CMake: Is it possible to build an executable from only static libraries and no source?

Question

I would like to build an executable from static libraries (i. e. .a-files) only. This is possible, because the main() function is contained in one of these libr

Answer 1

There is no way to do it without a hack. You need at least one *.c or *.cpp file.

What I do is make a dummy null.cpp file (zero bytes) and use that. You can also use /dev/null but that only works on Linux.

file(WRITE null.cpp "")

add_executable(tester
    null.cpp
)

target_link_libraries(tester
    -Wl,--whole-archive
    libtest1
    libtest2
    libtest3
    libtest4
    -Wl,--no-whole-archive
    gtest_main
)

Answer 2

There are mainly two reasons why a source file is enforced by CMake:

1. To determine the LINKER_LANGUAGE from the file ending(s)
2. Not all compilers do support an object/library only link step (for details see below)

And if you move the main() function to library please keep the following in mind: Why does the order in which libraries are linked sometimes cause errors in GCC?

So if you build the libraries with CMake in the same project, I would recommend to change your libraries (at least the one containing your main() function) to an object library:

cmake_minimum_required(VERSION 2.8.8)

project(NoSourceForExe)

file(WRITE main.cc "int main() { return 0; }")

add_library(MyLibrary OBJECT main.cc)
add_executable(MyExecutable $<TARGET_OBJECTS:MyLibrary>)

The add_library() documentation lists a warning here:

Some native build systems may not like targets that have only object files, so consider adding at least one real source file to any target that references $<TARGET_OBJECTS:objlib>.

But those are rare and listed in Tests/ObjectLibrary/CMakeLists.txt:

# VS 6 and 7 generators do not add objects as sources so we need a
# dummy object to convince the IDE to build the targets below.
...
# Xcode does not seem to support targets without sources.

Not knowing which host OS(s) you are targeting, you may just give it a try.

References

• CMake Object Lib containing main
• CMake/Tutorials/Object Library