distributing MSYS2 programs https://stackoverflow.com/a/79222920/12291425

GNUmakefile

targets := prog.exe my_library.dll my_plugin.dll

CXX := g++
CXXFLAGS :=  -c   -g -O0  -MMD -pedantic -Wall -Wextra -Wconversion -Wno-unused -Wno-unused-parameter -Werror -Wfatal-errors -UNDEBUG  -std=c++17 -Wno-missing-braces -Wno-sign-conversion

all: $(targets)

.PHONY: all

%.dll: %.o
	$(CXX) -shared -g -O0 -o $@ $<

# by implicit rule, my_plugin.o will be compiled from my_plugin.cpp

# by implicit rule, my_library.o will be compiled from my_library.cpp

prog.exe: prog.o my_library.dll
	g++ -o prog.exe prog.o -g -O0 -L. -lmy_library

files := $(wildcard *.dll *.o *.d *.exe)
distfiles := $(wildcard dist/*.dll dist/*.exe)
clean:
	rm $(files) 2>/dev/null || true
	rm $(distfiles) 2>/dev/null || true

.PHONY: clean

dist: all
	rm -r dist 2>/dev/null || true
	mkdir dist 2>/dev/null || true
	cp *.dll *.exe dist
	@# extra $ is needed in awk in makefile
	cd dist; ldd $(targets) | grep /ucrt64 | awk '{print $$3}' | xargs -i cp {} .
	@# if you make sure you are running UCRT, you can remove the following line
	cd dist; ldd $(targets) | grep /mingw64 | awk '{print $$3}' | xargs -i cp {} .
	cd dist; explorer . || true

.PHONY: dist

my_library.cpp

#include <iostream>

__declspec(dllexport)
int
my_library_function(int arg)
{
  std::cout << "~~~~ entering " << __func__ << " ~~~~\n";
  std::cout << "arg=" << arg << '\n';
  std::cout << "~~~~ leaving " << __func__ << " ~~~~\n";
  return 2*arg;
}

my_plugin.cpp

#include <iostream>

extern "C" __declspec(dllexport)
int
my_plugin_function(int arg)
{
  std::cout << "~~~~ entering " << __func__ << " ~~~~\n";
  std::cout << "arg=" << arg << '\n';
  std::cout << "~~~~ leaving " << __func__ << " ~~~~\n";
  return 2*arg;
}

prog.cpp

#include <windows.h>
#include <iostream>

__declspec(dllimport)
int
my_library_function(int arg);

int
main()
{
  std::cout << "~~~~ entering " << __func__ << " ~~~~\n";
  int result=my_library_function(123);
  std::cout << "result=" << result << '\n';
  std::cout << "~~~~ still in " << __func__ << " ~~~~\n";
  HINSTANCE lib=LoadLibrary("my_plugin.dll");
  if(lib)
  {
    FARPROC symbol=GetProcAddress(lib, "my_plugin_function");
    if(symbol)
    {
      int (*fnct)(int)=NULL;
      memcpy(&fnct, &symbol, sizeof(fnct));
      int result=fnct(123);
      std::cout << "result=" << result << '\n';
    }
    FreeLibrary(lib);
  }
  std::cout << "~~~~ leaving " << __func__ << " ~~~~\n";
  return 0;
}