[CMake] FindThreads and toolchains

[Dave Flogeras]

I am using CMake 3.7.2 and playing around with the FindThread finder.  The
following simple CMakeLists.txt works fine on various compilers: GCC,
clang, MSVC (tried 2013 and 2015).



CMAKE_MINIMUM_REQUIRED( VERSION 3.1 )

ADD_EXECUTABLE( mythread main.cpp )
SET( THREADS_PREFER_PTHREAD_FLAG TRUE )
FIND_PACKAGE( Threads REQUIRED )
TARGET_LINK_LIBRARIES( thread Threads::Threads )
TARGET_COMPILE_OPTIONS( thread INTERFACE Threads::Threads )



However, when I attempt to cross-compile for ARM with "cmake
-DCMAKE_TOOLCHAIN_FILE=blah ..", I get the following:



-- The C compiler identification is GNU 4.9.4
-- The CXX compiler identification is GNU 4.9.4
-- Check for working C compiler:
/usr/bin/armv6j-hardfloat-linux-gnueabi-gcc-4.9.4
-- Check for working C compiler:
/usr/bin/armv6j-hardfloat-linux-gnueabi-gcc-4.9.4 -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler:
/usr/bin/armv6j-hardfloat-linux-gnueabi-g++-4.9.4
-- Check for working CXX compiler:
/usr/bin/armv6j-hardfloat-linux-gnueabi-g++-4.9.4 -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Check if compiler accepts -pthread
CMake Error: TRY_RUN() invoked in cross-compiling mode, please set the
following cache variables appropriately:
   THREADS_PTHREAD_ARG (advanced)
For details see /tmp/thread/build/TryRunResults.cmake
-- Check if compiler accepts -pthread - no
-- Found Threads: TRUE
-- Configuring incomplete, errors occurred!
See also "/tmp/thread/build/CMakeFiles/CMakeOutput.log".
See also "/tmp/thread/build/CMakeFiles/CMakeError.log".


I can verify that the compiler does indeed accept -pthread, and IIRC might
require it for pthreads to work.

I can see from the CMakeError.log that it might be forgetting to specify
-lpthread when trying to compile the test:


Run Build Command:"/usr/bin/gmake" "cmTC_13694/fast"
/usr/bin/gmake -f CMakeFiles/cmTC_13694.dir/build.make
CMakeFiles/cmTC_13694.dir/build
gmake[1]: Entering directory '/tmp/thread/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_13694.dir/CheckSymbolExists.c.o
/usr/bin/armv6j-hardfloat-linux-gnueabi-gcc-4.9.4
--sysroot=/mnt/data/chroots/pie/    -o
CMakeFiles/cmTC_13694.dir/CheckSymbolExists.c.o   -c
/tmp/thread/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_13694
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_13694.dir/link.txt
--verbose=1
/usr/bin/armv6j-hardfloat-linux-gnueabi-gcc-4.9.4
--sysroot=/mnt/data/chroots/pie/
CMakeFiles/cmTC_13694.dir/CheckSymbolExists.c.o  -o cmTC_13694 -rdynamic
CMakeFiles/cmTC_13694.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x38): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
gmake[1]: *** [CMakeFiles/cmTC_13694.dir/build.make:98: cmTC_13694] Error 1
gmake[1]: Leaving directory '/tmp/thread/build/CMakeFiles/CMakeTmp'
gmake: *** [Makefile:126: cmTC_13694/fast] Error 2

File /tmp/thread/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#include <pthread.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef pthread_create
  return ((int*)(&pthread_create))[argc];
#else
  (void)argc;
  return 0;
#endif
}




Is this a bug? Or am I not using the find correctly?
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