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<div class="moz-cite-prefix">Am 10.02.20 um 14:23 schrieb Kyle
Edwards via CMake:<br>
</div>
<blockquote type="cite"
cite="mid:1581341028.31213.1.camel@kitware.com">
<pre class="moz-quote-pre" wrap="">On Mon, 2020-02-10 at 12:49 +0000, hex wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">hello,
My build step is not thread-safe (the instruction in the COMMAND
part). Every build step depends on one source file:
add_custom_command(
DEPENDS on_source.file
OUTPUT
object_file_for_source_file
COMMAND not-thread-safe-compiler --build on_source.file
COMMENT
"Building Source object"
)
add_custom_target DEPENDS on OUTPUT from all custom command's, but in
order to prevent parallel execution when cmake is called:
cmake --build . --target all -j30
I need to build each step in any order, but still not as a dependency
as this would trigger unnecessary recompilation. Using
MAIN_DEPENDENCY defines hierarchical dependency, but this won't help
in this situation.
I cannot figure out how to prevent parallel execution of COMMAND in
CMake. The only option I see is a single command
add_custom_target(tgt
DEPENDS on_all_source.files
COMMAND not-thread-safe-compiler --build on_source1.file
COMMAND not-thread-safe-compiler --build on_source2.file
COMMAND not-thread-safe-compiler --build on_source3.file
. . .
COMMENT
"Building target"
)
this would rebuild every source, though.
How do I set this up?
thank you
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
It sounds like you want order-only dependencies: dependencies the
command waits for before building, but whose changes do not actually
trigger a rebuild. As far as I know, add_custom_command() doesn't have
support for this - perhaps it could be added.</pre>
</blockquote>
<p>Actually, I think it kind of does, at least with the Ninja
generator. If you do something like</p>
<p>add_custom_output(OUTPUT fileA COMMAND ...)<br>
add_custom_target(fileA_target DEPENDS fileA)</p>
<p>add_custom_output(OUTPUT fileB DEPENDS fileA COMMAND ...)<br>
add_custom_output(OUTPUT fileC DEPENDS fileA_target COMMAND ...)</p>
<p>then both fileB and fileC will depend on fileA existing, but
fileC will not be rebuilt if fileA changes, while fileB will be.<br>
</p>
<p>Disclaimer - I couldn't find this behavior described in the
documentation, but it is what I've observed after spending
extensive amounts of time messing about with add_custom_command()
and add_custom_target() to build the documentation to our product
with CMake 3.16. Also, I've only tested this with the Ninja
generator, so it may behave differently with other generators.</p>
<p>With kind regards,<br>
Eric Dönges<br>
</p>
<blockquote type="cite"
cite="mid:1581341028.31213.1.camel@kitware.com">
</blockquote>
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