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  <title>Allow main function in named modules</title>
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<h1 class="title" style="text-align:center">Allow main function in named
modules</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3422R1</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-11-22</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      Core Language Evolution Group<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Chuanqi Xu<br>&lt;<a href="mailto:chuanqi.xcq@alibaba-inc.com" class="email">chuanqi.xcq@alibaba-inc.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This paper proposes to allow using the
<code class="sourceCode cpp">main</code> function in named module.</p>
<h1 data-number="2" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>The motivation example is for test. Considering:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode c++"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">export</span> <span class="kw">module</span> A<span class="op">;</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> non_exported <span class="op">{</span> <span class="op">...</span> <span class="op">};</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="kw">export</span> <span class="kw">template</span> <span class="op">&lt;</span><span class="kw">class</span> C<span class="op">&gt;</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> exported <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>    non_exported <span class="op">...;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>    <span class="op">...</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span></code></pre></div>
<p>Then it is a question for how to wrtie the unittests for
<code class="sourceCode cpp">non_exported</code>.</p>
<p>The most stragiht forward solution is to write the unittests in the
same module so that we can have the full visibility for the module.
e.g.,</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>module A; // or `module A:test_non_exported;` to make it more expressive.</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>int main() {</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>    non_exported ...;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>    ...</span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
<p>However, the above code is illegal according to the current
wording.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>[basic.start.main]p3:</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>  A program that declares:</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>  - ...</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>  - a function main that belongs to the global scope and is attached to a named module, or</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a>  - ...</span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a>  is ill-formed.</span></code></pre></div>
<p>If we want to use <a href="https://eel.is/c++draft/module#unit-7.2.2">language linkage</a> to
attach the main function to the global module, we can’t do that neither
according to <a href="https://eel.is/c++draft/basic.start.main#3">[basic.start.main]p3</a>
too:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>The main function shall not be declared within a linkage-specification ([dcl.link]).</span></code></pre></div>
<p>And also if the whole program is made of modules, it’s annoying to
have a single-non-module file just for main.</p>
<p>So we propose allowing the use of a main function in named modules to
make the above use cases and other use cases easier and more
straightforward.</p>
<p>BTW, both Clang and GCC didn’t implement [basic.start.main]p3.2 and
somehow gives the main the function the “correct” linkage:
https://godbolt.org/z/5Pfjzf87P . And if we accept the proposal, we may
only need to do some conforming implementations. e.g., in clang, now the
main function in the above example is attached to the named module and
we should attach it to the global module:
https://godbolt.org/z/6YMTdaPaP</p>
<h1 data-number="3" id="proposed-behavior"><span class="header-section-number">3</span> Proposed behavior<a href="#proposed-behavior" class="self-link"></a></h1>
<p>The main function in the global scope should always be attached to
the global module.</p>
<h1 data-number="4" id="design-decisions"><span class="header-section-number">4</span> Design Decisions<a href="#design-decisions" class="self-link"></a></h1>
<p>There was a paper <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1203r0.html">P1203R0</a>
that attempts to do something similar to this paper.</p>
<p>The only difference is that P1203R0 proposes to allow attaching the
main function to named modules, while this paper proposes to always
attach the main function to the global module.</p>
<p>Another difference is that P1203R0 seeks to introduce a new feature
called <code class="sourceCode cpp">exported main<span class="op">()</span></code>,
which stipulates that: (1) if a module has an <code class="sourceCode cpp">exported main<span class="op">()</span></code>,
that serves as the main function of the program. (2) if the program
lacks an exported main() but has (potentially multiple) non-exported
main function, one can be chosen at link time (e.g., as a driver for a
test suite), which reflects the existing behavior.</p>
<p>While this approach looks promising, we realize that it may be
challenging to implement in practice.</p>
<p>On the one hand, the ODR issue is <strong>already</strong> forbidden
clearly in <a href="https://eel.is/c++draft/basic.def.odr#15">[basic.def.odr]</a>. The
toolchains have just failed to enforce this rigorously.</p>
<p>On the other hand, currently, if there are multiple main functions
during linking, the linker will select the first main function. Although
this is a hack, it is actually used in practice. We have utilized this
internally, and according to the minutes of P1203R0, Bloomberg has done
so as well. Thus, it is a technique to hijack the main function, even if
it is considered a hack. The challenge here is that we will need more
effort to implement (1) to instruct the linker to fetch the exported
main() instead of the first main function.</p>
<p>So, while it initially seems better, we have some concerns: (1) it
changes an assumption of the ecosystem: we can’t hijack the main
function by altering the linking order (which is a hack but is used in
practice). (2) it requires more implementation effort. From the authors’
experience, resources to implement new features are limited. This is
particularly true when it requires vendors to modify the ABI and
linkers, while most developers involved are frontend developers. The
cost of development in other areas is quite high, and we believe vendors
can better allocate their time elsewhere.</p>
<p>We hope this will be a small change that benefits people while
preserving the existing mechanism for loading programs. Thus, we have
aimed to keep our approach as simple as possible, similar to how we
handle allocation and deallocation functions, by always attaching the
main function to the global module.</p>
<h1 data-number="5" id="impacts"><span class="header-section-number">5</span> Impacts<a href="#impacts" class="self-link"></a></h1>
<h2 data-number="5.1" id="impact-on-users"><span class="header-section-number">5.1</span> Impact on users<a href="#impact-on-users" class="self-link"></a></h2>
<p>Since both Clang and GCC somehow perform the desired behavior, at
least the users of Clang and GCC won’t be affected. We didn’t check the
behavior of MSVC, but we believe it should have the same impact as the
standard didn’t allow it.</p>
<h2 data-number="5.2" id="impact-on-standard"><span class="header-section-number">5.2</span> Impact on standard<a href="#impact-on-standard" class="self-link"></a></h2>
<p>Given this change asks we allow something forbidden before, it
shouldn’t introduce any regression changes.</p>
<h2 data-number="5.3" id="impact-on-implementation"><span class="header-section-number">5.3</span> Impact on implementation<a href="#impact-on-implementation" class="self-link"></a></h2>
<p>We believe it is easy to change the modules ownership for the
specific main function.</p>
<p>And given both Clang and GCC do the wanted behavior, we only need to
do some conforming change in Clang and GCC.</p>
<h2 data-number="5.4" id="summary"><span class="header-section-number">5.4</span> Summary<a href="#summary" class="self-link"></a></h2>
<p>Given that we didn’t see any negative impact and considering the
scale of the change, we suggest applying this in C++20 or recommending
that vendors implement it in C++20.</p>
<h1 data-number="6" id="proposed-wording"><span class="header-section-number">6</span> Proposed Wording<a href="#proposed-wording" class="self-link"></a></h1>
<p>Modify p3 of <strong>§6.9.3.1 [basic.start.main]</strong> :</p>
A program that declares
<ul>
<li>
a variable main that belongs to the global scope, or
</li>
<li>
<span class="rm" style="color: #bf0303"><del>a function main that
belongs to the global scope and is attached to a named
module,or</del></span>
</li>
<li>
a function template main that belongs to the global scope, or
</li>
<li>
an entity named main with C language linkage (in any namespace)
</li>
</ul>
<p>is ill-formed. The name main is not otherwise reserved.</p>
<p>Modify p7.2 of <strong>§10.1 [module.unit]</strong>:</p>
Otherwise, if the declaration
<ul>
<li>
is a namespace-definition with external linkage <span class="add" style="color: #006e28"><ins>,</ins></span> or
</li>
<li>
appears within a linkage-specification ([dcl.link])<span class="add" style="color: #006e28"><ins>, or</ins></span>
</li>
<li>
<span class="add" style="color: #006e28"><ins>is a function named main
that belongs to the global scope</ins></span>
</li>
</ul>
<p>it is attached to the global module.</p>
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