How to combine a C++ object file in a C project compiled with gcc (not g++)?

Question

Yet another C/C++ integration question: I am trying to update some legacy C library (let's call it libcl.a) with functionality that I have in a C++ library (let's call it libcppl.a). The liblc.a library is being used all over in my environment and is linked into many C projects, using GCC (in the C compiler mode):

>> gcc prog.c -lcl

The libcl.a currently consists of the cl.o object file (created with gcc from cl.c+cl.h).

The libcppl.a consists of the cppl.o object file (created with g++ from cppl.cpp+cppl.h).

Because the existing applications are written in C, and the build scripts are using GCC, I'd like to keep the transition to the updated library as simple as possible. Thus, I want to keep using GCC as the main compiler but still be able to link with the updated library.

Finding this answer, I could link C++ objects into GCC C project using -lstdc++:

>> gcc -c cl.c   -o cl.o
>> g++ -c cppl.c -o cppl.o
>> ar rcs libcl.a cl.o cppl.o
>> gcc prog.c -lcl -lstdc++

However, I want to eliminate the explicit mention of the libstdc++ in the compile command line.

What I tried to do was to include libstdc++ in the cl library, by doing:

>> ar rcs libcl.a cl.o cppl.o /usr/lib/libstdc++.so.6

However, when building the application, I get:

>> gcc prog.c -lcl
In file included from cppl.cpp:2:
./libcl.a(cppl.o):(.eh_frame+0x12): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status

1. Why doesn't the gcc linker find the C++ standard library that was archived together with my objects?

2. Is there a limitation on using ar with libraries (as opposed to object files)?

3. Is there a way to overcome this problem?

Answer 1

You can separately compile prog.c using gcc, then use g++ to link:

# compile the main program to an object file
gcc  prog.c -o prog.o

#use g++ to link everything (it'll automatically pull in libstdc++)
g++ prog.o -lcl

Alternatively, you can compile and link prog.c in one step by explicitly telling g++ to compile prog.c as a C source file using the -x option:

g++ -x c prog.c -lcl

See https://stackoverflow.com/a/5854712/12711 for more information on the difference in behavior between gcc and g++.