## Summary Through no fault of the `FindOpenMP.cmake` module that I can find, between CMake 3.19.7 and CMake 3.20.0, the default inferred OpenMP library type has changed from shared to static. Since default distributions of `libgomp.a` are not `-fPIC`, this can fail shared library builds. ## Reproduce Shared library build: ```cmake find_package(OpenMP REQUIRED) [...] target_link_libraries([...] OpenMP::OpenMP_CXX) ``` On a `centos:7` docker image with `devtoolset-9`, looking at `CMakeFiles/CMakeOutput.log` where `FindOpenMP` parses the output of a trial `-fopenmp` compilation to figure out flags and link line: ``` [...] implicit libs: [stdc++;m;gomp;gcc_s;gcc;pthread;c;gcc_s;gcc] implicit dirs: [/opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9;/opt/rh/devtoolset-9/root/usr/lib64;/lib64;/usr/lib64;/opt/rh/devtoolset-9/root/usr/lib] ``` Then the `find_library` calls for each implicit lib name in `FindOpenMP.cmake`, around line 260: ```cmake find_library(OpenMP_${_OPENMP_IMPLICIT_LIB_PLAIN}_LIBRARY NAMES "${_OPENMP_IMPLICIT_LIB_NAME}" DOC "Path to the ${_OPENMP_IMPLICIT_LIB_PLAIN} library for OpenMP" HINTS ${OpenMP_${LANG}_IMPLICIT_LINK_DIRS} CMAKE_FIND_ROOT_PATH_BOTH NO_DEFAULT_PATH ) ``` Finding, per `CMakeCache.txt`: ``` //Path to the gomp library for OpenMP OpenMP_gomp_LIBRARY:FILEPATH=/opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/libgomp.a ``` Finally populating the link line with, per `link.txt`: ``` [...] /opt/rh/devtoolset-9/root/usr/lib/gcc/x86_64-redhat-linux/9/libgomp.a [...] ``` Which fails the link by relocation errors hinting at `libgomp.a` not having been built with `-fPIC`. This is faulty because the `-fopenmp` flag should imply shared linking with GCC as far as I know. CMake 3.19 does not show this behavior, finding `libgomp.so` instead. I can reproduce this on ubuntu docker images too. ## Suggested culprits I'm frankly stumped by the change in behavior. I've only delved into the CMake module files but haven't looked into the C++ source files. I don't think any changes between 3.19 and 3.20 in the `FindOpenMP.cmake` module can have caused this. My unqualified opinion on the matter is that `find_library` might have changed to cause this. I might also only have myself to blame through something I don't understand or didn't know about. ## Suggested fixes It might be possible to augment the `find_library` call with the right library prefixes and suffixes which could be detected by the true linked libraries with `ldd` or so. As far as I can tell, the compiler output that is parsed doesn't actually offer any clues as to which OpenMP library type was linked (short of tracking the spec file and imitating its logic), and only the resulting binary has that information. If that is true, then `FindOpenMP.cmake` is currently somewhat fragile. Perhaps the information of whether to link static or shared openmp is present on the link line, but the parsing module does not capture that information. Alternatively, the OpenMP target property population module code could be altered not to include any found `omp` library at all, since this is implied in the `-fopenmp` flag in the few compilers I know. There is also no way to select the OpenMP library type desired, so this may present an opportunity to improve the module.