/in/foo.c:2:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
/in/foo.c: In function 'main':
/in/foo.c:4:11: error: 'n' undeclared (first use in this function)
  int i, a[n];
           ^
/in/foo.c:4:11: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:20:9: error: 'j' undeclared (first use in this function)
     for(j=10; j>i; j--)
         ^
/in/foo.c:4:9: warning: unused variable 'a' [-Wunused-variable]
  int i, a[n];
         ^