我可以释放最初从另一个指针分配的内存吗？

Question

If I allocated memory from another pointer, then declare a pointer equal to the other pointer, can I then free the memory allocated with the first pointer by using free() on the new pointer?

Example:

typedef struct foo{
  int n;
  char c;
} foo;

foo* bar = malloc(sizeof(foo));
foo* tmp = bar; // declaring pointer identical to other pointer

free(tmp); // can I do this to free *bar?

Answer 1

Yes, it is completely fine.

Your implementation of malloc will most likely mark the chunk of memory as allocated before returning a pointer to it.

Now you can have as many pointer variables as you want which point to this chunk of memory and you can free that memory by calling free on any of them.

The pointer itself doesn't contain information about whether the memory it points to has been allocated. It just points to it.

int* i = malloc(sizeof(int) * 23);
int* j = i;

free(j); 
// free(i); // this is undefined behavior, the memory was already freed

Answer 2

Yes you can do what you are asking about. But it is not another pointer, what you are freeing is the same pointer. When malloc returns a pointer, it is essentially giving you an integer that is the address of the start of the chunk of memory that it has reserved for you. When you are done are done with the memory, you pass that address to free . As far as free is concerned, there is no difference between:

free(malloc(17));

,

void *p = malloc(17);
free(p);

or

void *p1 = malloc(17);
void *p2 = p1;  
free(p2);

In all of these cases the value returned from malloc is getting to free , and that is all that matters.