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<p>Showcases different variations of asynchronous APIs with examples of
using libcurl, specifically, doing 2 GET requests - both sequentially
and concurrently.</p>
<p>NO threads and/or multithreading involved to disconnect any
associations of coroutines or fibers with threads. Something is
intentionally simpler, while still having as much details as
possible.</p>
<p>Jump to <a href="#tasks">tasks</a>, <a
href="#future">std::future</a>, <a href="#coroutines">coroutines</a>, <a
href="#fibers">fibers</a>, <a href="#senders">senders</a>.</p>
<p><mark>Work In Progress</mark>.</p>
<header id="title-block-header">
<h1 class="title">Styles of Asynchronous API</h1>
</header>
<nav id="TOC" role="doc-toc">
<ul>
<li><a href="#intro" id="toc-intro"><span
class="toc-section-number">1</span> # introduction</a></li>
<li><a href="#cmake" id="toc-cmake"><span
class="toc-section-number">2</span> # setup with cmake +
libcurl</a></li>
<li><a href="#libcurl_easy" id="toc-libcurl_easy"><span
class="toc-section-number">3</span> # building blocking API</a>
<ul>
<li><a href="#serve" id="toc-serve"><span
class="toc-section-number">3.1</span> # run simple http server for
tests</a></li>
</ul></li>
<li><a href="#libcurl_multi" id="toc-libcurl_multi"><span
class="toc-section-number">4</span> # building classic C-style callbacks
API</a>
<ul>
<li><a href="#libcurl_multi_design" id="toc-libcurl_multi_design"><span
class="toc-section-number">4.1</span> # thoughts on the design</a></li>
<li><a href="#libcurl_c_style" id="toc-libcurl_c_style"><span
class="toc-section-number">4.2</span> # note on C-style API (vs
C++)</a></li>
<li><a href="#libcurl_multi_impl" id="toc-libcurl_multi_impl"><span
class="toc-section-number">4.3</span> # implementing with libcurl
multi</a></li>
</ul></li>
<li><a href="#sync" id="toc-sync"><span
class="toc-section-number">5</span> # blocking, synchronous
(App_Blocking)</a>
<ul>
<li><a href="#error_handling" id="toc-error_handling"><span
class="toc-section-number">5.1</span> # on error handling</a></li>
</ul></li>
<li><a href="#async-polling-tasks-app_tasks"
id="toc-async-polling-tasks-app_tasks"><span
class="toc-section-number">6</span> # async polling, tasks
(App_Tasks)</a></li>
<li><a href="#blocking-stdfuturepromise"
id="toc-blocking-stdfuturepromise"><span
class="toc-section-number">7</span> # blocking
std::future/promise</a></li>
<li><a href="#async-polling-stdfuturepromise"
id="toc-async-polling-stdfuturepromise"><span
class="toc-section-number">8</span> # async polling,
std::future/promise</a></li>
<li><a href="#async-callbacks-app_callbacks"
id="toc-async-callbacks-app_callbacks"><span
class="toc-section-number">9</span> # async, callbacks
(App_Callbacks)</a></li>
<li><a href="#async-callbacks-polling-tasks-handle"
id="toc-async-callbacks-polling-tasks-handle"><span
class="toc-section-number">10</span> # async, callbacks + polling
(tasks, handle)</a></li>
<li><a
href="#async-with-statefullimplicit-callback-state.on_x.subscribedelegates"
id="toc-async-with-statefullimplicit-callback-state.on_x.subscribedelegates"><span
class="toc-section-number">11</span> # async with statefull/implicit
callback (state.on_X.subscribe/delegates)</a></li>
<li><a href="#building-c20-coroutines-api"
id="toc-building-c20-coroutines-api"><span
class="toc-section-number">12</span> # building C++20 coroutines API</a>
<ul>
<li><a href="#c-coroutines-basic-task-type"
id="toc-c-coroutines-basic-task-type"><span
class="toc-section-number">12.1</span> # C++ coroutines, basic task
type</a></li>
<li><a href="#c-coroutines-basic-await"
id="toc-c-coroutines-basic-await"><span
class="toc-section-number">12.2</span> # C++ coroutines, basic
await</a></li>
<li><a href="#c-coroutines-await-callback-with-a-crash"
id="toc-c-coroutines-await-callback-with-a-crash"><span
class="toc-section-number">12.3</span> # C++ coroutines, await callback
with a crash</a></li>
</ul></li>
<li><a href="#coroutines-on-top-polling-tasks"
id="toc-coroutines-on-top-polling-tasks"><span
class="toc-section-number">13</span> # coroutines on top polling
tasks</a></li>
<li><a href="#fibers-win32-app_fibers"
id="toc-fibers-win32-app_fibers"><span
class="toc-section-number">14</span> # fibers (WIN32)
(App_Fibers)</a></li>
<li><a href="#senders" id="toc-senders"><span
class="toc-section-number">15</span> # senders</a></li>
<li><a href="#reactive-streams" id="toc-reactive-streams"><span
class="toc-section-number">16</span> # reactive streams</a></li>
</ul>
</nav>
<hr />
<h1 data-number="1" id="intro"><span
class="header-section-number">1</span> <a href="#intro">#
</a>introduction</h1>
<p>Lets begin with simple C-style API on top of <a
href="https://curl.se/libcurl/c/">libcurl C API</a> and build a program
that may look like this:</p>
<div class="sourceCode" id="cb1"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="co">// our CURL API</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="bu">std::</span>string<span class="op"> </span>CURL_get<span class="op">(</span><span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url<span class="op">);</span></span>
<span id="cb1-3"><a href="#cb1-3"></a></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="dt">int</span> main<span class="op">()</span></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a> <span class="at">const</span> <span class="bu">std::</span>string<span class="op"> </span>r1 <span class="op">=</span> CURL_get<span class="op">(</span><span class="st">"localhost:5001/file1.txt"</span><span class="op">);</span></span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="at">const</span> <span class="bu">std::</span>string<span class="op"> </span>r2 <span class="op">=</span> CURL_get<span class="op">(</span><span class="st">"localhost:5001/file2.txt"</span><span class="op">);</span></span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="cf">return</span> <span class="dt">int</span><span class="op">(</span>r1<span class="op">.</span>size<span class="op">()</span> <span class="op">+</span> r2<span class="op">.</span>size<span class="op">());</span> <span class="co">// handle results</span></span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="op">}</span></span></code></pre></div>
<p>This performs two GET requests sequentially. Everything executes
synchronously.</p>
<p>Next, lets have idiomatic C-style callbacks API, intentionally,
<strong>not</strong> C++ one, see <a href="#libcurl_c_style">the
note</a>, to run requests concurrently:</p>
<div class="sourceCode" id="cb2"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="co">// libcurl bookkeeping</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="kw">using</span> CURL_Async <span class="op">=</span> <span class="dt">void</span><span class="op">*;</span></span>
<span id="cb2-3"><a href="#cb2-3"></a>CURL_Async CURL_async_create<span class="op">();</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="dt">void</span> CURL_async_destroy<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="dt">void</span> CURL_async_tick<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb2-6"><a href="#cb2-6"></a></span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="co">// main async callback API</span></span>
<span id="cb2-8"><a href="#cb2-8"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span>CURL_Async curl_async</span>
<span id="cb2-9"><a href="#cb2-9"></a> <span class="op">,</span> <span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb2-10"><a href="#cb2-10"></a> <span class="op">,</span> <span class="dt">void</span><span class="op">*</span> user_data</span>
<span id="cb2-11"><a href="#cb2-11"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">));</span></span></code></pre></div>
<p>doing 2 GET requests is more involved now:</p>
<div class="sourceCode" id="cb3"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="dt">int</span> main<span class="op">()</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a> <span class="kw">struct</span> State</span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="op">{</span></span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="dt">int</span> count <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb3-6"><a href="#cb3-6"></a> <span class="bu">std::</span>string<span class="op"> </span>r1<span class="op">;</span></span>
<span id="cb3-7"><a href="#cb3-7"></a> <span class="bu">std::</span>string<span class="op"> </span>r2<span class="op">;</span></span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="op">};</span></span>
<span id="cb3-9"><a href="#cb3-9"></a></span>
<span id="cb3-10"><a href="#cb3-10"></a> CURL_Async curl_async <span class="op">=</span> CURL_async_create<span class="op">();</span></span>
<span id="cb3-11"><a href="#cb3-11"></a> State state<span class="op">;</span></span>
<span id="cb3-12"><a href="#cb3-12"></a> CURL_async_get<span class="op">(</span>curl_async<span class="op">,</span> <span class="st">"localhost:5001/file1.txt"</span><span class="op">,</span> <span class="op">&</span>state</span>
<span id="cb3-13"><a href="#cb3-13"></a> <span class="op">,</span> <span class="op">[](</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">)</span></span>
<span id="cb3-14"><a href="#cb3-14"></a> <span class="op">{</span></span>
<span id="cb3-15"><a href="#cb3-15"></a> State<span class="op">&</span> state <span class="op">=</span> <span class="op">*</span><span class="kw">static_cast</span><span class="op"><</span>State<span class="op">*>(</span>user_data<span class="op">);</span></span>
<span id="cb3-16"><a href="#cb3-16"></a> state<span class="op">.</span>count <span class="op">+=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb3-17"><a href="#cb3-17"></a> state<span class="op">.</span>r1 <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(</span>response<span class="op">);</span></span>
<span id="cb3-18"><a href="#cb3-18"></a> <span class="op">});</span></span>
<span id="cb3-19"><a href="#cb3-19"></a> CURL_async_get<span class="op">(</span>curl_async<span class="op">,</span> <span class="st">"localhost:5001/file2.txt"</span><span class="op">,</span> <span class="op">&</span>state</span>
<span id="cb3-20"><a href="#cb3-20"></a> <span class="op">,</span> <span class="op">[](</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">)</span></span>
<span id="cb3-21"><a href="#cb3-21"></a> <span class="op">{</span></span>
<span id="cb3-22"><a href="#cb3-22"></a> State<span class="op">&</span> state <span class="op">=</span> <span class="op">*</span><span class="kw">static_cast</span><span class="op"><</span>State<span class="op">*>(</span>user_data<span class="op">);</span></span>
<span id="cb3-23"><a href="#cb3-23"></a> state<span class="op">.</span>count <span class="op">+=</span> <span class="dv">1</span><span class="op">;</span></span>
<span id="cb3-24"><a href="#cb3-24"></a> state<span class="op">.</span>r2 <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(</span>response<span class="op">);</span></span>
<span id="cb3-25"><a href="#cb3-25"></a> <span class="op">});</span></span>
<span id="cb3-26"><a href="#cb3-26"></a> <span class="cf">while</span> <span class="op">(</span>state<span class="op">.</span>count <span class="op">!=</span> <span class="dv">2</span><span class="op">)</span> <span class="co">// wait for 2 requests to finish</span></span>
<span id="cb3-27"><a href="#cb3-27"></a> <span class="op">{</span></span>
<span id="cb3-28"><a href="#cb3-28"></a> CURL_async_tick<span class="op">(</span>curl_async<span class="op">);</span></span>
<span id="cb3-29"><a href="#cb3-29"></a> <span class="op">}</span></span>
<span id="cb3-30"><a href="#cb3-30"></a> CURL_async_destroy<span class="op">(</span>curl_async<span class="op">);</span></span>
<span id="cb3-31"><a href="#cb3-31"></a> <span class="cf">return</span> <span class="dt">int</span><span class="op">(</span>state<span class="op">.</span>r1<span class="op">.</span>size<span class="op">()</span> <span class="op">+</span> state<span class="op">.</span>r2<span class="op">.</span>size<span class="op">());</span></span>
<span id="cb3-32"><a href="#cb3-32"></a><span class="op">}</span></span></code></pre></div>
<p>There is a need to have a <code>State</code> for bookkeeping, pass it
as <code>void*</code> user data to access later and, finally, run an
event loop to give libcurl a chance to process requests. Note, however,
requests execute concurrently now, as in 2 requests are active at the
same time.</p>
<p>After this, lets build <a href="#tasks">tasks</a>, <a
href="#future">std::future</a>, <a href="#coroutines">coroutines</a>, <a
href="#fibers">fibers</a>, <a href="#senders">senders</a> and other
variations of asynchronous API on top of C-style callbacks above.</p>
<p>But before that, lets wrap <a
href="https://curl.se/libcurl/c/">libcurl C API</a> for our needs.</p>
<hr />
<h1 data-number="2" id="cmake"><span
class="header-section-number">2</span> <a href="#cmake"># </a>setup with
cmake + libcurl</h1>
<p>Source code: <a
href="https://github.com/grishavanika/async_api_styles/blob/main/00_cmake_libcurl/main.cc">main.cc</a>,
<a
href="https://github.com/grishavanika/async_api_styles/blob/main/00_cmake_libcurl/CMakeLists.txt">CMakeLists.txt</a>.</p>
<p>For <a href="https://github.com/microsoft/vcpkg">vcpkg</a>, there is
extensive <a
href="https://learn.microsoft.com/en-us/vcpkg/get_started/get-started">documentation</a>
available. In short:</p>
<div class="sourceCode" id="cb4"><pre
class="sourceCode numberSource bash numberLines"><code class="sourceCode bash"><span id="cb4-1"><a href="#cb4-1"></a><span class="fu">git</span> clone https://github.com/microsoft/vcpkg</span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="bu">cd</span> vcpkg</span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="ex">bootstrap-vcpkg.bat</span></span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="ex">::</span> for a later use, assume we are in <span class="kw">`</span><span class="ex">K:\vcpkg</span><span class="kw">`</span></span>
<span id="cb4-5"><a href="#cb4-5"></a><span class="bu">set</span> VCPKG_ROOT=K:<span class="dt">\v</span>cpkg</span>
<span id="cb4-6"><a href="#cb4-6"></a><span class="ex">::</span> to make <span class="kw">`</span><span class="ex">vcpkg</span><span class="kw">`</span> available</span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="bu">set</span> path=%VCPKG_ROOT%<span class="kw">;</span><span class="ex">%PATH%</span></span></code></pre></div>
<p>For the project (which is going to have <code>async_api_styles</code>
name), vcpkg <a
href="https://learn.microsoft.com/vcpkg/consume/manifest-mode">manifest
mode</a> is used. Together with <code>curl</code> setup, all required
steps are:</p>
<div class="sourceCode" id="cb5"><pre
class="sourceCode numberSource bash numberLines"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1"></a><span class="bu">cd</span> async_api_styles</span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="ex">vcpkg</span> new <span class="at">--application</span></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="ex">vcpkg</span> add port curl</span></code></pre></div>
<p>Note: to find exact <code>curl</code> package name,
<code>vcpkg search curl</code> was used which prints:</p>
<blockquote>
<p>curl 8.13.0#1 A library for transferring data with URLs</p>
</blockquote>
<p><a
href="https://github.com/grishavanika/async_api_styles/blob/main/00_cmake_libcurl/CMakeLists.txt">CMakeLists.txt</a>
now looks like this:</p>
<div class="sourceCode" id="cb6"><pre
class="sourceCode numberSource cmake numberLines"><code class="sourceCode cmake"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">cmake_minimum_required</span>(<span class="ot">VERSION</span> <span class="dt">3.24</span> <span class="ot">FATAL_ERROR</span>)</span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">project</span>(async_api_styles <span class="ot">LANGUAGES</span> <span class="ot">CXX</span>)</span>
<span id="cb6-3"><a href="#cb6-3"></a></span>
<span id="cb6-4"><a href="#cb6-4"></a><span class="kw">add_executable</span>(<span class="bn">00_cmake_libcurl</span> main.cc)</span>
<span id="cb6-5"><a href="#cb6-5"></a><span class="kw">find_package</span>(<span class="im">CURL</span> <span class="ot">REQUIRED</span>)</span>
<span id="cb6-6"><a href="#cb6-6"></a><span class="kw">target_link_libraries</span>(<span class="bn">00_cmake_libcurl</span> <span class="ot">PRIVATE</span> <span class="bn">CURL::libcurl</span>)</span></code></pre></div>
<p><code>find_package(CURL REQUIRED)</code> syntax together with
<code>CURL::libcurl</code> target name is found from the output log of
<code>vcpkg install curl</code> (or during CMake configuration run)
which prints:</p>
<div class="sourceCode" id="cb7"><pre
class="sourceCode numberSource numberLines"><code class="sourceCode"><span id="cb7-1"><a href="#cb7-1"></a>curl is compatible with built-in CMake targets:</span>
<span id="cb7-2"><a href="#cb7-2"></a></span>
<span id="cb7-3"><a href="#cb7-3"></a> find_package(CURL REQUIRED)</span>
<span id="cb7-4"><a href="#cb7-4"></a> target_link_libraries(main PRIVATE CURL::libcurl)</span></code></pre></div>
<p>To test that everything compiles and links, save <a
href="https://github.com/grishavanika/async_api_styles/blob/main/00_cmake_libcurl/main.cc">main.cc</a>:</p>
<div class="sourceCode" id="cb8"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="pp">#include </span><span class="im"><curl/curl.h></span></span>
<span id="cb8-2"><a href="#cb8-2"></a></span>
<span id="cb8-3"><a href="#cb8-3"></a><span class="dt">int</span> main<span class="op">()</span></span>
<span id="cb8-4"><a href="#cb8-4"></a><span class="op">{</span></span>
<span id="cb8-5"><a href="#cb8-5"></a> CURL<span class="op">*</span> curl <span class="op">=</span> curl_easy_init<span class="op">();</span></span>
<span id="cb8-6"><a href="#cb8-6"></a> <span class="ot">assert</span><span class="op">(</span>curl<span class="op">);</span></span>
<span id="cb8-7"><a href="#cb8-7"></a> curl_easy_cleanup<span class="op">(</span>curl<span class="op">);</span></span>
<span id="cb8-8"><a href="#cb8-8"></a><span class="op">}</span></span></code></pre></div>
<p>Finally, to invoke CMake configure, build and run (with vcpkg):</p>
<div class="sourceCode" id="cb9"><pre
class="sourceCode numberSource bash numberLines"><code class="sourceCode bash"><span id="cb9-1"><a href="#cb9-1"></a><span class="bu">cd</span> async_api_styles</span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="fu">cmake</span> <span class="at">-S</span> . <span class="at">-B</span> build ^</span>
<span id="cb9-3"><a href="#cb9-3"></a> <span class="ex">-DCMAKE_TOOLCHAIN_FILE=%VCPKG_ROOT%\scripts\buildsystems\vcpkg.cmake</span></span>
<span id="cb9-4"><a href="#cb9-4"></a><span class="fu">cmake</span> <span class="at">--build</span> build <span class="at">--config</span> Debug</span>
<span id="cb9-5"><a href="#cb9-5"></a><span class="ex">::</span> run test</span>
<span id="cb9-6"><a href="#cb9-6"></a><span class="ex">./build/Debug/00_cmake_libcurl.exe</span></span></code></pre></div>
<p>see <a
href="https://github.com/grishavanika/async_api_styles/blob/main/build.cmd">build.cmd</a>.</p>
<h1 data-number="3" id="libcurl_easy"><span
class="header-section-number">3</span> <a href="#libcurl_easy">#
</a>building blocking API</h1>
<p>Source code: <a
href="https://github.com/grishavanika/async_api_styles/blob/main/01_libcurl_blocking_easy/main.cc">main.cc</a>.</p>
<p>Our blocking, synchronous API for GET request is straightforward,
lets go with function that looks like this:</p>
<div class="sourceCode" id="cb10"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="bu">std::</span>string<span class="op"> </span>CURL_get<span class="op">(</span><span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url<span class="op">);</span></span></code></pre></div>
<p>libcurl comes with two different APIs, <a
href="https://curl.se/libcurl/c/">“easy” and “multi”</a>. Lets use easy
interface; libcurl examples available online, including official <a
href="https://curl.se/libcurl/c/simple.html">simple.c example</a> for a
start.</p>
<p>Everything together leads to the implementation below, where
<code>curl_easy_perform()</code> call is the main one that blocks the
execution until request complete; once complete, we can return
results:</p>
<div class="sourceCode" id="cb11"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a><span class="pp">#include </span><span class="im"><string></span></span>
<span id="cb11-2"><a href="#cb11-2"></a></span>
<span id="cb11-3"><a href="#cb11-3"></a><span class="pp">#include </span><span class="im"><curl/curl.h></span></span>
<span id="cb11-4"><a href="#cb11-4"></a></span>
<span id="cb11-5"><a href="#cb11-5"></a><span class="pp">#if defined(NDEBUG)</span></span>
<span id="cb11-6"><a href="#cb11-6"></a><span class="pp"># undef NDEBUG</span></span>
<span id="cb11-7"><a href="#cb11-7"></a><span class="pp">#endif</span></span>
<span id="cb11-8"><a href="#cb11-8"></a><span class="pp">#include </span><span class="im"><cassert></span></span>
<span id="cb11-9"><a href="#cb11-9"></a></span>
<span id="cb11-10"><a href="#cb11-10"></a><span class="at">static</span> <span class="dt">size_t</span> CURL_OnWriteCallback<span class="op">(</span><span class="dt">void</span><span class="op">*</span> ptr<span class="op">,</span> <span class="dt">size_t</span> size<span class="op">,</span> <span class="dt">size_t</span> nmemb<span class="op">,</span> <span class="dt">void</span><span class="op">*</span> data<span class="op">)</span></span>
<span id="cb11-11"><a href="#cb11-11"></a><span class="op">{</span></span>
<span id="cb11-12"><a href="#cb11-12"></a> <span class="bu">std::</span>string<span class="op">&</span> response <span class="op">=</span> <span class="op">*</span><span class="kw">static_cast</span><span class="op"><</span><span class="bu">std::</span>string<span class="op">*>(</span>data<span class="op">);</span></span>
<span id="cb11-13"><a href="#cb11-13"></a> response<span class="op">.</span>append<span class="op">(</span><span class="kw">static_cast</span><span class="op"><</span><span class="at">const</span> <span class="dt">char</span><span class="op">*>(</span>ptr<span class="op">),</span> size <span class="op">*</span> nmemb<span class="op">);</span></span>
<span id="cb11-14"><a href="#cb11-14"></a> <span class="cf">return</span> <span class="op">(</span>size <span class="op">*</span> nmemb<span class="op">);</span></span>
<span id="cb11-15"><a href="#cb11-15"></a><span class="op">}</span></span>
<span id="cb11-16"><a href="#cb11-16"></a></span>
<span id="cb11-17"><a href="#cb11-17"></a><span class="bu">std::</span>string<span class="op"> </span>CURL_get<span class="op">(</span><span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url<span class="op">)</span></span>
<span id="cb11-18"><a href="#cb11-18"></a><span class="op">{</span></span>
<span id="cb11-19"><a href="#cb11-19"></a> CURL<span class="op">*</span> curl <span class="op">=</span> curl_easy_init<span class="op">();</span></span>
<span id="cb11-20"><a href="#cb11-20"></a> <span class="ot">assert</span><span class="op">(</span>curl<span class="op">);</span></span>
<span id="cb11-21"><a href="#cb11-21"></a></span>
<span id="cb11-22"><a href="#cb11-22"></a> CURLcode status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl<span class="op">,</span> CURLOPT_URL<span class="op">,</span> url<span class="op">.</span>c_str<span class="op">());</span></span>
<span id="cb11-23"><a href="#cb11-23"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-24"><a href="#cb11-24"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl<span class="op">,</span> CURLOPT_FOLLOWLOCATION<span class="op">,</span> <span class="dv">1</span><span class="bu">L</span><span class="op">);</span></span>
<span id="cb11-25"><a href="#cb11-25"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-26"><a href="#cb11-26"></a> </span>
<span id="cb11-27"><a href="#cb11-27"></a> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">;</span></span>
<span id="cb11-28"><a href="#cb11-28"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl<span class="op">,</span> CURLOPT_WRITEFUNCTION<span class="op">,</span> CURL_OnWriteCallback<span class="op">);</span></span>
<span id="cb11-29"><a href="#cb11-29"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-30"><a href="#cb11-30"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl<span class="op">,</span> CURLOPT_WRITEDATA<span class="op">,</span> <span class="op">&</span>response<span class="op">);</span></span>
<span id="cb11-31"><a href="#cb11-31"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-32"><a href="#cb11-32"></a></span>
<span id="cb11-33"><a href="#cb11-33"></a> status <span class="op">=</span> curl_easy_perform<span class="op">(</span>curl<span class="op">);</span></span>
<span id="cb11-34"><a href="#cb11-34"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-35"><a href="#cb11-35"></a></span>
<span id="cb11-36"><a href="#cb11-36"></a> <span class="dt">long</span> response_code <span class="op">=</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb11-37"><a href="#cb11-37"></a> status <span class="op">=</span> curl_easy_getinfo<span class="op">(</span>curl<span class="op">,</span> CURLINFO_RESPONSE_CODE<span class="op">,</span> <span class="op">&</span>response_code<span class="op">);</span></span>
<span id="cb11-38"><a href="#cb11-38"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb11-39"><a href="#cb11-39"></a> <span class="ot">assert</span><span class="op">(</span>response_code <span class="op">==</span> <span class="dv">200</span><span class="bu">L</span><span class="op">);</span></span>
<span id="cb11-40"><a href="#cb11-40"></a></span>
<span id="cb11-41"><a href="#cb11-41"></a> curl_easy_cleanup<span class="op">(</span>curl<span class="op">);</span></span>
<span id="cb11-42"><a href="#cb11-42"></a> <span class="cf">return</span> response<span class="op">;</span></span>
<span id="cb11-43"><a href="#cb11-43"></a><span class="op">}</span></span></code></pre></div>
<p>Note on error handling: for now, we crash on any unexpected error -
as in “crash the whole application”. <code>assert()</code> is enabled
always <strong>intentionally</strong> to simplify both, the sample code
<strong>and</strong> debugging:</p>
<div class="sourceCode" id="cb12"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="co">// after all includes, main.cc</span></span>
<span id="cb12-2"><a href="#cb12-2"></a><span class="pp">#if defined(NDEBUG)</span></span>
<span id="cb12-3"><a href="#cb12-3"></a><span class="pp"># undef NDEBUG</span></span>
<span id="cb12-4"><a href="#cb12-4"></a><span class="pp">#endif</span></span>
<span id="cb12-5"><a href="#cb12-5"></a><span class="pp">#include </span><span class="im"><cassert></span></span></code></pre></div>
<p>This is “bad” for generic, low-level library API/code, but could be
fine sometimes. We’ll <a href="#error_handling">discuss error handling
later</a>.</p>
<p>To see the code in action, lets run our program:</p>
<div class="sourceCode" id="cb13"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="pp">#include </span><span class="im"><print></span></span>
<span id="cb13-2"><a href="#cb13-2"></a></span>
<span id="cb13-3"><a href="#cb13-3"></a><span class="dt">int</span> main<span class="op">()</span></span>
<span id="cb13-4"><a href="#cb13-4"></a><span class="op">{</span></span>
<span id="cb13-5"><a href="#cb13-5"></a> <span class="at">const</span> <span class="bu">std::</span>string<span class="op"> </span>r <span class="op">=</span> CURL_get<span class="op">(</span><span class="st">"localhost:5001/file1.txt"</span><span class="op">);</span></span>
<span id="cb13-6"><a href="#cb13-6"></a> <span class="bu">std::</span>println<span class="op">(</span><span class="st">"CURL_get(file1.txt): '</span><span class="sc">{}</span><span class="st">'"</span><span class="op">,</span> r<span class="op">);</span></span>
<span id="cb13-7"><a href="#cb13-7"></a><span class="op">}</span></span></code></pre></div>
<p>that.. should crash since we don’t have local HTTP server running to
serve <code>localhost:5001/file1.txt</code>. See the <a
href="#serve">next section on how to make it happen</a>.</p>
<p>Once done, we should see the sample <a
href="https://github.com/grishavanika/async_api_styles/blob/main/01_libcurl_blocking_easy/file1.txt">file1.txt</a>
content in the console output:</p>
<pre><code>CURL_get(file1.txt): 'content 1'
</code></pre>
<h2 data-number="3.1" id="serve"><span
class="header-section-number">3.1</span> <a href="#serve"># </a>run
simple http server for tests</h2>
<p>To run sample code, lets use Python to have simple HTTP server that
hosts files in the current directory, see <a
href="https://github.com/grishavanika/async_api_styles/blob/main/01_libcurl_blocking_easy/serve.cmd">serve.cmd</a>:</p>
<div class="sourceCode" id="cb15"><pre
class="sourceCode numberSource bash numberLines"><code class="sourceCode bash"><span id="cb15-1"><a href="#cb15-1"></a><span class="ex">python</span> <span class="at">-m</span> http.server 5001</span></code></pre></div>
<p>Given the directory that has <a
href="https://github.com/grishavanika/async_api_styles/blob/main/01_libcurl_blocking_easy/file1.txt">file1.txt</a>
and <a
href="https://github.com/grishavanika/async_api_styles/blob/main/01_libcurl_blocking_easy/file2.txt">file2.txt</a>,
<code>CURL_get("localhost:5001/file1.txt")</code> and
<code>CURL_get("localhost:5001/file2.txt")</code> should work and return
the content of the files, see <a href="#libcurl_easy">blocking libcurl
section</a>.</p>
<h1 data-number="4" id="libcurl_multi"><span
class="header-section-number">4</span> <a href="#libcurl_multi">#
</a>building classic C-style callbacks API</h1>
<p>Source code: <a
href="https://github.com/grishavanika/async_api_styles/blob/main/02_libcurl_callbacks_multi/main.cc">main.cc</a>.</p>
<h2 data-number="4.1" id="libcurl_multi_design"><span
class="header-section-number">4.1</span> <a
href="#libcurl_multi_design"># </a>thoughts on the design</h2>
<p>Now, lets imagine simplest possible asynchronous API. The difference
to <a href="#libcurl_easy">blocking API</a> is that we ask the system to
start a GET request and the response should arrive some time later. The
system invokes a user-provided <code>callback</code> to notify us once
everything is done:</p>
<div class="sourceCode" id="cb16"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span><span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb16-2"><a href="#cb16-2"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="bu">std::</span>string<span class="op">));</span></span></code></pre></div>
<p>we could use it like this:</p>
<div class="sourceCode" id="cb17"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1"></a><span class="co">// start a request:</span></span>
<span id="cb17-2"><a href="#cb17-2"></a>CURL_async_get<span class="op">(</span><span class="st">"localhost:5001/file2.txt"</span></span>
<span id="cb17-3"><a href="#cb17-3"></a> <span class="op">,</span> <span class="op">[](</span><span class="bu">std::</span>string<span class="op"> </span>response<span class="op">)</span></span>
<span id="cb17-4"><a href="#cb17-4"></a><span class="op">{</span></span>
<span id="cb17-5"><a href="#cb17-5"></a> <span class="co">// probably, some time later:</span></span>
<span id="cb17-6"><a href="#cb17-6"></a> <span class="bu">std::</span>println<span class="op">(</span><span class="st">"got response: </span><span class="sc">{}</span><span class="st">"</span><span class="op">,</span> response<span class="op">);</span></span>
<span id="cb17-7"><a href="#cb17-7"></a><span class="op">});</span></span></code></pre></div>
<p>There are multiple issues with the design above:</p>
<ol type="1">
<li>Where is the “system” that starts the request? It could be implicit,
hidden global singleton, but we can also ask a user to explicitly create
and pass it around.</li>
<li>The callback accepts only <code>response</code>, there is no way for
a user to access other data, without resorting to global singletons
again. When starting a request, user should be able to provide opaque
pointer to some data that system does not touch and simply gives it back
in callback.</li>
<li>When and from where the “system” invokes a <code>callback</code>?
There are multiple answers, but we go with user-controlled event loop
that drives the system.</li>
</ol>
<p>To solve first issue, lets have explicit API to create and destroy
the system:</p>
<div class="sourceCode" id="cb18"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1"></a><span class="kw">using</span> CURL_Async <span class="op">=</span> <span class="dt">void</span><span class="op">*;</span> <span class="co">// system's state</span></span>
<span id="cb18-2"><a href="#cb18-2"></a></span>
<span id="cb18-3"><a href="#cb18-3"></a>CURL_Async CURL_async_create<span class="op">();</span></span>
<span id="cb18-4"><a href="#cb18-4"></a><span class="dt">void</span> CURL_async_destroy<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span></code></pre></div>
<p>where <code>CURL_Async</code> is the system itself, since user does
not care what’s that exactly, it’s hidden under <code>void*</code>. User
could create the system, use it and, once not needed, destroy - to clean
up resources, if any.</p>
<p>To drive a system with event loop, user must call the next API:</p>
<div class="sourceCode" id="cb19"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb19-1"><a href="#cb19-1"></a><span class="dt">void</span> CURL_async_tick<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span></code></pre></div>
<p>This is the chance for a system to actually do some work over time
<strong>and</strong> invoke user-provided callbacks, if needed.</p>
<p>Lastly, to give a user some controll over data in the callback, we
pass opaque <code>void*</code> pointer around:</p>
<div class="sourceCode" id="cb20"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1"></a><span class="co">// main async callback API</span></span>
<span id="cb20-2"><a href="#cb20-2"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span>CURL_Async curl_async</span>
<span id="cb20-3"><a href="#cb20-3"></a> <span class="op">,</span> <span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb20-4"><a href="#cb20-4"></a> <span class="op">,</span> <span class="dt">void</span><span class="op">*</span> user_data</span>
<span id="cb20-5"><a href="#cb20-5"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">));</span></span></code></pre></div>
<p><code>user_data</code> could be anything, system gives it back when
invoking <code>callback</code>. This is user responsibility to ensure
that pointer is valid all the time while request is in progress.</p>
<p>Overall, everything included, we need to implement next API, see <a
href="#libcurl_multi_impl">below</a>:</p>
<div class="sourceCode" id="cb21"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb21-1"><a href="#cb21-1"></a><span class="co">// libcurl bookkeeping</span></span>
<span id="cb21-2"><a href="#cb21-2"></a><span class="kw">using</span> CURL_Async <span class="op">=</span> <span class="dt">void</span><span class="op">*;</span></span>
<span id="cb21-3"><a href="#cb21-3"></a>CURL_Async CURL_async_create<span class="op">();</span></span>
<span id="cb21-4"><a href="#cb21-4"></a><span class="dt">void</span> CURL_async_destroy<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb21-5"><a href="#cb21-5"></a><span class="dt">void</span> CURL_async_tick<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb21-6"><a href="#cb21-6"></a></span>
<span id="cb21-7"><a href="#cb21-7"></a><span class="co">// main async callback API</span></span>
<span id="cb21-8"><a href="#cb21-8"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span>CURL_Async curl_async</span>
<span id="cb21-9"><a href="#cb21-9"></a> <span class="op">,</span> <span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb21-10"><a href="#cb21-10"></a> <span class="op">,</span> <span class="dt">void</span><span class="op">*</span> user_data</span>
<span id="cb21-11"><a href="#cb21-11"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">));</span></span></code></pre></div>
<h2 data-number="4.2" id="libcurl_c_style"><span
class="header-section-number">4.2</span> <a href="#libcurl_c_style">#
</a>note on C-style API (vs C++)</h2>
<p>For <a href="#libcurl_multi_design">C-style API above</a>, with C++,
“the system” could be a class, callback could be
<code>std::function<></code> to accept anything, generally making
it less verbose, having something like this:</p>
<div class="sourceCode" id="cb22"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb22-1"><a href="#cb22-1"></a><span class="co">// the API:</span></span>
<span id="cb22-2"><a href="#cb22-2"></a><span class="kw">class</span> CURL_Async</span>
<span id="cb22-3"><a href="#cb22-3"></a><span class="op">{</span></span>
<span id="cb22-4"><a href="#cb22-4"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb22-5"><a href="#cb22-5"></a> <span class="dt">void</span> get<span class="op">(</span><span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url<span class="op">,</span> <span class="bu">std::</span>function<span class="op"><</span><span class="dt">void</span> <span class="op">(</span><span class="bu">std::</span>string<span class="op">)>);</span></span>
<span id="cb22-6"><a href="#cb22-6"></a> <span class="dt">void</span> tick<span class="op">();</span></span>
<span id="cb22-7"><a href="#cb22-7"></a><span class="op">};</span></span>
<span id="cb22-8"><a href="#cb22-8"></a></span>
<span id="cb22-9"><a href="#cb22-9"></a><span class="co">// the use:</span></span>
<span id="cb22-10"><a href="#cb22-10"></a>CURL_Async curl<span class="op">;</span></span>
<span id="cb22-11"><a href="#cb22-11"></a>curl<span class="op">.</span>get<span class="op">(</span><span class="st">"localhost:5001/file1.txt"</span><span class="op">,</span> <span class="op">[](</span><span class="bu">std::</span>string<span class="op"> </span>r<span class="op">)</span></span>
<span id="cb22-12"><a href="#cb22-12"></a><span class="op">{</span></span>
<span id="cb22-13"><a href="#cb22-13"></a> <span class="bu">std::</span>println<span class="op">(</span><span class="st">"</span><span class="sc">{}</span><span class="st">"</span><span class="op">,</span> r<span class="op">);</span></span>
<span id="cb22-14"><a href="#cb22-14"></a><span class="op">});</span></span>
<span id="cb22-15"><a href="#cb22-15"></a>curl<span class="op">.</span>tick<span class="op">();</span> <span class="co">// etc</span></span></code></pre></div>
<p>However, C-style API we have is defacto standard, familiar and
reconized for asynchronous APIs with callbacks (citation needed).</p>
<p>The rest of asynchronous APIs implementations below are built on top
of C-style callback API, as a basic building block to cover similar
callbacks-based APIs.</p>
<h2 data-number="4.3" id="libcurl_multi_impl"><span
class="header-section-number">4.3</span> <a href="#libcurl_multi_impl">#
</a>implementing with libcurl multi</h2>
<p>Source code: <a
href="https://github.com/grishavanika/async_api_styles/blob/main/02_libcurl_callbacks_multi/main.cc">main.cc</a>.</p>
<p>For implementation of <a href="#libcurl_multi_design">the
API</a>:</p>
<div class="sourceCode" id="cb23"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb23-1"><a href="#cb23-1"></a><span class="kw">using</span> CURL_Async <span class="op">=</span> <span class="dt">void</span><span class="op">*;</span></span>
<span id="cb23-2"><a href="#cb23-2"></a>CURL_Async CURL_async_create<span class="op">();</span></span>
<span id="cb23-3"><a href="#cb23-3"></a><span class="dt">void</span> CURL_async_destroy<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb23-4"><a href="#cb23-4"></a><span class="dt">void</span> CURL_async_tick<span class="op">(</span>CURL_Async curl_async<span class="op">);</span></span>
<span id="cb23-5"><a href="#cb23-5"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span>CURL_Async curl_async</span>
<span id="cb23-6"><a href="#cb23-6"></a> <span class="op">,</span> <span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb23-7"><a href="#cb23-7"></a> <span class="op">,</span> <span class="dt">void</span><span class="op">*</span> user_data</span>
<span id="cb23-8"><a href="#cb23-8"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">));</span></span></code></pre></div>
<p>internally, lets have <code>CURL_AsyncScheduler</code> class to
handle adding requests, updating/ticking libcurl event loop and, in
general, to represent our whole <code>CURL_Async</code> system
state:</p>
<div class="sourceCode" id="cb24"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb24-1"><a href="#cb24-1"></a><span class="kw">struct</span> CURL_AsyncScheduler</span>
<span id="cb24-2"><a href="#cb24-2"></a><span class="op">{</span></span>
<span id="cb24-3"><a href="#cb24-3"></a> CURL_AsyncScheduler<span class="op">();</span></span>
<span id="cb24-4"><a href="#cb24-4"></a> <span class="op">~</span>CURL_AsyncScheduler<span class="op">();</span></span>
<span id="cb24-5"><a href="#cb24-5"></a> <span class="co">// no copy, no move</span></span>
<span id="cb24-6"><a href="#cb24-6"></a> CURL_AsyncScheduler<span class="op">(</span><span class="at">const</span> CURL_AsyncScheduler<span class="op">&)</span> <span class="op">=</span> <span class="kw">delete</span><span class="op">;</span></span>
<span id="cb24-7"><a href="#cb24-7"></a></span>
<span id="cb24-8"><a href="#cb24-8"></a> <span class="kw">using</span> Callback <span class="op">=</span> <span class="bu">std::</span>function<span class="op"><</span><span class="dt">void</span> <span class="op">(</span>CURL<span class="op">*</span> curl_easy<span class="op">)>;</span></span>
<span id="cb24-9"><a href="#cb24-9"></a></span>
<span id="cb24-10"><a href="#cb24-10"></a> <span class="dt">void</span> tick<span class="op">();</span></span>
<span id="cb24-11"><a href="#cb24-11"></a> <span class="dt">void</span> add_request<span class="op">(</span>CURL<span class="op">*</span> curl_easy<span class="op">,</span> Callback on_finish<span class="op">);</span></span>
<span id="cb24-12"><a href="#cb24-12"></a></span>
<span id="cb24-13"><a href="#cb24-13"></a> <span class="co">// our state</span></span>
<span id="cb24-14"><a href="#cb24-14"></a> CURLM<span class="op">*</span> _multi_curl <span class="op">=</span> <span class="kw">nullptr</span><span class="op">;</span></span>
<span id="cb24-15"><a href="#cb24-15"></a> <span class="bu">std::</span>unordered_map<span class="op"><</span>CURL<span class="op">*,</span> Callback<span class="op">></span> _curl_to_callback<span class="op">;</span></span>
<span id="cb24-16"><a href="#cb24-16"></a><span class="op">};</span></span></code></pre></div>
<p>This is what we’ll return to a user as <code>CURL_Async</code>
pointer. Lets do it:</p>
<div class="sourceCode" id="cb25"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb25-1"><a href="#cb25-1"></a>CURL_Async CURL_async_create<span class="op">()</span></span>
<span id="cb25-2"><a href="#cb25-2"></a><span class="op">{</span></span>
<span id="cb25-3"><a href="#cb25-3"></a> CURL_AsyncScheduler<span class="op">*</span> scheduler <span class="op">=</span> <span class="kw">new</span><span class="op">(</span><span class="bu">std::</span>nothrow<span class="op">)</span> CURL_AsyncScheduler<span class="op">();</span></span>
<span id="cb25-4"><a href="#cb25-4"></a> <span class="ot">assert</span><span class="op">(</span>scheduler<span class="op">);</span></span>
<span id="cb25-5"><a href="#cb25-5"></a> <span class="cf">return</span> scheduler<span class="op">;</span></span>
<span id="cb25-6"><a href="#cb25-6"></a><span class="op">}</span></span>
<span id="cb25-7"><a href="#cb25-7"></a></span>
<span id="cb25-8"><a href="#cb25-8"></a><span class="dt">void</span> CURL_async_destroy<span class="op">(</span>CURL_Async curl_async<span class="op">)</span></span>
<span id="cb25-9"><a href="#cb25-9"></a><span class="op">{</span></span>
<span id="cb25-10"><a href="#cb25-10"></a> <span class="ot">assert</span><span class="op">(</span>curl_async<span class="op">);</span></span>
<span id="cb25-11"><a href="#cb25-11"></a> CURL_AsyncScheduler<span class="op">*</span> scheduler <span class="op">=</span> <span class="kw">static_cast</span><span class="op"><</span>CURL_AsyncScheduler<span class="op">*>(</span>curl_async<span class="op">);</span></span>
<span id="cb25-12"><a href="#cb25-12"></a> <span class="kw">delete</span> scheduler<span class="op">;</span></span>
<span id="cb25-13"><a href="#cb25-13"></a><span class="op">}</span></span></code></pre></div>
<p>Done. Now, user just needs to pass <code>CURL_Async</code> handle
around. Before implementing internals, lets have a helper function that
gets actual <code>CURL_AsyncScheduler</code> instance from opaque
handle:</p>
<div class="sourceCode" id="cb26"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb26-1"><a href="#cb26-1"></a>CURL_AsyncScheduler<span class="op">&</span> CURL_scheduler<span class="op">(</span>CURL_Async curl_async<span class="op">)</span></span>
<span id="cb26-2"><a href="#cb26-2"></a><span class="op">{</span></span>
<span id="cb26-3"><a href="#cb26-3"></a> CURL_AsyncScheduler<span class="op">*</span> scheduler <span class="op">=</span> <span class="kw">static_cast</span><span class="op"><</span>CURL_AsyncScheduler<span class="op">*>(</span>curl_async<span class="op">);</span></span>
<span id="cb26-4"><a href="#cb26-4"></a> <span class="ot">assert</span><span class="op">(</span>scheduler<span class="op">);</span></span>
<span id="cb26-5"><a href="#cb26-5"></a> <span class="cf">return</span> <span class="op">*</span>scheduler<span class="op">;</span></span>
<span id="cb26-6"><a href="#cb26-6"></a><span class="op">}</span></span></code></pre></div>
<p>It’s not exposed to the user in any way. Lets implement our main API
in terms of our internal scheduler:</p>
<div class="sourceCode" id="cb27"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb27-1"><a href="#cb27-1"></a><span class="dt">void</span> CURL_async_get<span class="op">(</span>CURL_Async curl_async</span>
<span id="cb27-2"><a href="#cb27-2"></a> <span class="op">,</span> <span class="at">const</span> <span class="bu">std::</span>string<span class="op">&</span> url</span>
<span id="cb27-3"><a href="#cb27-3"></a> <span class="op">,</span> <span class="dt">void</span><span class="op">*</span> user_data</span>
<span id="cb27-4"><a href="#cb27-4"></a> <span class="op">,</span> <span class="dt">void</span> <span class="op">(*</span>callback<span class="op">)(</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">))</span></span>
<span id="cb27-5"><a href="#cb27-5"></a><span class="op">{</span></span>
<span id="cb27-6"><a href="#cb27-6"></a> <span class="co">// 1. setup curl easy handle</span></span>
<span id="cb27-7"><a href="#cb27-7"></a> CURL<span class="op">*</span> curl_easy <span class="op">=</span> curl_easy_init<span class="op">();</span></span>
<span id="cb27-8"><a href="#cb27-8"></a> <span class="ot">assert</span><span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb27-9"><a href="#cb27-9"></a> CURLcode status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl_easy<span class="op">,</span> CURLOPT_URL<span class="op">,</span> url<span class="op">.</span>c_str<span class="op">());</span></span>
<span id="cb27-10"><a href="#cb27-10"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb27-11"><a href="#cb27-11"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl_easy<span class="op">,</span> CURLOPT_FOLLOWLOCATION<span class="op">,</span> <span class="dv">1</span><span class="bu">L</span><span class="op">);</span></span>
<span id="cb27-12"><a href="#cb27-12"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb27-13"><a href="#cb27-13"></a> </span>
<span id="cb27-14"><a href="#cb27-14"></a> <span class="co">// 2. write response data to separate std::string</span></span>
<span id="cb27-15"><a href="#cb27-15"></a> <span class="bu">std::</span>string<span class="op">*</span> state <span class="op">=</span> <span class="kw">new</span> <span class="bu">std::</span>string<span class="op">{};</span></span>
<span id="cb27-16"><a href="#cb27-16"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl_easy<span class="op">,</span> CURLOPT_WRITEFUNCTION<span class="op">,</span> CURL_OnWriteCallback<span class="op">);</span></span>
<span id="cb27-17"><a href="#cb27-17"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb27-18"><a href="#cb27-18"></a> status <span class="op">=</span> curl_easy_setopt<span class="op">(</span>curl_easy<span class="op">,</span> CURLOPT_WRITEDATA<span class="op">,</span> state<span class="op">);</span></span>
<span id="cb27-19"><a href="#cb27-19"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb27-20"><a href="#cb27-20"></a></span>
<span id="cb27-21"><a href="#cb27-21"></a> <span class="co">// 3. associate with multi handle/event loop</span></span>
<span id="cb27-22"><a href="#cb27-22"></a> CURL_scheduler<span class="op">(</span>curl_async<span class="op">).</span>add_request<span class="op">(</span>curl_easy</span>
<span id="cb27-23"><a href="#cb27-23"></a> <span class="op">,</span> <span class="op">[</span>state<span class="op">,</span> user_data<span class="op">,</span> callback<span class="op">](</span>CURL<span class="op">*</span> curl_easy<span class="op">)</span></span>
<span id="cb27-24"><a href="#cb27-24"></a> <span class="op">{</span></span>
<span id="cb27-25"><a href="#cb27-25"></a> <span class="dt">long</span> response_code <span class="op">=</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb27-26"><a href="#cb27-26"></a> <span class="at">const</span> CURLcode status <span class="op">=</span> curl_easy_getinfo<span class="op">(</span>curl_easy<span class="op">,</span> CURLINFO_RESPONSE_CODE<span class="op">,</span> <span class="op">&</span>response_code<span class="op">);</span></span>
<span id="cb27-27"><a href="#cb27-27"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb27-28"><a href="#cb27-28"></a> <span class="ot">assert</span><span class="op">(</span>response_code <span class="op">==</span> <span class="dv">200</span><span class="bu">L</span><span class="op">);</span></span>
<span id="cb27-29"><a href="#cb27-29"></a> curl_easy_cleanup<span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb27-30"><a href="#cb27-30"></a> <span class="bu">std::</span>string<span class="op"> </span>data <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(*</span>state<span class="op">);</span></span>
<span id="cb27-31"><a href="#cb27-31"></a> <span class="kw">delete</span> state<span class="op">;</span></span>
<span id="cb27-32"><a href="#cb27-32"></a> callback<span class="op">(</span>user_data<span class="op">,</span> <span class="bu">std::</span>move<span class="op">(</span>data<span class="op">));</span></span>
<span id="cb27-33"><a href="#cb27-33"></a> <span class="op">});</span></span>
<span id="cb27-34"><a href="#cb27-34"></a><span class="op">}</span></span></code></pre></div>
<p>There are few moving parts:</p>
<ol type="1">
<li>we create and setup curl easy handle in the same way as for blocking
call;</li>
<li>we allocate separate <code>std::string</code> to write the response
data to with the same <code>CURL_OnWriteCallback</code> callback as in
<a href="#libcurl_easy">blocking implementation</a>;</li>
<li>finally, we associate the request with event loop/multi handle</li>
</ol>
<p>It could be done another way around, eliminating the need for
separate <code>std::string</code> allocation and few more optimizations,
mainly with the help of <a
href="https://curl.se/libcurl/c/CURLOPT_PRIVATE.html">associating user
data with curl easy handle/CURLOPT_PRIVATE</a>. However, it’s good
enough for illustrative purposes.</p>
<p>After creation of curl easy handle, we associate it with curl multi
handle:</p>
<div class="sourceCode" id="cb28"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb28-1"><a href="#cb28-1"></a>CURL_AsyncScheduler<span class="op">::</span>CURL_AsyncScheduler<span class="op">()</span></span>
<span id="cb28-2"><a href="#cb28-2"></a><span class="op">{</span></span>
<span id="cb28-3"><a href="#cb28-3"></a> <span class="at">const</span> CURLcode status <span class="op">=</span> curl_global_init<span class="op">(</span>CURL_GLOBAL_ALL<span class="op">);</span></span>
<span id="cb28-4"><a href="#cb28-4"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLE_OK<span class="op">);</span></span>
<span id="cb28-5"><a href="#cb28-5"></a> _multi_curl <span class="op">=</span> curl_multi_init<span class="op">();</span></span>
<span id="cb28-6"><a href="#cb28-6"></a> <span class="ot">assert</span><span class="op">(</span>_multi_curl<span class="op">);</span></span>
<span id="cb28-7"><a href="#cb28-7"></a><span class="op">}</span></span>
<span id="cb28-8"><a href="#cb28-8"></a></span>
<span id="cb28-9"><a href="#cb28-9"></a>CURL_AsyncScheduler<span class="op">::~</span>CURL_AsyncScheduler<span class="op">()</span></span>
<span id="cb28-10"><a href="#cb28-10"></a><span class="op">{</span></span>
<span id="cb28-11"><a href="#cb28-11"></a> <span class="at">const</span> CURLMcode status <span class="op">=</span> curl_multi_cleanup<span class="op">(</span>_multi_curl<span class="op">);</span></span>
<span id="cb28-12"><a href="#cb28-12"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLM_OK<span class="op">);</span></span>
<span id="cb28-13"><a href="#cb28-13"></a> curl_global_cleanup<span class="op">();</span></span>
<span id="cb28-14"><a href="#cb28-14"></a><span class="op">}</span></span>
<span id="cb28-15"><a href="#cb28-15"></a></span>
<span id="cb28-16"><a href="#cb28-16"></a><span class="dt">void</span> CURL_AsyncScheduler<span class="op">::</span>add_request<span class="op">(</span>CURL<span class="op">*</span> curl_easy<span class="op">,</span> Callback on_finish<span class="op">)</span></span>
<span id="cb28-17"><a href="#cb28-17"></a><span class="op">{</span></span>
<span id="cb28-18"><a href="#cb28-18"></a> <span class="ot">assert</span><span class="op">(</span>on_finish<span class="op">);</span></span>
<span id="cb28-19"><a href="#cb28-19"></a> <span class="ot">assert</span><span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb28-20"><a href="#cb28-20"></a> <span class="ot">assert</span><span class="op">(!</span>_curl_to_callback<span class="op">.</span>contains<span class="op">(</span>curl_easy<span class="op">));</span></span>
<span id="cb28-21"><a href="#cb28-21"></a></span>
<span id="cb28-22"><a href="#cb28-22"></a> <span class="at">const</span> CURLMcode status <span class="op">=</span> curl_multi_add_handle<span class="op">(</span>_multi_curl<span class="op">,</span> curl_easy<span class="op">);</span></span>
<span id="cb28-23"><a href="#cb28-23"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLM_OK<span class="op">);</span></span>
<span id="cb28-24"><a href="#cb28-24"></a> _curl_to_callback<span class="op">[</span>curl_easy<span class="op">]</span> <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(</span>on_finish<span class="op">);</span></span>
<span id="cb28-25"><a href="#cb28-25"></a><span class="op">}</span></span></code></pre></div>
<p><code>_curl_to_callback</code> map is used to be able to retrieve
callback later, given curl easy handle (<code>CURL*</code>).</p>
<p>Our user-exposed <code>CURL_async_tick()</code> API is implemented in
terms of scheduler:</p>
<div class="sourceCode" id="cb29"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb29-1"><a href="#cb29-1"></a><span class="dt">void</span> CURL_async_tick<span class="op">(</span>CURL_Async curl_async<span class="op">)</span></span>
<span id="cb29-2"><a href="#cb29-2"></a><span class="op">{</span></span>
<span id="cb29-3"><a href="#cb29-3"></a> CURL_scheduler<span class="op">(</span>curl_async<span class="op">).</span>tick<span class="op">();</span></span>
<span id="cb29-4"><a href="#cb29-4"></a><span class="op">}</span></span>
<span id="cb29-5"><a href="#cb29-5"></a></span>
<span id="cb29-6"><a href="#cb29-6"></a><span class="dt">void</span> CURL_AsyncScheduler<span class="op">::</span>tick<span class="op">()</span></span>
<span id="cb29-7"><a href="#cb29-7"></a><span class="op">{</span></span>
<span id="cb29-8"><a href="#cb29-8"></a> <span class="dt">int</span> running_handles <span class="op">=</span> <span class="op">-</span><span class="dv">1</span><span class="op">;</span></span>
<span id="cb29-9"><a href="#cb29-9"></a> CURLMcode status <span class="op">=</span> curl_multi_perform<span class="op">(</span>_multi_curl<span class="op">,</span> <span class="op">&</span>running_handles<span class="op">);</span></span>
<span id="cb29-10"><a href="#cb29-10"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLM_OK<span class="op">);</span></span>
<span id="cb29-11"><a href="#cb29-11"></a> <span class="dt">int</span> msgs_in_queue <span class="op">=</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb29-12"><a href="#cb29-12"></a> <span class="cf">while</span> <span class="op">(</span>CURLMsg<span class="op">*</span> m <span class="op">=</span> curl_multi_info_read<span class="op">(</span>_multi_curl<span class="op">,</span> <span class="op">&</span>msgs_in_queue<span class="op">))</span></span>
<span id="cb29-13"><a href="#cb29-13"></a> <span class="op">{</span></span>
<span id="cb29-14"><a href="#cb29-14"></a> <span class="cf">if</span> <span class="op">(</span>m<span class="op">-></span>msg <span class="op">!=</span> CURLMSG_DONE<span class="op">)</span></span>
<span id="cb29-15"><a href="#cb29-15"></a> <span class="op">{</span></span>
<span id="cb29-16"><a href="#cb29-16"></a> <span class="cf">continue</span><span class="op">;</span></span>
<span id="cb29-17"><a href="#cb29-17"></a> <span class="op">}</span></span>
<span id="cb29-18"><a href="#cb29-18"></a> CURL<span class="op">*</span> curl_easy <span class="op">=</span> m<span class="op">-></span>easy_handle<span class="op">;</span></span>
<span id="cb29-19"><a href="#cb29-19"></a> <span class="ot">assert</span><span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb29-20"><a href="#cb29-20"></a> status <span class="op">=</span> curl_multi_remove_handle<span class="op">(</span>_multi_curl<span class="op">,</span> curl_easy<span class="op">);</span></span>
<span id="cb29-21"><a href="#cb29-21"></a> <span class="ot">assert</span><span class="op">(</span>status <span class="op">==</span> CURLM_OK<span class="op">);</span></span>
<span id="cb29-22"><a href="#cb29-22"></a> <span class="kw">auto</span> it <span class="op">=</span> _curl_to_callback<span class="op">.</span>find<span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb29-23"><a href="#cb29-23"></a> <span class="ot">assert</span><span class="op">(</span>it <span class="op">!=</span> _curl_to_callback<span class="op">.</span>end<span class="op">());</span></span>
<span id="cb29-24"><a href="#cb29-24"></a> Callback callback <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(</span>it<span class="op">-></span>second<span class="op">);</span></span>
<span id="cb29-25"><a href="#cb29-25"></a> <span class="ot">assert</span><span class="op">(</span>callback<span class="op">);</span></span>
<span id="cb29-26"><a href="#cb29-26"></a> <span class="op">(</span><span class="dt">void</span><span class="op">)</span>_curl_to_callback<span class="op">.</span>erase<span class="op">(</span>it<span class="op">);</span></span>
<span id="cb29-27"><a href="#cb29-27"></a> callback<span class="op">(</span>curl_easy<span class="op">);</span></span>
<span id="cb29-28"><a href="#cb29-28"></a> <span class="op">}</span></span>
<span id="cb29-29"><a href="#cb29-29"></a><span class="op">}</span></span></code></pre></div>
<p>The main part of event loop is the call to
<code>curl_multi_perform()</code>. Once done we ask for easy handle
requests that were completed, search for an associated callback for each
request and invoke it.</p>
<p>Note, there are no threads involved and it’s possible to create many
GET requests at once with multiple calls to
<code>CURL_async_get()</code> - libcurl will manage them all
together.</p>
<p>Again, it’s user responsibility to drive libcurl with a periodic
calls to <code>CURL_async_tick()</code>. Lets do single request with the
API above (<a
href="https://github.com/grishavanika/async_api_styles/blob/main/02_libcurl_callbacks_multi/main.cc">source
code</a>):</p>
<div class="sourceCode" id="cb30"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb30-1"><a href="#cb30-1"></a><span class="pp">#include </span><span class="im"><print></span></span>
<span id="cb30-2"><a href="#cb30-2"></a></span>
<span id="cb30-3"><a href="#cb30-3"></a><span class="dt">int</span> main<span class="op">()</span></span>
<span id="cb30-4"><a href="#cb30-4"></a><span class="op">{</span></span>
<span id="cb30-5"><a href="#cb30-5"></a> <span class="kw">struct</span> State</span>
<span id="cb30-6"><a href="#cb30-6"></a> <span class="op">{</span></span>
<span id="cb30-7"><a href="#cb30-7"></a> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">;</span></span>
<span id="cb30-8"><a href="#cb30-8"></a> <span class="dt">bool</span> done <span class="op">=</span> <span class="kw">false</span><span class="op">;</span></span>
<span id="cb30-9"><a href="#cb30-9"></a> <span class="op">};</span></span>
<span id="cb30-10"><a href="#cb30-10"></a> CURL_Async curl_async <span class="op">=</span> CURL_async_create<span class="op">();</span></span>
<span id="cb30-11"><a href="#cb30-11"></a> State state<span class="op">;</span></span>
<span id="cb30-12"><a href="#cb30-12"></a> CURL_async_get<span class="op">(</span>curl_async<span class="op">,</span> <span class="st">"localhost:5001/file1.txt"</span><span class="op">,</span> <span class="op">&</span>state</span>
<span id="cb30-13"><a href="#cb30-13"></a> <span class="op">,</span> <span class="op">[](</span><span class="dt">void</span><span class="op">*</span> user_data<span class="op">,</span> <span class="bu">std::</span>string<span class="op"> </span>response<span class="op">)</span></span>
<span id="cb30-14"><a href="#cb30-14"></a> <span class="op">{</span></span>
<span id="cb30-15"><a href="#cb30-15"></a> State<span class="op">&</span> state <span class="op">=</span> <span class="op">*</span><span class="kw">static_cast</span><span class="op"><</span>State<span class="op">*>(</span>user_data<span class="op">);</span></span>
<span id="cb30-16"><a href="#cb30-16"></a> state<span class="op">.</span>response <span class="op">=</span> <span class="bu">std::</span>move<span class="op">(</span>response<span class="op">);</span></span>
<span id="cb30-17"><a href="#cb30-17"></a> state<span class="op">.</span>done <span class="op">=</span> <span class="kw">true</span><span class="op">;</span></span>
<span id="cb30-18"><a href="#cb30-18"></a> <span class="op">});</span></span>
<span id="cb30-19"><a href="#cb30-19"></a> <span class="cf">while</span> <span class="op">(!</span>state<span class="op">.</span>done<span class="op">)</span></span>
<span id="cb30-20"><a href="#cb30-20"></a> <span class="op">{</span></span>
<span id="cb30-21"><a href="#cb30-21"></a> CURL_async_tick<span class="op">(</span>curl_async<span class="op">);</span></span>
<span id="cb30-22"><a href="#cb30-22"></a> <span class="op">}</span></span>
<span id="cb30-23"><a href="#cb30-23"></a> CURL_async_destroy<span class="op">(</span>curl_async<span class="op">);</span></span>
<span id="cb30-24"><a href="#cb30-24"></a></span>
<span id="cb30-25"><a href="#cb30-25"></a> <span class="bu">std::</span>println<span class="op">(</span><span class="st">"async response: '</span><span class="sc">{}</span><span class="st">'"</span><span class="op">,</span> state<span class="op">.</span>response<span class="op">);</span></span>
<span id="cb30-26"><a href="#cb30-26"></a><span class="op">}</span></span></code></pre></div>
<p>If <a href="#serve">python HTTP server</a> is running, our program
should print:</p>
<pre><code>async response: 'content 1'
</code></pre>
<h1 data-number="5" id="sync"><span
class="header-section-number">5</span> <a href="#sync"># </a>blocking,
synchronous (App_Blocking)</h1>
<h2 data-number="5.1" id="error_handling"><span
class="header-section-number">5.1</span> <a href="#error_handling">#
</a>on error handling</h2>
<h3 class="unnumbered unlisted" id="assume-success-always-tooling"><a
href="#assume-success-always-tooling"># </a>assume success always
(tooling)</h3>
<h3 class="unnumbered unlisted" id="implicit-return-empty-string"><a
href="#implicit-return-empty-string"># </a>implicit, return empty
string</h3>
<h3 class="unnumbered unlisted"
id="status-code-out-parameter-stdfilesystem-style"><a
href="#status-code-out-parameter-stdfilesystem-style"># </a>status code,
out parameter (std::filesystem-style)</h3>
<h3 class="unnumbered unlisted" id="optional"><a href="#optional">#
</a>optional</h3>
<h3 class="unnumbered unlisted" id="exceptions"><a href="#exceptions">#
</a>exceptions</h3>
<h3 class="unnumbered unlisted" id="resultvariant-like"><a
href="#resultvariant-like"># </a>result/variant-like</h3>
<h3 class="unnumbered unlisted" id="resulttuple-like"><a
href="#resulttuple-like"># </a>result/tuple-like</h3>
<h3 class="unnumbered unlisted" id="resultspecialized"><a
href="#resultspecialized"># </a>result/specialized</h3>
<h1 data-number="6" id="async-polling-tasks-app_tasks"><span
class="header-section-number">6</span> <a
href="#async-polling-tasks-app_tasks"># </a>async polling, tasks
(App_Tasks)</h1>
<h1 data-number="7" id="blocking-stdfuturepromise"><span
class="header-section-number">7</span> <a
href="#blocking-stdfuturepromise"># </a>blocking
std::future/promise</h1>
<h1 data-number="8" id="async-polling-stdfuturepromise"><span
class="header-section-number">8</span> <a
href="#async-polling-stdfuturepromise"># </a>async polling,
std::future/promise</h1>
<h1 data-number="9" id="async-callbacks-app_callbacks"><span
class="header-section-number">9</span> <a
href="#async-callbacks-app_callbacks"># </a>async, callbacks
(App_Callbacks)</h1>
<h1 data-number="10" id="async-callbacks-polling-tasks-handle"><span
class="header-section-number">10</span> <a
href="#async-callbacks-polling-tasks-handle"># </a>async, callbacks +
polling (tasks, handle)</h1>
<h1 data-number="11"
id="async-with-statefullimplicit-callback-state.on_x.subscribedelegates"><span
class="header-section-number">11</span> <a
href="#async-with-statefullimplicit-callback-state.on_x.subscribedelegates">#
</a>async with statefull/implicit callback
(state.on_X.subscribe/delegates)</h1>
<h1 data-number="12" id="building-c20-coroutines-api"><span
class="header-section-number">12</span> <a
href="#building-c20-coroutines-api"># </a>building C++20 coroutines
API</h1>
<p>Coroutines materials:</p>
<ul>
<li><a href="https://kirit.com/How%20C%2B%2B%20coroutines%20work">How
C++ coroutines work</a>.</li>
<li>All of <a href="https://lewissbaker.github.io/">Asymmetric
Transfer</a>, author of <a
href="https://github.com/lewissbaker/cppcoro">cppcoro</a>.</li>
</ul>
<p>In short, we’d like to have somehow to be able to write something
like this:</p>
<div class="sourceCode" id="cb32"><pre
class="sourceCode numberSource cpp numberLines"><code class="sourceCode cpp"><span id="cb32-1"><a href="#cb32-1"></a><span class="at">const</span> <span class="bu">std::</span>string<span class="op"> </span>response <span class="op">=</span> <span class="cf">co_await</span> CURL_await_get<span class="op">(</span></span>
<span id="cb32-2"><a href="#cb32-2"></a> curl_async<span class="op">,</span> <span class="st">"localhost:5001/file1.txt"</span><span class="op">);</span></span>
<span id="cb32-3"><a href="#cb32-3"></a><span class="co">// use `response` as a usual variable, no callbacks</span></span></code></pre></div>
<p>There are several moving and a bit unrelative parts to have working