cmake cxx flags only at compile time - cmake

Question

I wanted to know if there is a way to pass some flags to the compiler (g++) only at compile time and not at the linking.
Let me explain with an example: I would like to build my program this way:
g++ -c source1.cpp -o source1.o
g++ -c source2.cpp -o source2.o
g++ source2.o source1.o -o myProgram.c
to do this I have created a CMakeLists.txt file that contains:
PROJECT(myProgram)
include_directories("${CMAKE_SOURCE_DIR}/inc")
aux_source_directory("${CMAKE_SOURCE_DIR}/src" SRC_FILES)
add_executable(${PROJECT_NAME} ${SRC_FILES})
then add this line to put the "-c" flag to the compiler:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -c")
But in this way, cmake add the "-c" flag always, even when g++ is doing the linking, ie:
g++ -c source1.cpp -o source1.o
g++ -c source2.cpp -o source2.o
g++ -c source2.o source1.o -o myProgram.c
and logically myProgram will not be created because the g++ does not run linking (-c in the last instruction).
How do I use the flag "-c" (or any other flag) only at compile?

Answer 1

Instead of
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -c")
try
add_compile_options(-c)
No clue why though. You can use multiple space separated options in the same add_compile_options.

Answer 2

I am using this "CMakeLists.txt"
PROJECT(prima_prova)
cmake_minimum_required(VERSION 2.8)
set(CMAKE_VERBOSE_MAKEFILE 1)
include_directories("${CMAKE_SOURCE_DIR}/inc")
aux_source_directory("${CMAKE_SOURCE_DIR}/src" SRC_FILES)
add_executable(${PROJECT_NAME} ${SRC_FILES})
when I run cmake .. and make, I get this:
make[2]: uscita dalla directory "/home/massimi/Scrivania/test/build"
make -f CMakeFiles/prima_prova.dir/build.make CMakeFiles/prima_prova.dir/build
make[2]: ingresso nella directory "/home/massimi/Scrivania/test/build"
[ 25%] Building CXX object CMakeFiles/prima_prova.dir/src/f1.cpp.o
/usr/bin/c++ -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/f1.cpp.o -c /home/massimi/Scrivania/test/src/f1.cpp
[ 50%] Building CXX object CMakeFiles/prima_prova.dir/src/main.cpp.o
/usr/bin/c++ -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/main.cpp.o -c /home/massimi/Scrivania/test/src/main.cpp
[ 75%] Building CXX object CMakeFiles/prima_prova.dir/src/f2.cpp.o
/usr/bin/c++ -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/f2.cpp.o -c /home/massimi/Scrivania/test/src/f2.cpp
[100%] Linking CXX executable prima_prova
/usr/bin/cmake -E cmake_link_script CMakeFiles/prima_prova.dir/link.txt -- verbose=1
/usr/bin/c++ CMakeFiles/prima_prova.dir/src/f1.cpp.o CMakeFiles/prima_prova.dir/src/main.cpp.o CMakeFiles/prima_prova.dir/src/f2.cpp.o -o prima_prova -rdynamic
make[2]: uscita dalla directory "/home/massimi/Scrivania/test/build"
when I add this line in the file "CMakeLists.txt":
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -c")
when I run cmake .. and make, I get this:
make[2]: uscita dalla directory "/home/massimi/Scrivania/test/build"
make -f CMakeFiles/prima_prova.dir/build.make CMakeFiles/prima_prova.dir/build
make[2]: ingresso nella directory "/home/massimi/Scrivania/test/build"
[ 25%] Building CXX object CMakeFiles/prima_prova.dir/src/f1.cpp.o
/usr/bin/c++ -c -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/f1.cpp.o -c /home/massimi/Scrivania/test/src/f1.cpp
[ 50%] Building CXX object CMakeFiles/prima_prova.dir/src/main.cpp.o
/usr/bin/c++ -c -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/main.cpp.o -c /home/massimi/Scrivania/test/src/main.cpp
[ 75%] Building CXX object CMakeFiles/prima_prova.dir/src/f2.cpp.o
/usr/bin/c++ -c -I/home/massimi/Scrivania/test/inc -o CMakeFiles/prima_prova.dir/src/f2.cpp.o -c /home/massimi/Scrivania/test/src/f2.cpp
[100%] Linking CXX executable prima_prova
/usr/bin/cmake -E cmake_link_script CMakeFiles/prima_prova.dir/link.txt --verbose=1
/usr/bin/c++ -c CMakeFiles/prima_prova.dir/src/f1.cpp.o CMakeFiles/prima_prova.dir/src/main.cpp.o CMakeFiles/prima_prova.dir/src/f2.cpp.o -o prima_prova -rdynamic
c++: warning: CMakeFiles/prima_prova.dir/src/f1.cpp.o: linker input file unused because linking not done
The flag "-c" or any iother flag in the variable CMAKE_CXX_FLAGS has been added always, even during linking, not only when is compiling sources.

Answer 3

CMAKE_CXX_FLAGS variable is used in command line only when CMake is compiling sources. During linking CMAKE_EXE_LINKER_FLAGS (and flags set by modifying target properties) are used, even if g++ is used as linker executable.