foo.c:9:10: error: unknown type name 'BigInt'; did you mean 'int'?
 void Mul(BigInt *p, int x);
          ^~~~~~
          int
foo.c:10:10: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3);
          ^~~~~~
          int
foo.c:10:21: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3);
                     ^~~~~~
                     int
foo.c:10:33: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3);
                                 ^~~~~~
                                 int
foo.c:11:13: error: unknown type name 'BigInt'; did you mean 'int'?
 void OutPut(BigInt * p);
             ^~~~~~
             int
foo.c: In function 'main':
foo.c:15:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt x1;
  ^~~~~~
  struct 
foo.c:16:4: error: request for member 'digit' in something not a structure or union
  x1.digit[0] = 1;x1.n = 1;
    ^
foo.c:16:20: error: request for member 'n' in something not a structure or union
  x1.digit[0] = 1;x1.n = 1;
                    ^
foo.c:20:3: warning: implicit declaration of function 'Mul' [-Wimplicit-function-declaration]
   Mul(&x1,m1);
   ^~~
foo.c:22:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt x2;
  ^~~~~~
  struct 
foo.c:23:4: error: request for member 'digit' in something not a structure or union
  x2.digit[0] = 1;x2.n = 1;
    ^
foo.c:23:20: error: request for member 'n' in something not a structure or union
  x2.digit[0] = 1;x2.n = 1;
                    ^
foo.c:26:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt x3;
  ^~~~~~
  struct 
foo.c:27:4: error: request for member 'digit' in something not a structure or union
  x3.digit[0]=0;
    ^
foo.c:28:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt xx1;
  ^~~~~~
  struct 
foo.c:30:17: error: request for member 'n' in something not a structure or union
  for(i = 0; i<x2.n; i++)
                 ^
foo.c:32:3: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
   BigInt xx1;
   ^~~~~~
   struct 
foo.c:33:6: error: request for member 'n' in something not a structure or union
   xx1.n = x1.n; 
      ^
foo.c:33:13: error: request for member 'n' in something not a structure or union
   xx1.n = x1.n; 
             ^
foo.c:34:15: error: request for member 'n' in something not a structure or union
   for(k=0;k<x1.n;k++)
               ^
foo.c:35:7: error: request for member 'digit' in something not a structure or union
    xx1.digit[k] = x1.digit[k];
       ^
foo.c:35:21: error: request for member 'digit' in something not a structure or union
    xx1.digit[k] = x1.digit[k];
                     ^
foo.c:38:13: error: request for member 'digit' in something not a structure or union
   int l = x2.digit[i];
             ^
foo.c:43:3: warning: implicit declaration of function 'Add' [-Wimplicit-function-declaration]
   Add(&xx1,&x3,&x3);
   ^~~
foo.c:47:2: warning: implicit declaration of function 'OutPut' [-Wimplicit-function-declaration]
  OutPut(&x3);
  ^~~~~~
foo.c:29:8: warning: unused variable 'j' [-Wunused-variable]
  int v,j,k;
        ^
foo.c:28:9: warning: unused variable 'xx1' [-Wunused-variable]
  BigInt xx1;
         ^~~
foo.c: At top level:
foo.c:51:10: error: unknown type name 'BigInt'; did you mean 'int'?
 void Mul(BigInt *p, int x)
          ^~~~~~
          int
foo.c:68:10: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3)
          ^~~~~~
          int
foo.c:68:21: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3)
                     ^~~~~~
                     int
foo.c:68:33: error: unknown type name 'BigInt'; did you mean 'int'?
 void Add(BigInt *p1,BigInt *p2, BigInt *p3)
                                 ^~~~~~
                                 int
foo.c:100:13: error: unknown type name 'BigInt'; did you mean 'int'?
 void OutPut(BigInt *p)
             ^~~~~~
             int
foo.c: In function 'main':
foo.c:18:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d%d%d",&m1,&n1,&m2,&n2);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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