/in/foo.c:2:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
/in/foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:7:9: warning: unknown escape sequence: '\.'
  printf("2,5,3\.n");
         ^~~~~~~~~~