为什么这个简单的hello world代码段错误？

Question

Excuse the beginner level of this question. I have the following simple code, but it does not seem to run. It gets a segmentation fault. If I replace the pointer with a simple call to the actual variable, it runs fine... I'm not sure why.

struct node
{
 int x;
 struct node *left;
 struct node *right;
};

int main()
{
 struct node *root;
 root->x = 42;
 printf("Hello world. %d", root->x);
 getchar();
 return 0;
}

What is wrong with this code?

Answer 1

struct node *root;
root->x = 42;

You're dereferencing an uninitialized pointer. To allocate storage for the node:

struct node *root = malloc(sizeof(struct node));

You could also allocate a node on the stack:

struct node root;
root.x = 42;

Answer 2

In order to use a pointer to access something, the pointer must be pointing at that something. In order for the pointer to be pointing at that something, that something must exist. Creating a pointer does not create anything for it to point at. You must do so explicitly, either by dynamic allocation ( malloc() ), stack allocation (ie a local variable) or by pointing to something that already exists (eg a static instance, such as a global; a value that was passed in as a parameter; etc.).

Answer 3

After struct node *root; line add the

root = (sturct node*) malloc(sizeof(struct node));

Also, before Return 0 line add the

free(root);