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I directly started with managed languages and have barely any experience with C++, hence this question might be too basic.

In a managed language like .net, GC frees the memory. From what I read, in C++ this is done by calling delete. But what does it do to free memory? Does it it set all the bits at a memory location to zero? Or does it in some other way tell the operating system that the memory is available for reuse?

Update: I have been thru this before and I know what GC does. But thats not my question. I am not trying to ask how GC works. What I am trying to understand is, how do you tell some memory is free?

developer747
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  • *When a garbage collector frees memory, what does it do exactly?* It can do any of the things you enumerated plus more, as long as it complies with the CLR specification it doesn't matter what ends are used to achieve that means. – ta.speot.is Jul 06 '13 at 01:03
  • I'm still just trying to figure out who Will is, and why he needs to be freed. Just who is this Memory guy? – Hot Licks Jul 06 '13 at 01:36
  • Just so you all know, it was NOT me who downvoted all the answers. – developer747 Jul 06 '13 at 14:11

6 Answers6

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delete does three different things:

  1. Runs the destructor of the object (or of all objects in the array in the case of delete[]).

  2. Marks the chunk of memory previously used by the object as free.

  3. If possible, informs the operating system that a chunk of memory is free for other programs to use.

Your question is about #2 and #3 together, but they are very different things. To understand how #2 works, remember that the (typically) single "heap" provided by the operating system is segmented into smaller chunks of different sizes. When you allocate a chunk of memory with new, you get a pointer to a previously free part of the heap, and the runtime performs the necessary bookkeeping that marks that region as unavailable for further allocations. delete does the reverse: it performs the bookkeeping that marks the region as available again, optionally coalescing it with adjacent free regions to reduce fragmentation. Subsequent calls to new will consider that region when looking for free memory to return.

In other words, it is wrong to ask what happens with the memory when it is deallocated. The real magic happens in the bookkeeping region! To learn about implementation of generic allocators, google for implementation of malloc.

As for #3, it is an optional step and in many cases impossible to perform. It is only possible to "give back" freed memory that happens to reside at the very end of the allocated heap. A single allocation situated after a large region will remove the possibility of giving back.

user4815162342
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  • Since he's mentioning *managed languages* and *.NET*, I would say that this answer strays from the topic. – Lasse V. Karlsen Jul 06 '13 at 21:10
  • @LasseV.Karlsen I believe it's exactly the other way around: the OP specifically says that he understands how the GC works, but wants to know what it means to "free" some memory. In the 2nd paragraph he explicitly asks "what does [delete] do to free memory", so it's clear that the OP is curious about low-level allocation. – user4815162342 Jul 06 '13 at 21:16
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In C++, if you allocate memory using "new", that portion of memory will be allocated by the OS to that particular process, until you release that memory or until that process exits.

If some portion of memory allocated for a process means that OS does not allow other process to use that portion of memory until that process release that memory. In C++, you have to use "delete" to release memory.

By releasing memory portion, process just inform the OS that it does not use this portion any more so that OS can allocate that memory portion whenever other processes request memory. In that case content of the memory portion will not be changed.

SRF
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Garbage collection is just automatic memory management (so you never need to delete anything, the system will take care of it for you). I'm not 100% on whether it sets memory locations to 0, but I would assume it doesnt, since when you delete in c++, thats not what happens, it just allows the space to be used for storage. Writing zeros over everything is much more inefficient and not necessary. Here's some links that might be able to help explain this more thoroughly:

How does garbage collection and scoping work in C#?

http://en.wikipedia.org/wiki/Garbage_collection_(computer_science)

Community
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taylorc93
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  • so what exactly does delete do in C++? Does it in some way communicate to the operating system that this memory is free for use? – developer747 Jul 06 '13 at 01:04
  • @developer747: It calls the destructor of the object to do any clean-up that the code needs, then calls the function `operator delete` on the pointer, which in turn deallocates the memory by calling a function like `free`, which in turns might let the operating system know the memory is now freed. – user541686 Jul 06 '13 at 01:06
  • To put it simply, yes. To free memory in c++, you can either reset your pointers to `null` (effectively writing zeroes over that memory) or call `delete`, which just returns that memory to the heap and allows it to be reused – taylorc93 Jul 06 '13 at 01:07
  • @taylorc93:So freeing memory is setting all bits to 0s, correct? – developer747 Jul 06 '13 at 01:18
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No, it does not set the bits to zero. In a very simplified explanation,

First the garbage collector must determine, not what objects are no longer accessible ("not reachable"), but which ones are still accessible or reachable. It does this by simply listing all object roots. A root is a memory location containing a pointer to a reference object (an object on the heap). Then, recursively, it flags as "reachable" every object referenced by a root, or referenced by a field or property of a object already flagged as reachable.
There are four types of roots.

  1. static variables containing reference objects
  2. reference objects on the stack for any currently active thread.
  3. reference types in method parameters
  4. reference objects pointed to by CPU registers.

After determining what reference objects are still accessible (reachable) by any code in the App Domain, it takes all those objects that are still reachable, and if there are any gaps in physical memory between them, it "defragments" them by moving some of them so they are all contiguous, then it sets the pointer which represents the "end" od "used" memory to the end of this new compressed defragmented list. All new memory allocations, for newly instantiated objects, are then allocated from the memory immediately after this pointer location.

If there are no gaps in the memory used by the reachable objects, it just resets the pointer to the end of the last reachable object in the list.

Charles Bretana
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  • I understand the defragmentation part as explained in http://www.youtube.com/watch?v=rp8PvFvSO_c . But what I am trying to understand is, what determines that memory is free? Is it what is in the memory (all 0s) or is there some tracking done by the operating system? – developer747 Jul 06 '13 at 01:06
  • No worries.. I thought you wanted to know how the memory was released, niot how to determine which memory to release. I will edit to include that as well. – Charles Bretana Jul 06 '13 at 02:31
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Inside each application, dynamic memory are managed by "heaps". When your code asks for a block of memory, it asks the heap manager to allocate a block of memory, when your application frees that block of memory, it returns it back to the heap manager. In a traditional application, you must explicitly return each memory you allocated. Otherwise you will eventually run out of memory.

In languages like C# or Java, the runtime offers a garbage collector. A garbage collector automatically identify "unreachable" memory block and free them. An unreable memory block is a block that is no longer referenced by any variables. For example, if you have a global variable p1 that points to a block of memory, because p1 is global, so it is visible to anywhere in your code, then it is always reachable. Thus it will never be released by garbage collector. On the other hand, if you have local variable p2 in one of your function Foo, vairable p2 is no longer reachable after Foo has returned. The garbage collector is able to identify such variables and free any memory block pointed by them.

As application/garbage collector interact with the heap, the heap may decide to ask for more memory from the OS or return it to the OS. The OS manages all these memory request from different process and it then decide how to allocate the actual physical memory to different process.

user2555312
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No, deleting a pointer does not set the bytes to zero. It's not in the standard of course, but it would be a performance overhead and serious implementations don't bother doing it, and it does not even make sense, when the memory is used for complex objects (floats, objects, strings, etc)

You can always try it out. Declare a pointer to an int, write an integer, delete the pointer. Then read the content of the deleted pointer again. Does it have the same content?

int *ptr = new int;
*ptr = 13;
cout << "Before delete: " << *ptr << endl;
delete ptr;
cout << "After delete: " << *ptr << endl;

Yes probably it will, BUT ptr is just a dangling pointer you have there, the memory has been returned to the system and it will be available when you allocate memory again, it's likely that when you allocate another int* it will be pointing where ptr was pointing.

Stefanos T.
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