$ cat cygmsgbox.cpp
#include <stdio.h>
#include <unistd.h>
#include <windows.h>

int main(int, char **) {

   int PID = fork();
   if (PID) {
      MessageBox(NULL,
                 "Congratulations, it's a boy!",
                 "Proud parent",
                 MB_OK | MB_ICONINFORMATION
                 );
      puts("Parent messagebox closed");
   }
   else {
      MessageBox(NULL,
                 "Hello, are you my mother?",
                 "New child",
                 MB_OK | MB_ICONINFORMATION
                 );
      puts("Child messagebox closed");
   }
   return 0;
}

$ gcc cygmsgbox.cpp -lstdc++

$ ./a.exe
Child messagebox closed
Parent messagebox closed