c - C中结构内部结构的问题

Question

我有这段代码:

struct node{
   double *coordinates; 
};

struct vptree{
   struct node vp;
   double md;
   int idx;
   struct vptree *inner;
   struct vptree *outer;
};

void buildvp(double *X, int n, int d){

   for(int i=0;i<n;i++){
      for(int j=0;j<d;j++){
        printf("%lf   ",X[i*d+j]);
      }
      printf("\n");
   }
   struct vptree *tree;
   printf("That 's ok! 1\n");
   tree->vp.coordinates=(double *)malloc(d*sizeof(double));
   printf("That 's ok! 2\n");

   for(int i=0;i<d;i++){
      tree->vp.coordinates[i]=X[(n-1)*d+i];
      printf("for i=%d the X= %lf",i,tree->vp.coordinates[i]);  
   }
}


int main(){
   double X[8][2]={{2,5},{1,1},{0,0},{1,3},{2,5},{1,1},{-2,8},{6,-1}};
   buildvp(&X,8,2);
   return 0;
}

当矩阵X为7x2或5x2时，程序没问题。当矩阵 X 为 8x2 或 6x2 时，程序不会打印消息“That's ok 2”。我意识到结构有问题(特别是在命令中tree->vp.coordinates=(double *)malloc(d*sizeof(double))) 但我无法修复它。你有什么想法吗？

Answer 1

   struct vptree *tree;
   printf("That 's ok! 1\n");
   tree->vp.coordinates=(double *)malloc(d*sizeof(double));

tree 未指向有效内存，这就是您在取消引用时出现未定义行为的原因。

也许您首先需要为其分配内存，然后再释放。

   struct vptree *tree = malloc(sizeof *tree);
     …
   free(tree);

或将其声明为普通变量。

   struct vptree tree;
   // access it using (.) operator
   tree.vp.coordinates=(double *)malloc(d*sizeof(double));