foo.c:7:9: error: unknown type name 'BigInt'; did you mean 'int'?
 int Mul(BigInt *p,int x){
         ^~~~~~
         int
foo.c:21:11: error: unknown type name 'BigInt'; did you mean 'int'?
 int MyMul(BigInt *x1,BigInt *x2,BigInt *x3){
           ^~~~~~
           int
foo.c:21:22: error: unknown type name 'BigInt'; did you mean 'int'?
 int MyMul(BigInt *x1,BigInt *x2,BigInt *x3){
                      ^~~~~~
                      int
foo.c:21:33: error: unknown type name 'BigInt'; did you mean 'int'?
 int MyMul(BigInt *x1,BigInt *x2,BigInt *x3){
                                 ^~~~~~
                                 int
foo.c:38:13: error: unknown type name 'BigInt'; did you mean 'int'?
 void Output(BigInt *p){
             ^~~~~~
             int
foo.c: In function 'main':
foo.c:45:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt x1,x2;
  ^~~~~~
  struct 
foo.c:46:4: error: request for member 'digit' in something not a structure or union
  x1.digit[0]=1,x1.n=1;
    ^
foo.c:46:18: error: request for member 'n' in something not a structure or union
  x1.digit[0]=1,x1.n=1;
                  ^
foo.c:46:15: warning: left-hand operand of comma expression has no effect [-Wunused-value]
  x1.digit[0]=1,x1.n=1;
               ^
foo.c:47:4: error: request for member 'digit' in something not a structure or union
  x2.digit[0]=1,x2.n=1;
    ^
foo.c:47:18: error: request for member 'n' in something not a structure or union
  x2.digit[0]=1,x2.n=1;
                  ^
foo.c:47:15: warning: left-hand operand of comma expression has no effect [-Wunused-value]
  x2.digit[0]=1,x2.n=1;
               ^
foo.c:49:5: error: request for member 'n' in something not a structure or union
   x1.n=Mul(&x1,a);
     ^
foo.c:49:8: warning: implicit declaration of function 'Mul' [-Wimplicit-function-declaration]
   x1.n=Mul(&x1,a);
        ^~~
foo.c:51:5: error: request for member 'n' in something not a structure or union
   x2.n=Mul(&x2,c);
     ^
foo.c:52:2: error: unknown type name 'BigInt'; use 'struct' keyword to refer to the type
  BigInt x3;
  ^~~~~~
  struct 
foo.c:53:4: error: request for member 'n' in something not a structure or union
  x3.n=MyMul(&x1,&x2,&x3);
    ^
foo.c:53:7: warning: implicit declaration of function 'MyMul' [-Wimplicit-function-declaration]
  x3.n=MyMul(&x1,&x2,&x3);
       ^~~~~
foo.c:54:10: error: request for member 'digit' in something not a structure or union
  while(x3.digit[x3.n]==0&&x3.n>=2)x3.n--;
          ^
foo.c:54:19: error: request for member 'n' in something not a structure or union
  while(x3.digit[x3.n]==0&&x3.n>=2)x3.n--;
                   ^
foo.c:54:29: error: request for member 'n' in something not a structure or union
  while(x3.digit[x3.n]==0&&x3.n>=2)x3.n--;
                             ^
foo.c:54:37: error: request for member 'n' in something not a structure or union
  while(x3.digit[x3.n]==0&&x3.n>=2)x3.n--;
                                     ^
foo.c:55:14: error: request for member 'n' in something not a structure or union
  for(int i=x3.n;i>=1;i--)printf("%d",x3.digit[i]);
              ^
foo.c:55:40: error: request for member 'digit' in something not a structure or union
  for(int i=x3.n;i>=1;i--)printf("%d",x3.digit[i]);
                                        ^
foo.c:44:14: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  int a,b,c,d;scanf("%d%d%d%d",&a,&b,&c,&d);
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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