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| <div class="section" id="cmp0058"> |
| <span id="policy:CMP0058"></span><h1>CMP0058<a class="headerlink" href="#cmp0058" title="Permalink to this headline">ΒΆ</a></h1> |
| <p>Ninja requires custom command byproducts to be explicit.</p> |
| <p>When an intermediate file generated during the build is consumed |
| by an expensive operation or a large tree of dependents, one may |
| reduce the work needed for an incremental rebuild by updating the |
| file timestamp only when its content changes. With this approach |
| the generation rule must have a separate output file that is always |
| updated with a new timestamp that is newer than any dependencies of |
| the rule so that the build tool re-runs the rule only when the input |
| changes. We refer to the separate output file as a rule’s <em>witness</em> |
| and the generated file as a rule’s <em>byproduct</em>.</p> |
| <p>Byproducts may not be listed as outputs because their timestamps are |
| allowed to be older than the inputs. No build tools (like <code class="docutils literal"><span class="pre">make</span></code>) |
| that existed when CMake was designed have a way to express byproducts. |
| Therefore CMake versions prior to 3.2 had no way to specify them. |
| Projects typically left byproducts undeclared in the rules that |
| generate them. For example:</p> |
| <div class="highlight-cmake"><div class="highlight"><pre><span></span><span class="nb">add_custom_command</span><span class="p">(</span> |
| <span class="s">OUTPUT</span> <span class="s">witness.txt</span> |
| <span class="s">COMMAND</span> <span class="o">${</span><span class="nv">CMAKE_COMMAND</span><span class="o">}</span> <span class="s">-E</span> <span class="s">copy_if_different</span> |
| <span class="o">${</span><span class="nv">CMAKE_CURRENT_SOURCE_DIR</span><span class="o">}</span><span class="s">/input.txt</span> |
| <span class="s">byproduct.txt</span> <span class="c"># timestamp may not change</span> |
| <span class="s">COMMAND</span> <span class="o">${</span><span class="nv">CMAKE_COMMAND</span><span class="o">}</span> <span class="s">-E</span> <span class="s">touch</span> <span class="s">witness.txt</span> |
| <span class="s">DEPENDS</span> <span class="o">${</span><span class="nv">CMAKE_CURRENT_SOURCE_DIR</span><span class="o">}</span><span class="s">/input.txt</span> |
| <span class="p">)</span> |
| <span class="nb">add_custom_target</span><span class="p">(</span><span class="s">Provider</span> <span class="s">DEPENDS</span> <span class="s">witness.txt</span><span class="p">)</span> |
| <span class="nb">add_custom_command</span><span class="p">(</span> |
| <span class="s">OUTPUT</span> <span class="s">generated.c</span> |
| <span class="s">COMMAND</span> <span class="s">expensive-task</span> <span class="s">-i</span> <span class="s">byproduct.txt</span> <span class="s">-o</span> <span class="s">generated.c</span> |
| <span class="s">DEPENDS</span> <span class="o">${</span><span class="nv">CMAKE_CURRENT_BINARY_DIR</span><span class="o">}</span><span class="s">/byproduct.txt</span> |
| <span class="p">)</span> |
| <span class="nb">add_library</span><span class="p">(</span><span class="s">Consumer</span> <span class="s">generated.c</span><span class="p">)</span> |
| <span class="nb">add_dependencies</span><span class="p">(</span><span class="s">Consumer</span> <span class="s">Provider</span><span class="p">)</span> |
| </pre></div> |
| </div> |
| <p>This works well for all generators except <span class="target" id="index-0-generator:Ninja"></span><a class="reference internal" href="../generator/Ninja.html#generator:Ninja" title="Ninja"><code class="xref cmake cmake-generator docutils literal"><span class="pre">Ninja</span></code></a>. |
| The Ninja build tool sees a rule listing <code class="docutils literal"><span class="pre">byproduct.txt</span></code> |
| as a dependency and no rule listing it as an output. Ninja then |
| complains that there is no way to satisfy the dependency and |
| stops building even though there are order-only dependencies |
| that ensure <code class="docutils literal"><span class="pre">byproduct.txt</span></code> will exist before its consumers |
| need it. See discussion of this problem in <a class="reference external" href="https://github.com/martine/ninja/issues/760">Ninja Issue 760</a> |
| for further details on why Ninja works this way.</p> |
| <p>Instead of leaving byproducts undeclared in the rules that generate |
| them, Ninja expects byproducts to be listed along with other outputs. |
| Such rules may be marked with a <code class="docutils literal"><span class="pre">restat</span></code> option that tells Ninja |
| to check the timestamps of outputs after the rules run. This |
| prevents byproducts whose timestamps do not change from causing |
| their dependents to re-build unnecessarily.</p> |
| <p>Since the above approach does not tell CMake what custom command |
| generates <code class="docutils literal"><span class="pre">byproduct.txt</span></code>, the Ninja generator does not have |
| enough information to add the byproduct as an output of any rule. |
| CMake 2.8.12 and above work around this problem and allow projects |
| using the above approach to build by generating <code class="docutils literal"><span class="pre">phony</span></code> build |
| rules to tell Ninja to tolerate such missing files. However, this |
| workaround prevents Ninja from diagnosing a dependency that is |
| really missing. It also works poorly in in-source builds where |
| every custom command dependency, even on source files, needs to |
| be treated this way because CMake does not have enough information |
| to know which files are generated as byproducts of custom commands.</p> |
| <p>CMake 3.2 introduced the <code class="docutils literal"><span class="pre">BYPRODUCTS</span></code> option to the |
| <span class="target" id="index-0-command:add_custom_command"></span><a class="reference internal" href="../command/add_custom_command.html#command:add_custom_command" title="add_custom_command"><code class="xref cmake cmake-command docutils literal"><span class="pre">add_custom_command()</span></code></a> and <span class="target" id="index-0-command:add_custom_target"></span><a class="reference internal" href="../command/add_custom_target.html#command:add_custom_target" title="add_custom_target"><code class="xref cmake cmake-command docutils literal"><span class="pre">add_custom_target()</span></code></a> |
| commands. This option allows byproducts to be specified explicitly:</p> |
| <div class="highlight-cmake"><div class="highlight"><pre><span></span><span class="nb">add_custom_command</span><span class="p">(</span> |
| <span class="s">OUTPUT</span> <span class="s">witness.txt</span> |
| <span class="s">BYPRODUCTS</span> <span class="s">byproduct.txt</span> <span class="c"># explicit byproduct specification</span> |
| <span class="s">COMMAND</span> <span class="o">${</span><span class="nv">CMAKE_COMMAND</span><span class="o">}</span> <span class="s">-E</span> <span class="s">copy_if_different</span> |
| <span class="o">${</span><span class="nv">CMAKE_CURRENT_SOURCE_DIR</span><span class="o">}</span><span class="s">/input.txt</span> |
| <span class="s">byproduct.txt</span> <span class="c"># timestamp may not change</span> |
| <span class="s">...</span> |
| </pre></div> |
| </div> |
| <p>The <code class="docutils literal"><span class="pre">BYPRODUCTS</span></code> option is used by the <span class="target" id="index-1-generator:Ninja"></span><a class="reference internal" href="../generator/Ninja.html#generator:Ninja" title="Ninja"><code class="xref cmake cmake-generator docutils literal"><span class="pre">Ninja</span></code></a> generator |
| to list byproducts among the outputs of the custom commands that |
| generate them, and is ignored by other generators.</p> |
| <p>CMake 3.3 and above prefer to require projects to specify custom |
| command byproducts explicitly so that it can avoid using the |
| <code class="docutils literal"><span class="pre">phony</span></code> rule workaround altogether. Policy <code class="docutils literal"><span class="pre">CMP0058</span></code> was |
| introduced to provide compatibility with existing projects that |
| still need the workaround.</p> |
| <p>This policy has no effect on generators other than <span class="target" id="index-2-generator:Ninja"></span><a class="reference internal" href="../generator/Ninja.html#generator:Ninja" title="Ninja"><code class="xref cmake cmake-generator docutils literal"><span class="pre">Ninja</span></code></a>. |
| The <code class="docutils literal"><span class="pre">OLD</span></code> behavior for this policy is to generate Ninja <code class="docutils literal"><span class="pre">phony</span></code> |
| rules for unknown dependencies in the build tree. The <code class="docutils literal"><span class="pre">NEW</span></code> |
| behavior for this policy is to not generate these and instead |
| require projects to specify custom command <code class="docutils literal"><span class="pre">BYPRODUCTS</span></code> explicitly.</p> |
| <p>This policy was introduced in CMake version 3.3. |
| CMake version 3.8.2 warns when it sees unknown dependencies in |
| out-of-source build trees if the policy is not set and then uses |
| <code class="docutils literal"><span class="pre">OLD</span></code> behavior. Use the <span class="target" id="index-0-command:cmake_policy"></span><a class="reference internal" href="../command/cmake_policy.html#command:cmake_policy" title="cmake_policy"><code class="xref cmake cmake-command docutils literal"><span class="pre">cmake_policy()</span></code></a> command to set |
| the policy to <code class="docutils literal"><span class="pre">OLD</span></code> or <code class="docutils literal"><span class="pre">NEW</span></code> explicitly. The policy setting |
| must be in scope at the end of the top-level <code class="docutils literal"><span class="pre">CMakeLists.txt</span></code> |
| file of the project and has global effect.</p> |
| <div class="admonition note"> |
| <p class="first admonition-title">Note</p> |
| <p class="last">The <code class="docutils literal"><span class="pre">OLD</span></code> behavior of a policy is |
| <span class="target" id="index-0-manual:cmake-policies(7)"></span><a class="reference internal" href="../manual/cmake-policies.7.html#manual:cmake-policies(7)" title="cmake-policies(7)"><code class="xref cmake cmake-manual docutils literal"><span class="pre">deprecated</span> <span class="pre">by</span> <span class="pre">definition</span></code></a> |
| and may be removed in a future version of CMake.</p> |
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