Am 10.02.20 um 14:23 schrieb Kyle Edwards via CMake:
On Mon, 2020-02-10 at 12:49 +0000, hex wrote:
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
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.
Actually, I think it kind of does, at least with the Ninja generator. If
you do something like
add_custom_output(OUTPUT fileA COMMAND ...)
add_custom_target(fileA_target DEPENDS fileA)
add_custom_output(OUTPUT fileB DEPENDS fileA COMMAND ...)
add_custom_output(OUTPUT fileC DEPENDS fileA_target COMMAND ...)
then both fileB and fileC will depend on fileA existing, but fileC will
not be rebuilt if fileA changes, while fileB will be.
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.
With kind regards,
Eric Dönges
*Dr. Eric Dönges*
Senior Software Engineer
MVTec Software GmbH | Arnulfstr. 205 | 80634 Munich | Germany
doen...@mvtec.com <mailto:doen...@mvtec.com> | Tel: +49 89 457 695-0 |
www.mvtec.com <http://www.mvtec.com>
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