Call Linux shared-object C (foreign) functions directly via ctypes inside Python

With Python ctypes, Python can directly call (foreign) C functions defined in shared object (.so) or dynamic library on Linux. This note documents the compiling and linking of C functions in Ubuntu 24.04.2 LTS (GNU/Linux 6.6.87.1-microsoft-standard-WSL2 x86_64).

Build dynamic lib (*.so) on Linux

Complex makefile (see a mini version below) are utilized to create a *.so file in two steps. To make it easy to undertsand, let's assume there are three .cpp (and three corresponding .h) files in the current dir:

• LunarDate.cpp      // C++ classes implement convertions between lunar and solar dates
• DateHelper.cpp      // C++ help functions used in LunarDate.cpp
• solarLunar.cpp      // C functions (using the LunarDate and SolarDate classes) to be exported to Python

1. Compile object files one-by-one:
   • clang++ -fPIC -fdeclspec -I. -D__WSL_C__ -c LunarDate.cpp -o LunarDate.o
   • clang++ ...
   • clang++ ...
2. Link the three object files into a shared object file:
   • clang++ -shared DateHelper.o LunarDate.o solarLunar.o -o libSolarLunar.so

Notes:

• Compiler option [-fPIC] is necessary for shared lib
• Compiler option [-fdeclspec] is needed for calling C functions directly in Python
• The pre-processor [__WSL_C__] takes care of Linux or Windows (see solarLunar.cpp)
• Linker option [-shared] works with [-fPIC] together
• solarLunar.cpp is actually used in convertions between Gregorian and Chinese lunar dates

Python calls C functions

To call C functions in Python, you must pass the correct argument types (i.e., byte string, c_double, ctypes Structure, or ctypes array) and specify the return type. Check out examples directly in c4Python.py for what most of you need to know.

My actual make.mini file

To create the libSolarLunar.so for Python, type:

make -f make.mini dlib

DYNA_LIB = lib/libSolarLunar.so
OBJS = _obj_DEBUG/vcprjs/qlcdll/lunarPrj/solarLunar.o \
    _obj_DEBUG/utility/date/DateHelper.o \
    _obj_DEBUG/utility/date/LunarDate.o    

COMPILE.cxx = clang++ -I../utility  -c

# rule for making shared object
dlib:$(DYNA_LIB)

$(DYNA_LIB):$(OBJS)		
	-mkdir -p lib
	clang++ -shared  $(OBJS)     -o $@

# rule for making object from C++ sources
objs:$(OBJS)
	
_obj_DEBUG/%.o:../%.cpp
ifeq ($(dir $@), $(wildcard $(dir $@)))
	-mkdir -p $(dir $@)
endif
	$(COMPILE.cxx) -fPIC -fdeclspec -D__WSL_C__ $< -o $@