ld报告对nm找到的符号的未定义引用

Question

I am building a shared library with the following instructions :

cc -shared -Wl,-soname,libmy.so.0 [lots of .o files] -o libmy.so.0
ln -f -s libmy.so.0 libmy.so

I don't know if this is important, but this library is written in C.

I then try to link with this library, from a C++ program :

/usr/bin/c++ -g CMakeFiles/codegen.dir/client/codegen.cpp.o CMakeFiles/codegen.dir
 /namenode/shared_objects.cpp.o  -o codegen -rdynamic -L/path/to/libmy -lpthread
 -lboost_system-mt -lboost_filesystem-mt -lboost_unit_test_framework-mt
 -lboost_serialization-mt -lmy -Wl,-rpath,/path/to/libmy

But ld reports an error :

CMakeFiles/codegen.dir/client/codegen.cpp.o: In function `main':
[...]/src/client/codegen.cpp:46:
   undefined reference to `alloc_code(int, int, int, int, int)'

Even if alloc_code appears to be in the shared library:

$ nm libmy.so | grep alloc_code
0000000000002c80 T alloc_code

Note that libmy.so and my codegen program are compiled using different compiler flags (one is compiled in debug mode while the other is compiled in optimized mode), but I don't think the issue comes from here.

What could make ld unable to link codegen with libmy.so ?

Answer 1

Your C function needs to be declared to C++ as this:

#ifdef __cplusplus
extern "C" {
#endif

void alloc_code(int, int, int, int, int);

#ifdef __cplusplus
}
#endif

Otherwise, the compiler and thus linker assume C++ linkage (where multiple functions can exist with the same name so long as their arguments differ) - and will search for the "C++-mangled" variant of the function.

When using extern "C" you tell the compiler and linker to use the unmangled "C" name.

To demonstrate this, let's write the same "library" and compile it with both a C compiler and a C++ compiler:

/* Library Source Code */
int library_function(void) {
    return 0;
}

Then compile with C:

gcc -o libdummy.so -shared -fPIC dummy-library.c
nm libdummy.so | grep library_function

Which outputs:

0000000000000640 T library_function

Running the same thing with a C++ compiler yields:

g++ -o libdummy.so -shared -fPIC dummy-library.c
nm libdummy.so

=>

0000000000000670 T _Z16library_functionv