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When trying to use OpenMP in a 32bit build with find_package(OpenMP REQUIRED), it tries to link with the 64bit library instead. It should be reproducible with this:

cmake_minimum_required(VERSION 3.20)

project(test_32bit_openmp LANGUAGES CXX)

find_package(OpenMP REQUIRED)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -m32")

include(CheckCXXSourceCompiles)

set(CMAKE_REQUIRED_FLAGS "-m32")
set(CMAKE_REQUIRED_LIBRARIES OpenMP::OpenMP_CXX)

check_cxx_source_compiles(
"
#include <omp.h>
int main(void) {

  omp_get_max_threads();
  return 0;
}
"
    RESULT_VAR
)

if(NOT RESULT_VAR)
    message(SEND_ERROR "check_cxx_source_compiles with openmp failed!")
endif()

error logs show:

Performing C++ SOURCE FILE Test RESULT_VAR failed with the following output:
Change Dir: cmake-openmp/build-test/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_2716f && [1/2 100.4/sec] Building CXX object CMakeFiles/cmTC_2716f.dir/src.cxx.o
[2/2 149.2/sec] Linking CXX executable cmTC_2716f
FAILED: cmTC_2716f 
: && /usr/bin/c++ -m32 -m32  CMakeFiles/cmTC_2716f.dir/src.cxx.o -o cmTC_2716f  /usr/lib/gcc/x86_64-linux-gnu/7/libgomp.so  /usr/lib/x86_64-linux-gnu/libpthread.so && :
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/7/libgomp.so: error adding symbols: file in wrong format
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.


Source file was:

#include <omp.h>
int main(void) {

  omp_get_max_threads();
  return 0;
}

I have the 32bit library available:

$ find /usr/lib/gcc -name libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/11/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/9/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/7/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/7/x32/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/7/32/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/10/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/10/x32/libgomp.so
/usr/lib/gcc/x86_64-linux-gnu/10/32/libgomp.so

Current workaround is to set -DOpenMP_gomp_LIBRARY:FILEPATH=/usr/lib/gcc/x86_64-linux-gnu/7/32/libgomp.so Which isn't ideal.