使用 realloc 收缩分配的 memory

Question

Simple question about the realloc function in C: If I use realloc to shrink the memory block that a pointer is pointing to, does the "extra" memory get freed? Or does it need to be freed manually somehow?

For example, if I do

int *myPointer = malloc(100*sizeof(int));
myPointer = realloc(myPointer,50*sizeof(int));
free(myPointer);

Will I have a memory leak?

Answer 1

No, you won't have a memory leak. realloc will simply mark the rest "available" for future malloc operations.

But you still have to free myPointer later on. As an aside, if you use 0 as the size in realloc , it will have the same effect as free on some implementations . As Steve Jessop and R.. said in the comments, you shouldn't rely on it.

Answer 2

There is definitely not a memory leak, but any of at least 3 things could happen when you call realloc to reduce the size:

1. The implementation splits the allocated memory block at the new requested length and frees the unused portion at the end.
2. The implementation makes a new allocation with the new size, copies the old contents to the new location, and frees the entire old allocation.
3. The implementation does nothing at all.

Option 3 would be a rather bad implementation, but perfectly legal; there's still no "memory leak" because the whole thing will still be freed if you later call free on it.

As for options 1 and 2, which is better depends a lot on whether you favor performance or avoiding memory fragmentation. I believe most real-world implementations will lean towards doing option 1.

Answer 3

The new code still leaks the original allocation if the realloc fails. I expect most implementations won't ever fail to shrink a block, but it's allowed. The correct way to call realloc, whether growing or shrinking the block, is void *tmp = realloc(myPointer, 50*sizeof(int)); if (.tmp) { /* handle error somehow, myPointer still points to the old block; which is still allocated */ } myPointer = tmp.. – Steve Jessop 48 mins ago

Hey, I couldn't figure out how to reply to your comment, sorry.

Do I need to cast tmp to the type of myPointer? In this case, do I need to write

myPointer = (int*)tmp

Also, in this case, when I do free(myPointer) The memory pointed at by tmp will be freed as well, right? So no need to do

free(myPointer)
free(tmp)

Answer 4

In the way you have given your code, yes, it might have a leak. The idea of realloc is that it can return you a new location of your data. Like you do it in your question you lose that pointer that realloc sends you.

int *myPointer2 = realloc(myPointer,50*sizeof(int));
assert(myPointer2); 
myPointer = myPointer2;

Answer 5

I guess the process for realloc() is, it first dealocates the previous memory block then, allocates again (My college teacher told that). If that remains the case, it would have already freed the extra 50 bytes. And if there is data in 100 byte memory, will get assigned to 50 reallocated bytes memory, which may, lead to loss of data. So there shouldn't be a Memory Leak.