On Mon, Oct 7, 2019, at 6:55 AM, Craig Scott wrote:
>
>
> On Mon, Oct 7, 2019 at 9:32 PM Setzer Sebastian (CM-CI2/ECS2) via CMake
> <cmake@cmake.org> wrote:
>> Dear list,
>> The manual says:
>> https://cmake.org/cmake/help/latest/command/add_custom_target.html
>> Reference files and outputs of custom commands created with
>> add_custom_command() command calls in the same directory (CMakeLists.txt
>> file). They will be brought up to date when the target is built.
>> Use the add_dependencies() command to add dependencies on other targets.
>>
>> This is different from what the manual of add_custom_command says, and
>> would be surprising for all users who only read the manual of
>> add_custom_command and then think they know what the parameter means for
>> add_custom_target.
>>
>> But on the other hand, when I test with this:
>> --------------------------------------------------
>> cmake_minimum_required (VERSION 3.14)
>> project(dependency_test LANGUAGES)
>>
>> add_custom_target(T1
>> COMMAND echo T1
>> )
>> add_custom_target(T2
>> COMMAND echo T2
>> DEPENDS T1
>> )
>>
>> # cmake -GNinja -B build .
>> # ninja T2
>> --------------------------------------------------
>> Then T1 and T2 are built, so contrary to what the manual says, it seems to
>> work.
>>
>> Is the manual just outdated (Maybe behavior has changed and only manual for
>> add_custom_command has been updated)?
>> Should it be the same as for add_custom_command, or are there really some
>> differences?
>
> As it happens, I just updated the docs for these in the last day or so. You
> can find it in the not-yet-merged merge request here:
>
> https://gitlab.kitware.com/cmake/cmake/merge_requests/3891
>
> I wrote an explanation of the specific behavior of the dependencies in the
> associated issue here:
>
> https://gitlab.kitware.com/cmake/cmake/issues/19771#note_635547
>
> There is a difference between the two, but it's subtle and specific to the
> sort of scenario I mentioned in that issue comment.
>
> --
> Craig Scott
> Melbourne, Australia
> https://crascit.com
>
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I found the wording for add_custom_command and add_custom_target to be
confusing and had to look at examples (and books) to understand it better.
The DEPENDS field is kind of different for the two commands.
It may be that add_custom_target "DEPENDS" can accept other custom targets; it
seems from your example it does (which wouldn't surprise me from what I
remember from the CMake source code).
For add_custom_target:
The phrase "They will be brought up to date when the target is built." had me
thinking that this was going to force the custom command to always run.
Its more of a "The commands of add_custom_target() outputs will be run as
needed before any add_custom_target() commands are run."
If a file listed in DEPENDS does not exist on disk then behavior is defined by
the build tool and may or may not throw an error. I recall that Make and Ninja
throws errors and MSBuild will have a warning.
Then there is the SOURCES field which acts like:
Similar to ``DEPENDS`` with the addition that the specified files are checked
to exist on disk and will be added to IDE project files for convenience in
editing.
"add_custom_target() sometimes builds SOURCES for Visual Studio generators but
never for Makefiles"
https://gitlab.kitware.com/cmake/cmake/issues/10360
For add_custom_command it acts like (and this includes the trick that is used
to make it always run):
If ``DEPENDS`` is not specified the command will run whenever the ``OUTPUT`` is
missing or it will run whenever a file-level dependency is updated. If the
command does not create one of the listed ``OUTPUT`` then the rule will always
run. For example defining an output file called ALL or FORCE that is never
created.
Then for the add_custom_command signature that is used for build events. You
don't need to put the event. It defaults to POST_BUILD. This is different from
the documentation that shows it is a required field.
--
Fred
--
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