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

Let's get our definitions correct, first. A memory leak is when memory is dynamically allocated, eg with malloc(), and all references to the memory are lost without the corresponding free. An easy way to make one is like this:

#define BLK ((size_t)1024)
while(1){
    void * vp = malloc(BLK);
}

Note that every time around the while(1) loop, 1024 (+overhead) bytes are allocated, and the new address assigned to vp; there's no remaining pointer to the previous malloc'ed blocks. This program is guaranteed to run until the heap runs out, and there's no way to recover any of the malloc'ed memory. Memory is "leaking" out of the heap, never to be seen again.

What you're describing, though, sound like

int main(){
    void * vp = malloc(LOTS);
    // Go do something useful
    return 0;
}

You allocate the memory, work with it until the program terminates. This is not a memory leak; it doesn't impair the program, and all the memory will be scavenged up automagically when the program terminates.

Generally, you should avoid memory leaks. First, because like altitude above you and fuel back at the hangar, memory that has leaked and can't be recovered is useless; second, it's a lot easier to code correctly, not leaking memory, at the start than it is to find a memory leak later.

Answer 2

In theory no, in practise it depends.

It really depends on how much data the program is working on, how often the program is run and whether or not it is running constantly.

If I have a quick program that reads a small amount of data makes a calculation and exits, a small memory leak will never be noticed. Because the program is not running for very long and only uses a small amount of memory, the leak will be small and freed when the program exists.

On the other hand if I have a program that processes millions of records and runs for a long time, a small memory leak might bring down the machine given enough time.

As for third party libraries that have leaks, if they cause a problem either fix the library or find a better alternative. If it doesn't cause a problem, does it really matter?

Answer 3

This was already discussed ad nauseam. Bottom line is that a memory leak is a bug and must be fixed. If a third party library leaks memory, it makes one wonder what else is wrong with it, no? If you were building a car, would you use an engine that is occasionally leaking oil? After all, somebody else made the engine, so it's not your fault and you can't fix it, right?

Answer 4

Its really not a leak if its intentional and its not a problem unless its a significant amount of memory, or could grow to be a significant amount of memory. Its fairly common to not cleanup global allocations during the lifetime of a program. If the leak is in a server or long running app, grows over time, then its a problem.

Answer 5

I see the same problem as all scenario questions like this: What happens when the program changes, and suddenly that little memory leak is being called ten million times and the end of your program is in a different place so it does matter? If it's in a library then log a bug with the library maintainers, don't put a leak into your own code.

Answer 6

I can count on one hand the number of "benign" leaks that I've seen over time.

So the answer is a very qualified yes.

An example. If you have a singleton resource that needs a buffer to store a circular queue or deque but doesn't know how big the buffer will need to be and can't afford the overhead of locking or every reader, then allocating an exponentially doubling buffer but not freeing the old ones will leak a bounded amount of memory per queue/deque. The benefit for these is they speed up every access dramatically and can change the asymptotics of multiprocessor solutions by never risking contention for a lock.

I've seen this approach used to great benefit for things with very clearly fixed counts such as per-CPU work-stealing deques, and to a much lesser degree in the buffer used to hold the singleton /proc/self/maps state in Hans Boehm's conservative garbage collector for C/C++, which is used to detect the root sets, etc.

While technically a leak, both of these cases are bounded in size and in the growable circular work stealing deque case there is a huge performance win in exchange for a bounded factor of 2 increase in the memory usage for the queues.