I'm using CppUTest to test the C code defined in a fornol.c source file. That file defines the main production main() function.
I also have an AllTests.cpp file that also has a main() function, but that main() is supposed to be used only when running the unit tests.
AllTests.cpp gets compiled to a .o file, whereas fornol.c gets compiled to a libfornol.a archive.
Then CppUTest tries to link everything together, but here is what I get instead:
Linking fornol_tests
cc -o fornol_tests objs/tests/AllTests.o objs/tests/FornolTests.o lib/libfornol.a ../../CppUTest/lib/libCppUTest.a ../../CppUTest/lib/libCppUTestExt.a -lstdc++ -lgcov
lib/libfornol.a(fornol.o): In function `main':
/home/dlindelof/Work/endor/nol/fornol/fornol.c:453: multiple definition of `main'
objs/tests/AllTests.o:/home/dlindelof/Work/endor/nol/fornol/tests/AllTests.cpp:4: first defined here
collect2: ld returned 1 exit status
It looks as if the main() function defined in fornol.c and present in the archive libfornol.a conflicts with the main() defined in AllTests.cpp. But my understanding was that archive/library files are searched only if/when a given symbol hasn't been referenced yet. It should therefore not be a problem to have the same symbol defined more than once, provided all definitions are in archive/library files.
What am I doing wrong here?
You need to remove the main() from AllTests.cpp and put it in its own source file.
When a linker links in a library, it can't split object files in the library; it has to either link or omit each object file in the library as a unit. (I know LLVM is different, but that's another topic.) This is why, if you look at the source for a library like glibc, each function gets its own source file.
So what's happening to you is that the linker needs to pull in an object file (fornol.o) from the library (libfornol.a) to satisfy dependencies, but that object file carries a duplicate symbol with it (main).
It's perfectly okay to put test code in a library (we do this routinely where I work), but keep it in its own source files (we traditionally use main.cc). (This is a better test anyway, because test code should not have access to static-declared symbols.)