Trouble shooting
Finding MPI
Most implementations of MPI come with a compiler wrapper. It is a good practice to provide the CMake compiler wrapper to CMake. CMake will often figure out that the compiler provided is a MPI wrapper, set the MPI_FOUND variable to true and understand that no additional options or flags are required when building the project.
Using find_package
The command find_package can be used to find any package or library the project depends on. It will look for the package in a pre-defined set of directories and will return the first one it finds. If CMake can find a package, it will import new targets which you can later used in your project.
You can find a list of packages supported by CMake developers on the main documentation.
To obtain a list of supported packages the --help-module-list argument can be used together with filtering for entries that start with "Find":
If CMake cannot find the package and you marked it as required, it will throw a fatal error. In such situation, or if a wrong version is found, you can specify the location of the package by setting the variable <PACKAGE_NAME>_ROOT. CMake will then look for the package in that folder.
Warning
Be aware that that behaviour of find_package has changed over the years and older versions of CMake use different rules to find packages. For example, some older versions will ignore the <PACKAGE_NAME>_ROOT variable and others will ignore it but print a policy warning.
Cmake policy warning
Sometimes CMake's output reports the message:
Error
CMake Warning (dev) at CMakeLists.txt:6 (find_package):
Policy CMP0074 is not set: find_package uses
Environment variable MPI_ROOT is set to:
/mnt/lustre/indy2lfs/sw/intel/compilers_and_libraries_2020.4.304/linux/mpi/intel64
For compatibility, CMake is ignoring the variable. This warning is for project developers. Use -Wno-dev to suppress it.
This appears when a default behaviour changes between different CMake versions. Whenever a behavioural change is introduced in CMake the old default will be kept for a few versions and the above deprecation warning printed. Package developers can set the new behaviour by setting the corresponding policy to new. Reading the warning and the policy definition might help asses whether the deprecation will affect your build or not.
Tracing
When debugging a CMake installation, you may find useful to print out the commands being executed by CMake. This is possible by adding to the cmake command the flag --trace-source=${CMAKE_FILE}, where ${CMAKE_FILE} is the name of the file containing the CMake code. For instance to debug CMake code contained in the CMakeLists.txt file you can use:
By adding the flag --trace-expand each variable will be substituted with their current value.