CMake Tutorial
In Step 6 of the CMake tutorial, it says "To create a source distribution you would type" cpack --config CPackSourceConfig.cmake. I tried this command and get four tarballs: output.tar.bz, output.tar.gz, output.tar.xzand output.tar.Z.
The tarball is just a compression of the current directory. The Makefile packed contains absolute path of my current system. It means I need to run cmake again if I distribute it to other system.
So what is the point of using cpack?
One of the point of using CPack for create source package is to not including version-control files into resulted tarball (see CPACK_SOURCE_IGNORE_FILES variable).
Also it is possible to futher modify list of files, intended for source package. E.g., one can exclude internal documentation (which is useful for developers only) but include some pre-generated files, so user doesn't require to have additional tools, like Flex/Bison, to build source package.
Though CPack (and especially its source package functionality) is poorly documented (see, e.g., this mailing and this bugreport), source package generation is relatively flexible.
Related
I'm trying to use find_package to include libraries in CMake.
This question talks about how to tell CMake to link to the GMP library (external). I am trying to follow the steps of the answer there but do not have any of the <name>Config.cmake or <name>-config.cmake files, as mentioned by some of the comments, which appears to be the default. The answer does not mention any solution for when you don't know how to get/find these files. The comments to that answer link to an old website (external) with a lot of broken links, that describes a list of Load Modules. It's unclear to me where these modules come from and how to get them.
According to the official CMake documentation (external), if the configuration files are not found, find_package falls back from "Module Mode" to "Config Mode". I don't understand what this means and in what cases this would be relevant, especially since the documentation discourages reading about "Config Mode".
The documentation says that
The file is first searched in the CMAKE_MODULE_PATH, then among the Find Modules provided by the CMake installation.
I am still confused about whether these configuration files are supposed to come with CMake or with the library in question and where they are supposed to be located. Probably both are possible but how does one know in a specific case?
Example code, trying to follow modern best practices:
# CMakeLists.txt (not working)
cmake_minimum_required(VERSION 3.2) # I have no idea what version I actually need
project (GMP_demo_project)
# Enable C++17 standard
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
find_package(GMP REQUIRED)
# Create the executable from sources
add_executable(GMP_demo GMP_demo.cpp)
target_link_libraries(GMP_demo gmp gmpxx)
The code outputs an error message along the lines of
CMake Error at CMakeLists.txt:10 (find_package):
By not providing "FindGMP.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "GMP", but
CMake did not find one.
Could not find a package configuration file provided by "GMP" with any of
the following names:
GMPConfig.cmake
gmp-config.cmake
Add the installation prefix of "GMP" to CMAKE_PREFIX_PATH or set "GMP_DIR"
to a directory containing one of the above files. If "GMP" provides a
separate development package or SDK, be sure it has been installed.
Question: How does one, in general, obtain and organize these configuration files (CMake Load Modules)? How can one expect another user to have these files on his system? My question is intended to be general and only use GMP as an example (although I am in fact interested in being able to use it).
Just as an aside, I can compile, link and execute my demo code just fine using gcc GMP_demo.cpp -lstdc++ -lgmp after having installed GMP as suggested by the library documentation. The problem is just getting CMake to do it. I can also just give CMake the absolute path of the library, which would of course be much easier but not portable (assuming one can get find_package to actually work and be portable with reasonable amounts of work).
How does one, in general, obtain and organize these configuration files (CMake Load Modules)?
Broadly speaking, there are three buckets these fall into:
Files provided directly by the package. This is the ideal solution, and would be what CMake calls Config mode. There would be a file called GMPConfig.cmake which cmake could find by searching preconfigured paths, or by providing a specific path at configuration time (cmake -DGMP_Dir=/path/to/GMP/install/root). The advantages of this approach are that generation of GMPConfig.cmake is mostly automatic, and the libraries can include things like installation paths and compilation flags. The disadvantage is that the library develops have to actually go to the effort of leveraging modern CMake, and not everybody does this.
Files provided directly by CMake. For common packages (e.g., boost) CMake ships FindXXX.cmake files that search well-known paths and take care of this for you. These work identically to the above from an end-user perspective, but which Find modules are available depends on the version of CMake you have installed.
Files provided by some random person that are copy/pasted into projects. How these works depends on the person who wrote it, so you'll have to read their documentation. Use your favorite search engine and try to find FindGMP.cmake, then drop it in a module folder somewhere and update CMAKE_MODULE_PATH appropriately.
How can one expect another user to have these files on his system?
It's your job to install whatever dependencies a package requires. Anything using modern CMake (bullet 1 listed above) should install the XXXConfig.cmake file as part of its installation. If a library is built by something other than CMake, you'd have to either hope for bullet #2, or find/write your own FindXXX.cmake file (bullet #3).
For your specific case, you might be better off with find_library, since your sample compilation line looks like it just needs to link.
I'm attempting to make a CMake package for Crypto++ inclusion in CMake projects, this will end up in the noloader/cryptopp-cmake repo if it gets done.
The ultimate goal is to come up with a working cross-platform FindCryptoPP.cmake file which can be dropped in the Crypto++ source directory to do things like:
find_package(CryptoPP REQUIRED)
target_link_libraries(libbiocoin cryptopp-static)
Or:
find_package(CryptoPP REQUIRED)
target_link_libraries(libbiocoin cryptopp-shared)
In a finished application and have it "just work."
My current best solution within a CMake application is to build Crypto++ for the platform, stick the resulting archive or library in a lib directory, reference that within the CMakeLists.txt and pull it in that way, but of course that requires packaging a binary distribution of the compiled Crypto++ for every platform targeted by the application, which would be nasty to maintain and generally bad even if it weren't crypto code.
It's better to provide a CMake configuration file. find_package will look for a configuration file if no FindFoo.cmake find script is provided. One advantage over a find script is that you won't end with different, maybe conflicting versions of the find script.
See https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html, especially the section Create Layout.
Separating packages into main and -devel parts looks like a good established practice, which allows to save some space for those who are not building software. On Linux, a library that uses SONAME versioning can have two or more files representing a single library, e.g.:
libABC.so - linker script, e.g. INPUT (libABC.so.1)
libABC.so.1 - real binary
Where .so file is really used at the build time only, redirecting actual dependency to the versioned file. If so, shouldn't it be placed into e.g. libABC-devel.rpm along with include files? It looks inappropriate to put both files to a run-time package, e.g. libABC.rpm.
E.g. one observation, if you inspect ls /usr/lib/lib*.so*, many of versioned files do not have corresponding lib*.so pair.
I also noticed that e.g. Fedora and Conda-forge maintainers replace these linker scripts with symlinks (1, 2) and package them along with the main binary. Is there any reason to do that despite that original library builds linker scripts instead?
I'll accept an answer which summarizes arguments for one or another way of packaging linker scripts into runtime/devel for sake of discussion with package maintainers.
After running the cmake command once to generate a build system, when, if ever, should I rerun the cmake command?
The generated build systems can detect changes in the associated CMakeLists.txt files and behave accordingly. You can see the logic for doing so in generated Makefiles. The exact rules for when this will happen successfully are mysterious to me.
When should I rerun cmake? Does the answer depend on the generator used?
This blog post (under heading: "Invoking CMake multiple times") points out the confusion over this issue and states that the answer is actually 'never', regardless of generator, but I find that surprising. Is it true?
The answer is simple:
The cmake binary of course needs to re-run each time you make changes to any build setting, but you wont need to do it by design; hence "never" is correct regarding commands you have to issue.
The build targets created by cmake automatically include checks for each file subsequently [=starting from the main CMakeLists.txt file] involved or included generating the current set of Makefiles/VS projects/whatever. When invoking make (assuming unix here) this automatically triggers a previous execution of cmake if necessary; so your generated projects include logic to invoke cmake itself! As all command-line parameters initially passed (e.g. cmake -DCMAKE_BUILD_TYPE=RELEASE .. will be stored in the CMakeCache.txt, you dont need to re-specify any of those on subsequent invocations, which is why the projects also can just run cmake and know it still does what you intended.
Some more detail:
CMake generates book-keeping files containing all files that were involved in Makefile/Project generation, see e.g. these sample contents of my <binary-dir>/CMakeFiles/Makefile.cmake file using MSYS makefiles:
# The top level Makefile was generated from the following files:
set(CMAKE_MAKEFILE_DEPENDS
"CMakeCache.txt"
"C:/Program Files (x86)/CMake/share/cmake-3.1/Modules/CMakeCCompiler.cmake.in"
"C:/Program Files (x86)/CMake/share/cmake-3.1/Modules/RepositoryInfo.txt.in"
"<my external project bin dir>/release/ep_tmp/IRON-cfgcmd.txt.in"
"../CMakeFindModuleWrappers/FindBLAS.cmake"
"../CMakeFindModuleWrappers/FindLAPACK.cmake"
"../CMakeLists.txt"
"../CMakeScripts/CreateLocalConfig.cmake"
"../Config/Variables.cmake"
"../Dependencies.cmake"
"CMakeFiles/3.1.0/CMakeCCompiler.cmake"
"CMakeFiles/3.1.0/CMakeRCCompiler.cmake")
Any modification to any of these files will trigger another cmake run whenever you choose to start a build of a target. I honestly dont know how fine-grained those dependencies tracking goes in CMake, i.e. if a target will just be build if any changes somewhere else wont affect the target's compilation. I wouldn't expect it as this can get messy quite quickly, and repeated CMake runs (correctly using the Cache capabilities) are very fast anyways.
The only case where you need to re-run cmake is when you change the compiler after you started a project(MyProject); but even this case is handled by newer CMake versions automatically now (with some yelling :-)).
additional comment responding to comments:
There are cases where you will need to manually re-run cmake, and that is whenever you write your configure scripts so badly that cmake cannot possibly detect files/dependencies you're creating. A typical scenario would be that your first cmake run creates files using e.g. execute_process and you would then include them using file(GLOB ..). This is BAD style and the CMake Docs for file explicitly say
Note: We do not recommend using GLOB to collect a list of source files from your source tree. If no CMakeLists.txt file changes when a source is added or removed then the generated build system cannot know when to ask CMake to regenerate.
Btw this comment also sheds light on the above explained self-invocation by the generated build system :-)
The "proper" way to treat this kind of situations where you create source files during configure time is to use add_custom_command(OUTPUT ...), so that CMake is "aware" of a file being generated and tracks changes correctly. If for some reason you can't/won't use add_custom_command, you can still let CMake know of your file generation using the source file property GENERATED. Any source file with this flag set can be hard-coded into target source files and CMake wont complain about missing files at configure time (and expects this file to be generated some time during the (first!) cmake run.
Looking into this topic for reading the version information from a debian/changelog file (generation phase), I ran in the topic that cmake execution should be triggered as debian/changelog is modified. So I had the need to add debian/changelog to CMAKE_MAKEFILE_DEPENDS.
In my case, debian/changelog is read through execute_process. Execute_process unfortunately gives no possibility to add files processed to CMAKE_MAKEFILE_DEPENDS. But I found that running configure_file will do it. Actually I am really missing something like DEPENDENCIES in execute_process.
However, as I had the need to configure the debian/changelog file for my needs, the solution came implicitly to me.
I actually also found a hint about this in the official documentation of configure_file:
"If the input file is modified the build system will re-run CMake to re-configure the file and generate the build system again."
So using configure_file should be a safe to trigger the re-run of cmake.
From a user perspective, I would expect other commands to extend CMAKE_MAKEFILE_DEPENDS, too. E.g. execute_process (on demand) but also file(READ) (implicitly like configure_file). Perhaps there are others. Each read file is likely to influence the generation phase. As an alternative it would be nice to have a command to just extend the dependency list (hint for the cmake developers, perhaps one comes along).
A quote from the official documentation:
"Specify rules to run at install time."
What exactly is install time?
The problem for me: I am on Linux, software is installed from packages that are just dependencies and data. There is no CMake that can do anything here. So installation time of software is out of scope from CMake. So what exactly do they mean?
Building a CMake project can roughly be divided into three phases:
Configure time. This includes everything that happens while running cmake itself. This phase is concerned with inspecting certain properties of the host system and generating the specific build files for that platform under the selected configuration.
Build time. This includes everything that happens while actually building your project from the files generated by CMake (like, when running cmake --build or make). This is where all of the actual compilation and linking happens, so at the end of the build phase, you have a usable binary.
Install time. This includes everything that happens when running the INSTALL target generated by CMake (like, when running cmake --build --target install or make install). This takes care of copying the binaries that were generated into the build tree to a different directory. Note that the build tree contains a lot of stuff that is no longer needed once the build is completed if you are only interested in running the binary. Examples include all intermediate build artifacts, like the build files generated during the configure phase or the intermediate object files created during the build phase. Furthermore, the install phase might include additional steps to ensure that the binaries produced during the build are portable. For instance, on Linux systems you might want to remove the build directory from the shared library search path in the binary and replace it with a portable equivalent. So the install phase might do more than just copy all the important files to a new directory. It could also include additional steps that change the binaries to make them more portable.
Note that the last phase is optional. If you do not want to support calling make install but prefer another deployment mechanism, you simply don't use the install command in your CMake script and no INSTALL target will be generated.
I'd like to expand the answer, which ComicSansMS gave you, a little bit.
As he mentioned - CMake generates an extra target called install for the make tool (when you use a Makefile-based generator).
It may look weird for you as a package system is used for Linux. However the install target is still useful or even necessary:
When you develop your application you may need to install (move binaries and possibly some include files) to a certain location so some of your projects may see each other. For example, you may develop a library and a set of non-related applications which use it. Then this library must be installed somewhere to be visible. It doesn't mean you need to put it to the /usr directory; you may use your /home.
The process of Linux package preparation requires an install step. For example, the RPM packaging system does three main steps when the rpm package file is being built: the project is configured, then is compiled and linked and finally is being installed to a certain location. All files from this location are being packed to the rpm file.