If I programmed — without knowing it — a memory leak, and the application terminates, is the leaked memory freed?
Yes, a "memory leak" is simply memory that a process no longer has a reference to, and thus can no longer free. The OS still keeps track of all the memory allocated to a process, and will free it when that process terminates.
In the vast majority of cases the OS will free the memory - as is the case with normal "flavors" of Windows, Linux, Solaris, etc. However it is important to note that in specialized environments such as various Real-Time Operating Systems the memory may not be freed when the program is terminated.
The OS executing your program usually does cleanup memory that is not freed explicitly and handles that are not closed explicitly, but this is not guaranteed by the C++ standard. You may find some embedded device that do not free up your memory leaks.
That being said Windows and all distros of Linux that I've ever seen do free up memory leaks.
You can easily create a huge loop of memory leaks though to test it out yourself. Watch your RAM usage grow and then close your program. You will see that the RAM usage goes back down.
Another consideration when using C++ is that if you aren't deleting your heap allocated memory then your destructors are also not being called. Sometimes you will have other side effects as well if your destructors aren't called.
Are you running on a desktop OS (Windows, Linux etc.)? If so, yes, in general the system will free any memory associated with the program when the program exits.
Usually, yes. Some systems support things like shared memory blocks that aren't automatically freed when a program exits though. Most still keep a reference count and delete it when all the programs that opened it exit, but a few don't (e.g., 16-bit Windows had a few types of items that would remain allocated even when nothing referred to them -- though it usually crashed for other reasons before enough of this accumulated to cause a problem...)
As far as I know, a modern operating system will free this memory once the program terminates.
Depends on what memory you leaked. Some memory can't be reclaimed by the OS. Most memory on most OSes however will be automatically reclaimed when the process exits.
Related
I read wikipedia article saying
Memory leaks may not be serious or even detectable by normal means. In
modern operating systems, normal memory used by an application is
released when the application terminates. This means that a memory
leak in a program that only runs for a short time may not be noticed
and is rarely serious.
OS automatically release normal memory when terminating. Therefore, if memory leak was not serious by all means, leaked memory by program may not be matter after termination.
But it mentions only in case of "normal memory" and I got worried.
Could anyone explain what non-normal memory means?
How about few philosophical reasons?😀
For starters, if you ever need to redesign concept of lifetime in your program later, such as switch to a service, dll, multidoc support, or something like that, addressing this will become a must and an extra cost. Secondly, if the program does not free memory, chances are it does not release other critical resources, file locks on servers, for example (pure speculation, of course, I do not know what your program does).
But memory release on process termination will be guaranteed, outside of real-time systems on specialized hardware. Those can behave very differently.
In general; A program that leaks memory is only a problem while that program is running. It will take up more memory than it needs and may even run out. But, as soon as the program terminates, the OS kernel reclaims all memory that the program ever allocated, so it will all be free again and available for other uses.
It's actually a fairly common trick to intentionally leak memory on shutdown in some cases. If you know that your objects destructors don't do any work that is of any consequence if the program is shutting down anyway, then it can be faster to just leak the objects and let the OS clean up when you are terminating, rather than running all the destructors. However; only do that if you have a good reason to and really know what you are doing and why you are doing it.
As we know the heap is used for dynamic allocation of memory for an application. How is the heap memory cleared(and hence avoiding memory leaks) in case of abnormal application termination?
Consider the following scenarios:
Say an application crashes all of a sudden on Windows or Linux.
We force kill an application in linux: kill -9 <process_name>
A C++ program in Visual Studio throws an error in the middle of execution.
Is heap management and cleanup any different in the above cases? [Please add more use-case scenarios which might be of interest here]
This question came up in my mind since we always talk about ensuring no memory leak happens in our code. Now how do we handle scenarios where we force close an application which might result into a program exit without calling the memory free-up calls.
And if such memory leak happens repeatedly, is it possible that the OS becomes short of heap memory? Or does the OS have a way of handling it...
Assuming the OS is a typical implementation of Unix or Windows, the heap memory is released by the OS when the application is killed, no matter what method it is killed by.
Obviuously, other OS's may not do exactly this, and it's up to each OS to solve this problem in a meaningful way - I'm not aware of any OS that doesn't "clean up after killed processes", but I'm sure such a thing may exist in some corner of this world.
Edit: There may be OTHER resoources that aren't quite so easily released, such as shared memory or semaphores used by multiple. But most OS's tend to deal with those by releasing the reference of the application being killed, and letting other processes that wait for any "waitable object" (mutex, semaphore, etc) will be "let run".
"The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits" so killing/closing an application abnormally/normally won't leak any memory.
As for dynamic memory management you should use RAII(smart pointers is one example) to take care of memory leaks and management during exceptions and so on.
In cases where your application exits, the OS simply reclaims back all the memory it gave to the process. The OS doesn't understand leaks, it simply takes back what it gave to a process. So there is no leak per se. All memory is reclaimed. You might leak other resources(file descriptors etc) but a clever use of RAII should guard you against that.
It doesn't matter how your process closes, any remaining memory allocated from that process is freed by the OS memory manager when it closes. It's good practice for you to free all the memory you allocate before your process dies, but the available heap for the OS/other applications is the same after your process closes either way.
Just for fun I created a project that created about 5 GB of memory and did not delete it. As long as the application is running that "memory leak" is there. The second I close my application the memory within 2 seconds is back down to normal as if my program never ran. So the questions have to be asked.
Does Windows 7 clean up memory leaks from bad programs when they are done?
Do all Windows versions do this?
Will Linux and Mac OS X environments do this?
When the program terminates, the operating system reclaims all the memory that had previously been allocated to it. Cleaning up memory leaks may be a perceived by-product of this, but the OS does not actually see it that way. It does not know that the program had been leaking memory, just that it had allocated it.
Once a process in which your application runs exits, the OS reclaims all the memory allocated to the process.
This is typically true for all operating systems not just Windows 7 or Windows for that matter.
Note that, you may observe different behavior for other leaking resources like file handles etc, usually OS'es dont reclaim those. So, it is usually(Yes,there are exceptions) a good practice to make your own application clear the mess(deallocate the allocated resource) that it made instead of delegating it to the OS.
Not only does the program manages memory but does the OS too. And it reclaims all the memory allocated to a program after it exist. It doesn't interfere in between the execution of the program (Other than for paging and swapping). This control over memory of the OS helps the OS from crashing from memory leaks to a point.
Memory management is the act of managing computer memory. The
essential requirement of memory management is to provide ways to
dynamically allocate portions of memory to programs at their request,
and freeing it for reuse when no longer needed. This is critical to
the computer system.
BSD Unix normally starts reclaiming memory when the percentage of free memory drops below 5% and continues reclaiming until the free memory percentage reaches 7%
Yes (Windows 7 does reclaim all memory allocated to a program when the program exits, regardless of how it exits — under control or when crashing).
Yes (for any version of Windows that's recent enough to be still running).
Yes (Unix, Linux, Mac OS X, BSD all reclaim all memory allocated to a program when the program exits, regardless of how it exits).
A few old operating systems did not reacquire resources when a program exited. I believe AmigaOS was one; another, I believe, was the old Mac OS (Mac OS 9 and earlier). However, substantially all true multi-tasking systems have to reclaim memory (and resources in general) when the process to which it was allocated exits.
This is not the case for all OS's, for example, I do not believe WinXP will behave this way.
Though for most modern OS's it is now the case. I believe all current versions of Linux, Windows and MacOS do this.
For windows, I'm pretty sure it was introduced in Windows Vista. At the time it was a rather exciting improvement as there were A LOT of dodgey windows applications out there that didn't manage their memory well. At the time it was a big win for windows, but it had come late to the party (as usual), as Linux and MacOS were already doing it long before.
Having said that, I'm sure you appreciate that you definitely still need to manage your memory properly within your application and not simply rely upon your OS to clean up after you. Applications need to be efficient and predictable with their memory use during their runtime as well.
When a C/C++ program containing the dynamically allocated memory(using malloc/new) without free/delete calls is terminated, what happens to that dynamically allocated memory?
Does the operating system takes back the memory or does that memory becomes unaccessible to other programs?
I don't think that there are any guarantees in the language standard, but modern operating systems which support sparse virtual memory and memory protection (such as MacOS X, Linux, all recent version of Windows, and all currently manufactured phone handsets) automatically clean up after badly-behaved processes (when they terminate) and free the memory for you. The memory remains unavailable, however as long as the program is running.
If you're programming on microcontrollers, on MacOS 9 or earlier, DOS, or Windows 3.x, then you might need to be concerned about memory leaks making memory permanently unavailable to the whole operating system.
Most modern operating systems employ a memory manager, and all userland processes only see so-called virtual memory, which is not related to actual system memory in a way that the program could inspect. This means that programs cannot simply read another process's memory or kernel memory. It also means that the memory manager will completely "free" all memory that has been assigned to a process when that process terminates, so that memory leaks within the program do not usually "affect" the rest of the system (other than perhaps forcing a huge amount of disk swapping and perhaps some "out of memory" behaviour).
This doesn't mean that it's in any way OK to treat memory leaks light-heartedly, it only means that no single program can casually corrupt other processes on modern multi-tasking operating systems (deliberate abuse of administrative privileges notwithstanding, of course).
Short answer: yes, the OS will free this memory.
Most operating systems will free this memory, however it is bad practice to rely upon this behaviour. Some operating systems will not free this memory. For example, embedded systems. For portability, always free all the memory you allocate.
C/C++ doesn't have garbage collection feature. If you allocate memory and doesn't free it up, it's of no use while the program execution continues. This is called memory leak. Once the execution finishes, OS takes back this memory and is again available for use.
During the program's execution, you cannot count on the operating reclaiming the memory. That would be a garbage collection feature found in many other languages such as Java and C#. While garbage collected c++ is possible, I don't believe any mainstream implementations use it.
On the other hand, once your program completes its execution, the OS should reclaim the memory used by the program. So during execution the memory remains off-limits, but is accessible again after the program exits.
I have a doubt about the role of the operating system in regards to a process lifetime right now. I am using Linux.
Suppose that I have an application that creates a set of objects in the heap using new. During the lifetime of the application I have no need to delete any of those objects except at the exit the application or on an exception before exiting to do the clean-up.
Suppose I don't call delete at the end of the application for all those objects, do usually the OS reclaim/free all the heap allocated to make it available again at the process exits? If the process exit because of an exception or call to return or exit, does this always occurs?
If this is true this means that if I don't call delete there won't be any impact on the OS or on the other applications running on a machine. Right?
I usually use boost shared pointers or use delete but I would like just to clarify this doubt in a OS/Linux context
Kind Regards
AFG
That is correct. Any leak of memory after the lifetime of a process on a protected mode operating system is as a really nasty bug in the kernel (sometimes processes crash).
Having said that, the simplest way to check for memory leaks is to ensure that the heap has exactly the same number of allocated cells at the end of execution as it has at the beginning of execution. If you do not delete on exit, you have no way of checking this and will never discover legitimate memory leaks.
Modern OSes reclaim all resources of a closed process. Not only memory, but also file handles, etc.
No fear, the OS reclaims all the memory. What you need to watch out for is leaving some persistent resources, such as files, in an indeterminate state.
Just FYI my programming language deliberately fails to free memory on exit by default: it's faster that way. However RAII is not permitted. In C++ if you use RAII then you need to take more care.
Fast answer is no damage for OS if program don't call destructors of created objects or not freeing memory or other OS handles. Maximum impact is lost part of files which program has written before exiting.
Therefore Herb Satter in their book write about technique that short-lived applications specially doesn't free memory and don't call destructors for maximum speed of execution.
But better that programs normally handles their used OS resources.
(Sorry for my English)
You need to be more careful with other resources like file handles, database connections, etc. However, all memory is definitely reclaimed.