C error which worked fine

[florin coter]

hello world.
i wrote the following WRONG code:
..
        FILE *open(), *fp;
        int close();

        fp = fopen(...);
        fprintf( fp, ...);
        fclose(fp);
..
i used turboc 1.5. all went fine (no error note), untill i read the error.
the undeclared fopen sould have been assumed of type int and not FILE *...
so why did it work ok?
thanks, florin.