是否应该将指针传递给free（）指向已分配空间的开头？

Question

Say I used malloc to request a free space. The variable ptr points to the created memory space. In my code the value of ptr is changed to access different locations in my allocated space.

Calling free(ptr) , after ptr has been changed, would not be correct, no?. What I should do instead is to create a replica of ptr after malloc . This new pointer should be used instead in the free function call. Is that right?

Cheers

Answer 1

Calling free(ptr) , after ptr has been changed, would not be correct, no?

That is the correct understanding. It would not be correct.

What i should do instead is to create a replica of ptr after malloc . This new pointer should be used instead in the free function call. Is that right?

You have the following choice here:

1. Keep the original variable and use a copy to point to different locations of the malloc ed data. Call free using original variable.

2. Make a copy, use the original variable to point to different locations of the malloc ed data. Call free using copy.

3. Keep the original pointer. Use an index to access different locations of the malloc ed data. Call free using original variable.

Regardless of the way you access different locations of the data returned by malloc , the value of the pointer that was returned by malloc must be used in the call to free .

Answer 2

Yes, your understanding and suggested solution are correct. Moreover, according to the C11 standard,

7.22.3.3 The free function

...if the argument does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to free or realloc , the behavior is undefined

So trying to free somewhere in the middle of the memory you allocated causes undefined behavior and likely breaks your program.

Answer 3

As per Chapter 7.20.3.2, c99 standard, 2nd paragraph [emphasis mine]

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the calloc , malloc , or realloc function, or if the space has been deallocated by a call to free or realloc , the behavior is undefined.

So, you cannot pass a pointer to free() which is not returned by malloc() or family. You're very right.

To get your job done, at the beginning, you've to make a copy of the orginal pointer returned by malloc() and once you're done with using the memory, you need to call free() with the previously stored value.

Answer 4

Yes. You understood right. When ptr is changed to point to different location then calling free will memory allocated by malloc will not be deallocated.
You need to keep a copy of the returned pointer.