Are memory leaks ever ok? [closed]

Question

Is it ever acceptable to have a memory leak in your C or C++ application?

What if you allocate some memory and use it until the very last line of code in your application (for example, a global object's destructor)? As long as the memory consumption doesn't grow over time, is it OK to trust the OS to free your memory for you when your application terminates (on Windows, Mac, and Linux)? Would you even consider this a real memory leak if the memory was being used continuously until it was freed by the OS.

What if a third party library forced this situation on you? Would refuse to use that third party library no matter how great it otherwise might be?

I only see one practical disadvantage, and that is that these benign leaks will show up with memory leak detection tools as false positives.

Answer 1

I'm sure that someone can come up with a reason to say Yes, but it won't be me. Instead of saying no, I'm going to say that this shouldn't be a yes/no question. There are ways to manage or contain memory leaks, and many systems have them.

There are NASA systems on devices that leave the earth that plan for this. The systems will automatically reboot every so often so that memory leaks will not become fatal to the overall operation. Just an example of containment.

Answer 2

While most answers concentrate on real memory leaks (which are not OK ever, because they are a sign of sloppy coding), this part of the question appears more interesting to me:

What if you allocate some memory and use it until the very last line of code in your application (for example, a global object's deconstructor)? As long as the memory consumption doesn't grow over time, is it OK to trust the OS to free your memory for you when your application terminates (on Windows, Mac, and Linux)? Would you even consider this a real memory leak if the memory was being used continuously until it was freed by the OS.

If the associated memory is used, you cannot free it before the program ends. Whether the free is done by the program exit or by the OS does not matter. As long as this is documented, so that change don't introduce real memory leaks, and as long as there is no C++ destructor or C cleanup function involved in the picture. A not-closed file might be revealed through a leaked FILE object, but a missing fclose() might also cause the buffer not to be flushed.

So, back to the original case, it is IMHO perfectly OK in itself, so much that Valgrind, one of the most powerful leak detectors, will treat such leaks only if requested. On Valgrind, when you overwrite a pointer without freeing it beforehand, it gets considered as a memory leak, because it is more likely to happen again and to cause the heap to grow endlessly.

Then, there are not nfreed memory blocks which are still reachable. One could make sure to free all of them at the exit, but that is just a waste of time in itself. The point is if they could be freed before. Lowering memory consumption is useful in any case.

Answer 3

Generally a memory leak in a stand alone application is not fatal, as it gets cleaned up when the program exits.

What do you do for Server programs that are designed so they don't exit?

If you are the kind of programmer that does not design and implement code where the resources are allocated and released correctly, then I don't want anything to do with you or your code. If you don't care to clean up your leaked memory, what about your locks? Do you leave them hanging out there too? Do you leave little turds of temporary files laying around in various directories?

Leak that memory and let the program clean it up? No. Absolutely not. It's a bad habit, that leads to bugs, bugs, and more bugs.

Clean up after yourself. Yo momma don't work here no more.

Answer 4

I don't consider it to be a memory leak unless the amount of memory being "used" keeps growing. Having some unreleased memory, while not ideal, is not a big problem unless the amount of memory required keeps growing.

Answer 5

Many people seem to be under the impression that once you free memory, it's instantly returned to the operating system and can be used by other programs.

This isn't true. Operating systems commonly manage memory in 4KiB pages. malloc and other sorts of memory management get pages from the OS and sub-manage them as they see fit. It's quite likely that free() will not return pages to the operating system, under the assumption that your program will malloc more memory later.

I'm not saying that free() never returns memory to the operating system. It can happen, particularly if you are freeing large stretches of memory. But there's no guarantee.

The important fact: If you don't free memory that you no longer need, further mallocs are guaranteed to consume even more memory. But if you free first, malloc might re-use the freed memory instead.

What does this mean in practice? It means that if you know your program isn't going to require any more memory from now on (for instance it's in the cleanup phase), freeing memory is not so important. However if the program might allocate more memory later, you should avoid memory leaks - particularly ones that can occur repeatedly.

Also see this comment for more details about why freeing memory just before termination is bad.

A commenter didn't seem to understand that calling free() does not automatically allow other programs to use the freed memory. But that's the entire point of this answer!

So, to convince people, I will demonstrate an example where free() does very little good. To make the math easy to follow, I will pretend that the OS manages memory in 4000 byte pages.

Suppose you allocate ten thousand 100-byte blocks (for simplicity I'll ignore the extra memory that would be required to manage these allocations). This consumes 1MB, or 250 pages. If you then free 9000 of these blocks at random, you're left with just 1000 blocks - but they're scattered all over the place. Statistically, about 5 of the pages will be empty. The other 245 will each have at least one allocated block in them. That amounts to 980KB of memory, that cannot possibly be reclaimed by the operating system - even though you now only have 100KB allocated!

On the other hand, you can now malloc() 9000 more blocks without increasing the amount of memory your program is tying up.

Even when free() could technically return memory to the OS, it may not do so. free() needs to achieve a balance between operating quickly and saving memory. And besides, a program that has already allocated a lot of memory and then freed it is likely to do so again. A web server needs to handle request after request after request - it makes sense to keep some "slack" memory available so you don't need to ask the OS for memory all the time.

Answer 6

I believe the answer is no, never allow a memory leak, and I have a few reasons which I haven't seen explicitly stated. There are great technical answers here but I think the real answer hinges on more social/human reasons.

(First, note that as others mentioned, a true leak is when your program, at any point, loses track of memory resources that it has allocated. In C, this happens when you malloc() to a pointer and let that pointer leave scope without doing a free() first.)

The important crux of your decision here is habit. When you code in a language that uses pointers, you're going to use pointers a lot. And pointers are dangerous; they're the easiest way to add all manner of severe problems to your code.

When you're coding, sometimes you're going to be on the ball and sometimes you're going to be tired or mad or worried. During those somewhat distracted times, you're coding more on autopilot. The autopilot effect doesn't differentiate between one-off code and a module in a larger project. During those times, the habits you establish are what will end up in your code base.

So no, never allow memory leaks for the same reason that you should still check your blind spots when changing lanes even if you're the only car on the road at the moment. During times when your active brain is distracted, good habits are all that can save you from disastrous missteps.

Beyond the "habit" issue, pointers are complex and often require a lot of brain power to track mentally. It's best to not "muddy the water" when it comes to your usage of pointers, especially when you're new to programming.

There's a more social aspect too. By proper use of malloc() and free(), anyone who looks at your code will be at ease; you're managing your resources. If you don't, however, they'll immediately suspect a problem.

Maybe you've worked out that the memory leak doesn't hurt anything in this context, but every maintainer of your code will have to work that out in his head too when he reads that piece of code. By using free() you remove the need to even consider the issue.

Finally, programming is writing a mental model of a process to an unambiguous language so that a person and a computer can perfectly understand said process. A vital part of good programming practice is never introducing unnecessary ambiguity.

Smart programming is flexible and generic. Bad programming is ambiguous.