Compiling 32-bit vs 64-bit project using CMake - 64-bit

Question

How do I specify that CMake should use a different link_directories value depending on whether the target is 32-bit or 64-bit? For example, 32-bit binaries need to link with 32-bit Boost, 64-bit binaries need to link with 64-bit Boost.

Answer 1

You do something along these lines
if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
set( BOOST_LIBRARY "/boost/win64/lib" )
else( CMAKE_SIZEOF_VOID_P EQUAL 8 )
set( BOOST_LIBRARY "/boost/win32/lib" )
endif( CMAKE_SIZEOF_VOID_P EQUAL 8 )
set( CMAKE_EXE_LINKER_FLAGS ${BOOST_LIBRARY} )

Answer 2

I know it's quite old question. But it's still on top when you search with Google "cmake 32 64".
I have answer similar to user434507's answer but a little bit more readable in my opinion (I don't like if-else construction in cmake, it looks ugly):
math(EXPR BITS "8*${CMAKE_SIZEOF_VOID_P}")
set(BOOST_LIBRARY "/boost/win${BITS}/lib")
set(CMAKE_EXE_LINKER_FLAGS ${BOOST_LIBRARY})
This will point BOOST_LIBRARY path to /boost/win32/lib or /boost/win64/lib, depending on your architecture.

Answer 3

For Boost specifically, you should use
FIND_LIBRARY(Boost 1.44 COMPONENTS ...)
Then the CMake variable Boost_LIBRARY_DIRS will contain the correct library path, which has to be set using LINK_DIRECTORIES, e.g.
LINK_DIRECTORIES(${Boost_LIBRARY_DIRS})
The more general case is correctly described in user434507's answer.

Answer 4

Based on rominf I turned up following solution (for Windows).
I install boost libraries into: C:\Boost_32 and C:\Boost_64
In CMakeLists.txt
math(EXPR BITS "8*${CMAKE_SIZEOF_VOID_P}")
set(BOOST_ROOT C:/Boost_${BITS})
find_package(Boost 1.64.0 COMPONENTS ... )
INCLUDE_DIRECTORIES( ${Boost_INCLUDE_DIR} )
LINK_DIRECTORIES(${Boost_LIBRARY_DIR})
Explanation:
CMAKE_SIZEOF_VOID_P is equal to 4 on 32bit platform, and 8 on 64bit platform.
Expression 8*${CMAKE_SIZEOF_VOID_P} will evaluate to 32 or 64,
respectively.
C:/Boost_${BITS} turns into C:/Boost_32 or C:/Boost_64 automagically
Advantages:
You don't need conditionals (and in my CMakeLists there are too many already),
It is 90% how you 'should' include Boost with CMake.