/in/foo.c:4:19: error: expected ':', ',', ';', '}' or '__attribute__' before '=' token
  int digit[100000]={0};
                   ^
/in/foo.c:7:10: error: unknown type name 'num'
 void mul(num *p,int x)
          ^~~
/in/foo.c:22:10: error: unknown type name 'num'
 int mul2(num *x,num *y,num *z)
          ^~~
/in/foo.c:22:17: error: unknown type name 'num'
 int mul2(num *x,num *y,num *z)
                 ^~~
/in/foo.c:22:24: error: unknown type name 'num'
 int mul2(num *x,num *y,num *z)
                        ^~~
/in/foo.c: In function 'main':
/in/foo.c:47:2: error: unknown type name 'num'
  num a,b,c;
  ^~~
/in/foo.c:47:2: note: use 'struct' keyword to refer to the type
/in/foo.c:49:3: error: request for member 'digit' in something not a structure or union
  a.digit[0]=x1;b.digit[0]=y1;
   ^
/in/foo.c:49:17: error: request for member 'digit' in something not a structure or union
  a.digit[0]=x1;b.digit[0]=y1;
                 ^
/in/foo.c:52:3: warning: implicit declaration of function 'mul' [-Wimplicit-function-declaration]
   mul(&a,x1);
   ^~~
/in/foo.c:58:3: error: request for member 'n' in something not a structure or union
  c.n=mul2(&a,&b,&c);
   ^
/in/foo.c:58:6: warning: implicit declaration of function 'mul2' [-Wimplicit-function-declaration]
  c.n=mul2(&a,&b,&c);
      ^~~~
/in/foo.c:59:13: error: request for member 'n' in something not a structure or union
  for(int i=c.n-1;i>=0;i--)
             ^
/in/foo.c:61:16: error: request for member 'digit' in something not a structure or union
   printf("%d",c.digit[i]);
                ^