指向指针空 C 的指针

Question

I have a question about a double pointer in C. For example:

int** point = malloc(sizeof(int**));
int* point2 = malloc(sizeof(int*));
*point2 = 5;
*point = point2;
free(point2);
printf("%d", *point);

My question is, if I have a double pointer pointing to another pointer (point2) and I make a free on (point2), the double pointer (point) is going to point to a memory address that has nothing. So I should do * point = NULL?

Answer 1

Strictly speaking, you don't have to assign NULL to *point . However, it is a very good idea to do it anyway, because it helps you distinguish valid pointers from invalid when you debug your program.

the double pointer ( point ) is going to point to a memory address that has nothing.

Memory always has something in it. However, it's not always under your control, so it may not be legal to read its content.

For instance, when you free point2 , the value stored in point is not going to disappear. However, it would become illegal to use that value, as it becomes indeterminate.