/in/foo.c:8:1: error: unknown type name 'Bigint'
 Bigint d;
 ^~~~~~
/in/foo.c:8:1: note: use 'struct' keyword to refer to the type
/in/foo.c:9:10: error: unknown type name 'Bigint'
 void Mul(Bigint* p, int x)
          ^~~~~~
/in/foo.c:25:13: error: unknown type name 'Bigint'
 void Change(Bigint* p, int k, char ans[])
             ^~~~~~
/in/foo.c: In function 'main':
/in/foo.c:50:3: error: request for member 'n' in something not a structure or union
  d.n = 1;
   ^
/in/foo.c:51:3: error: request for member 'a' in something not a structure or union
  d.a[0] = 1;
   ^
/in/foo.c:55:3: warning: implicit declaration of function 'Mul' [-Wimplicit-function-declaration]
   Mul(&d, x);
   ^~~
/in/foo.c:57:2: warning: implicit declaration of function 'Change' [-Wimplicit-function-declaration]
  Change(&d, k, ans);
  ^~~~~~