C++ Creating a dynamic library using source, static archives and other dynamic libraries

Question

In my use case, I have YAML-CPP, SQLite3, and my 'data.cpp' file that I want to all be combined into the same dynamic library, 'libdata.so'.

I first compiled yaml-cpp (as an archive):

mkdir -p "build"
cd "build"
cmake ..
make -j5

to get 'libyaml-cpp.a'.

I then compile sqlite3:

gcc -c -o libsqlite3.a sqlite3.c -lpthread -ldl

to get 'libsqlite3.a'. I know that this a C-based file, and there are differences between it and C++, but I've read that it shouldn't make too much difference here. I also know that I'm using -lpthread -ldl which is for dynamic loading, but I'm not sure how to get around it.

My question is: Can I compile my 'data.cpp' file with YAMP-CPP and SQLite3 such that they all exist in the same 'libdata.so' output file (where the linker will use the YAML-CPP and SQLite3 functions contained in 'libdata.so' when they're called by 'data.cpp')?

I have tried:

g++ -c -fPIC -o libdata.so \
  -Wl,--whole-archive libsqlite3.a \
  -Wl,--whole-archive libyaml-cpp.a \
  -ldl -lpthread \
  data.cpp

(for the sake of the snippet, all files reside in the same directory)

UPDATE
I added the suggestion from botje to the line and it helped in part. After more research, I found a few more pieces that progressed further:

gcc -DSQLITE_OMIT_LOAD_EXTENSION -c -fPIC -lpthread -o libsqlite3.a sqlite3.c

mkdir -p "build"
cd "build"
env CFLAGS='-fPIC' CXXFLAGS='-fPIC' cmake ..
make -j$(CORES)
cd ..
cp "build/libyaml-cpp.a" ./

g++ -shared -fPIC -o libdata.so \
  -L./ \
  -Wl,-Bdynamic data.cpp \
  -Wl,-Bstatic -lsqlite3 -lyaml-cpp \
  -Wl,-Bdynamic -lpthread

g++ -L./ -ldata -o tester tester.cpp

The library now compiles, however, when I try to link against it with 'tester.cpp', I get the error:

/usr/bin/ld: libdata.so: undefined reference to YAML::detail...

I'm guessing this may be a flag ordering issue, but I'm not sure what order it should be then. Placing the flags for SQLite3 and YAML-CPP before the data.cpp argument fails to compile the shared library.

Answer 1

After some more research, here's the method that worked for me (with extra verbosity):

# Compile SQLite3:
# - Disable the plugin loader (removes the libdl dependency)
# - Compile only (-c)
# - Use Position Independent Code (-fPIC)
# - Add the PThread library
# - After compilation, archive object (for completeness)
gcc -DSQLITE_OMIT_LOAD_EXTENSION -c -fPIC -pthread -o sqlite3.o sqlite3.c


# Compile YAML-CPP
# - Create (and enter) a build directory
# - Run CMAKE with -fPIC enabled
# - Run MAKE
# - Exit and copy archive from build directory
mkdir -p "build"
cd "build"
env CFLAGS='-fPIC' CXXFLAGS='-fPIC' cmake ..
make -j$(CORES)
cd ..
cp build/libyaml-cpp.a libyaml-cpp.a


# Compile Shared Library
# - Ensure shared (-shared) (also prevents looking for a 'main')
# - Use Position Independent Code (-fPIC)
# - Use current directory for locating libraries
# - Set target CPP file
# - STATICALLY link from SQLite3 and YAML-CPP archives
# - DYNAMICALLY link from PThread library (used by SQLite3 for thread-safe access)
g++ -shared -fPIC -o libdata.so data.cpp \
  -L./ \
  -Wl,-Bstatic -l:sqlite3.o -lyaml-cpp \
  -Wl,-Bdynamic -pthread


# Compile Test Program
# - Specify current directory for includes and libraries
# - Link dynamically to 'libdata.so' 
g++ -I./ -L./ -ldata -o tester tester.cpp

The last issue I encountered ended up being a missing include directory for YAML-CPP.

A couple of notes for credit:

• @Botje: For pointing out that I need -shared and not -c in the compilation of a shared library. (libdata.so)
• @Maxim Egorushkin: For linking to a very useful document on the matter.

One thing to note as well, is that when linking against a C library in a C++ program, you may need to use 'extern "C"' (as elaborated in the linked page). This is especially important when using the SQLite3 library.