Thank you very much for help. I am ashamed for misinforming you.... the problem with compilation of code SETCODE(, IP, IPaddress, 10, ipEqual(a, b)); appeared after I have inserted line "return 0;" to the end of int main(int argc, char **argv) function in files set.cpp and uc.cpp I have inserted them because compiler was saying that "uc.cpp(53): error C4716: 'SDL_main' : must return a value" uc.cpp: 51: printf("count=%d\n", count); // print how many were sent 52: mySDL_Quit(); // clean up and quit 53: } That is why I inserted "return 0;" to the 53 line. And from this point other compilation problems come. Maybe, I shouldn't insert "return 0"? I supposed that was not serious change... Or it is serious? Thank you, Nasikat Tynaeva.