</span> <span class="o">=</span> <span class="n">stream</span><span class="p">.</span><span class="n">get_schema</span><span class="p">(</span><span class="o">&</span><span class="n">stream</span><span class="p">,</span> <span class="o">&</span><span class="n">schema</span><span class="p">);</span> |
| <span class="k">if</span> <span class="p">(</span><span class="n">errcode</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span> |
| <span class="n">handle_error</span><span class="p">(</span><span class="n">errcode</span><span class="p">,</span> <span class="o">&</span><span class="n">stream</span><span class="p">);</span> |
| <span class="p">}</span> |
| |
| <span class="kt">int64_t</span> <span class="n">num_rows</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> |
| |
| <span class="c1">// Iterate over results: loop until error or end of stream</span> |
| <span class="k">while</span> <span class="p">((</span><span class="n">errcode</span> <span class="o">=</span> <span class="n">stream</span><span class="p">.</span><span class="n">get_next</span><span class="p">(</span><span class="o">&</span><span class="n">stream</span><span class="p">,</span> <span class="o">&</span><span class="n">chunk</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span> <span class="o">&&</span> |
| <span class="n">chunk</span><span class="p">.</span><span class="n">release</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> |
| <span class="c1">// Do something with chunk...</span> |
| <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Result chunk: got %lld rows</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">chunk</span><span class="p">.</span><span class="n">length</span><span class="p">);</span> |
| <span class="n">num_rows</span> <span class="o">+=</span> <span class="n">chunk</span><span class="p">.</span><span class="n">length</span><span class="p">;</span> |
| |
| <span class="c1">// Release chunk</span> |
| <span class="n">chunk</span><span class="p">.</span><span class="n">release</span><span class="p">(</span><span class="o">&</span><span class="n">chunk</span><span class="p">);</span> |
| <span class="p">}</span> |
| |
| <span class="c1">// Was it an error?</span> |
| <span class="k">if</span> <span class="p">(</span><span class="n">errcode</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span> <span class="p">{</span> |
| <span class="n">handle_error</span><span class="p">(</span><span class="n">errcode</span><span class="p">,</span> <span class="o">&</span><span class="n">stream</span><span class="p">);</span> |
| <span class="p">}</span> |
| |
| <span class="n">fprintf</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Result stream ended: total %lld rows</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">num_rows</span><span class="p">);</span> |
| |
| <span class="c1">// Release schema and stream</span> |
| <span class="n">schema</span><span class="p">.</span><span class="n">release</span><span class="p">(</span><span class="o">&</span><span class="n">schema</span><span class="p">);</span> |
| <span class="n">stream</span><span class="p">.</span><span class="n">release</span><span class="p">(</span><span class="o">&</span><span class="n">stream</span><span class="p">);</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </div> |
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