Hi Eric,
thanks for the comprehensive reply to my query. My follow up comments in
line below.
On 25/05/2019 15:47, Eric Noulard wrote:
Le sam. 25 mai 2019 à 13:51, Bill Somerville <b...@classdesign.com
<mailto:b...@classdesign.com>> a écrit :
Hi Robert,
thanks for that, the target name change does seem to help but I am
still unable to achieve my goal. Here is a simplified example that
demonstrates the problem:
cmake_minimum_required (VERSION 3.1.0 FATAL_ERROR)
project (demo LANGUAGES NONE)
add_custom_target (prog_target COMMAND ${CMAKE_COMMAND} -E touch
prog${CMAKE_EXECUTABLE_SUFFIX})
add_executable (prog IMPORTED)
add_dependencies (prog prog_target)
set_target_properties (prog PROPERTIES IMPORTED_LOCATION
${CMAKE_CURRENT_BINARY_DIR}/prog${CMAKE_EXECUTABLE_SUFFIX})
install (TARGETS prog RUNTIME DESTINATION bin)
which gives the following error at CMake configuration:
CMake Error at CMakeLists.txt:7 (install):
install TARGETS given target "prog" which does not exist.
-- Configuring incomplete, errors occurred!
So the target that 'add_executable(name IMPORTED)' creates is not
a real executable target. I can change its properties but the
'install(TARGETS ...)' command thinks it does not exist. Note that
a executable target is a very simple demonstration and I
understand that I can use 'install(PROGRAM ...)' just about as
easily, but when it comes to a shared library it gets a lot more
complex when using, exporting, and instlling it, and it seems that
IMPORTED targets fall well short of useful when they are actually
produced by the current CMake project.
I can understand that an IMPORTED target is perhaps not meant to
be installable
Robert will give his advice on that but I bet IMPORTED target were
never meant to be drop-in *replacement* of genuine target.
They were meant to ease *reference to* object/lib:executable that are
outside the build of the project.
e.g the doc says:
https://cmake.org/cmake/help/latest/command/add_library.html#imported-libraries
"An IMPORTED library target references a library file located outside
the project"
Nmelly a target that is "already installed somewhere" and that you
want to reference in your CMake build.
That is fine but I don't see why there is a problem with making IMPORTED
targets better CMake citizens, it should be down to the CMake script
author as to whether they take advantage of that. What is there now is a
lie and the add_*(target ... IMPORTED ...) commands are not doing what
they should IMHO.
but if so then your statement that "The goal that you have is
fully supported by CMake" seems to be incorrect. To reiterate, I
am trying to use foreign tools to make binary targets and wish to
have CMake treat them *as if* they were created by supported
languages like C, ++, or Fortran. Am I still missing something?
My opinion is that IMPORTED target are not designed (as of today) for
that purpose.
Agreed.
When you want to do what you seem to want you need to either:
1) add a "new LANGUAGE" (you seem to be willing to add golang)
see:
https://stackoverflow.com/questions/7978517/how-do-i-get-cmake-to-work-with-the-go-programming-language
and may be: https://github.com/aadidenko/golang-cmake-example (or
fork of this)
I am not sure that adding a new CMake supported language is right for
Go. For the Go ecosystem the Go build tools do the right thing and CMake
is largely unnecessary. Particularly the Go build system deals with
dependencies internally so I don't see how it could be added to CMake
without using the lower level Go tools and having to list many source
dependencies that the high level Go build system deals with without
fuss. Note that I am dealing with cgo which allows Go language static or
shared libraries to interoperate with other languages via a C API. This
is where CMake needs to join in because a project using cgo is likely to
build tests, tools, and libraries in languages that CMake does support.
As far as I can see the Go build system support is weak in providing a
"package" from, for example, a cgo shared library, it just builds the
shared object and header files and stops there.
2) define a set of custom macros that mimic genuine target behaviour.
may be see: https://github.com/cpconduce/go_cmake
This is exactly where I started but it only covers tools written in Go
and uses install(PROGRAMS ...) to install them. It uses
add_custom_command() which means all source dependencies must be
specified for reliable rebuilds, which is effectively discarding the
dependency management of the Go build system. It would be very fragile
in a large project and doesn't address the cgo capabilities to create
libraries. My queries here are exactly because I am trying to extend
this approach to tools and libraries that play well in the CMake
ecosystem, without sacrificing the advantages of the Go build system. I
can, and have, written CMake functions that use add_custom_target() to
defer Go builds to the Go build system, but am failing so far to bridge
the results with CMake. Clearly at present IMPORTED targets do not work
for this, so I guess I am asking how to do this another way, e.g. by
setting all the necessary target properties to make my targets proper
CMake citizens?
If you want to have a look at existing similar example shippped with
CMake, have a look at UseJava.cmake module
(https://cmake.org/cmake/help/latest/module/UseJava.html)
You'll see that you have
add_jar, which is similar to add_executable and create a custom target
and
install_jar which is similar to install on genuine target but plays
with target properties and install(FILES...) in order to mimic that.
see: https://github.com/Kitware/CMake/blob/master/Modules/UseJava.cmake
This looks more interesting and complete but the first thing I note is
that it does not really integrate with the CMake ecosystem, instead it
has a parallel set of commands (add|install|find|export)_jar() that
mimic the CMake equivalents. That may work well for Java, even including
a foreign function interface as it seems to support JNI, but there is a
key difference. A Java class file is linked purely at runtime so there
is no need for CMake to know about how to link the JAR file to a native
executable target (it uses add_library(target INTERFACE ...) to
integrate the generated API headers). With Go and cgo the generated
library *is a native library*, just as Go tools are native binaries, and
to use them seamlessly in CMake they have to behave as any other
imported or built executable target, a parallel set of CMake commands
that do not integrate the target will not get the job done.
I'm not developing with golang but AFAIK go has a builtin "build
system" so bringing go as a language in CMake may not be the best option,
but again I am no Go expert.
You are correct and this is exactly why I am looking for a way to hook
the products of the Go build system into CMake rather than trying to add
it as a supported language. I do not know enough abut the low level Go
build system commands to understand if they are suitable for driving
from CMake like it does for C, C++, or Fortran compilers and the target
system linker but that seems to be the wrong approach, I'd rather use
the top level 'go build' command and tell CMake about what I expect it
to build and what top level targets I want to be up to date before that
step. This seems to be a simple add_custom_target() invocation. There is
a small penalty that the 'go build' command will always run but that's
fine since I expect 'go build' to do minimal re-builds based on its own
dependency tracking.
The approach I am current trying is to treat cgo libraries as INTERFACE
libraries and having my CMake scripts reference the library binaries by
their path by misusing target_link_libraries(so_library_target INTERFACE
...) to hook things up. This doesn't help with installing the library
target but that can be done using install(PROGRAMS ...). That at least
allows install(EXPORT ...) to work, I think - I have not yet tried
importing the shared library into anther CMake project.
The only reference of that kind of thing I found in the CMake mailing
list is oldish:
https://cmake.org/pipermail/cmake-developers/2011-August/013715.html
That link shows exactly why using the Go build tools as a custom target
command is right IMHO, it also doesn't address using the product of the
build in the same or other projects.
May be adding the support for "install" command for IMPORTED target is
doable but this seems to be a feature request to be discussed on
developer mailing list;
I think there is justification since a CMake script author decides
whether to take advantage of it. It seems to me that there is stuff that
is internal to the CMake C++ application similar to properties but not
addressable by the script author. Of course it all depends how complex
that stuff is and whether if can be addressed by either some new target
properties or extra arguments to the add_(executable|library)(target
IMPORTED) commands. The latter seems easiest as the internal logic and
data model need not be exposed for misuse by script authors.
If there really is not a practical way to build a library or tool with a
foreign toolchain in CMake and have it integrated like the ones produced
by supported languages then I will raise an issue on the development
forum, but TBH I am disappointed that manipulating existing target
properties is not sufficient. Custom targets seem to fall well short of
what they promise.
Regards,
Eric
Regards
Bill Somerville.
Regards
Bill Somerville.
On 24/05/2019 20:23, Robert Maynard wrote:
Hi,
The goal that you have is fully supported by CMake. You have just run
into a bug in CMake, and you should report this to
https://gitlab.kitware.com/cmake/cmake/issues .
Basically at a very high level the name out the add_executable target
`callback_generator` is the same as the internal name that CMake is
using for the add_custom_command. This than causes some logic in CMake
(cmTargetTraceDependencies) to incorrectly build another link between
the add_custom_command and your add_executable.
The easiest way to solve this issue is name the add_executable target
to have a different name than the output of your custom command minus
file extension. So maybe something like `callback_generator_exec`
--
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