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.
Related
I've run into memory leaks many times. Usually when I'm malloc-ing like there's no tomorrow, or dangling FILE *s like dirty laundry. I generally assume (read: hope desperately) that all memory is cleaned up at least when the program terminates. Are there any situations where leaked memory won't be collected when the program terminates, or crashes?
If the answer varies widely from language-to-language, then let's focus on C(++).
Please note hyperbolic usage of the phrase, 'like there's no tomorrow', and 'dangling ... like dirty laundry'. Unsafe* malloc*ing can hurt the ones you love. Also, please use caution with dirty laundry.
No. Operating systems free all resources held by processes when they exit.
This applies to all resources the operating system maintains: memory, open files, network connections, window handles...
That said, if the program is running on an embedded system without an operating system, or with a very simple or buggy operating system, the memory might be unusable until a reboot. But if you were in that situation you probably wouldn't be asking this question.
The operating system may take a long time to free certain resources. For example the TCP port that a network server uses to accept connections may take minutes to become free, even if properly closed by the program. A networked program may also hold remote resources such as database objects. The remote system should free those resources when the network connection is lost, but it may take even longer than the local operating system.
The C Standard does not specify that memory allocated by malloc is released when the program terminates. This done by the operating system and not all OSes (usually these are in the embedded world) release the memory when the program terminates.
As all the answers have covered most aspects of your question w.r.t. modern OSes, but historically, there is one that is worth mentioning if you have ever programmed in the DOS world. Terminant and Stay Resident (TSR) programs would usually return control to the system but would reside in memory which could be revived by a software / hardware interrupt. It was normal to see messages like "out of memory! try unloading some of your TSRs" when working on these OSes.
So technically the program terminates, but because it still resides on memory, any memory leak would not be released unless you unload the program.
So you can consider this to be another case apart from OSes not reclaiming memory either because it's buggy or because the embedded OS is designed to do so.
I remember one more example. Customer Information Control System (CICS), a transaction server which runs primarily on IBM mainframes is pseudo-conversational. When executed, it processes the user entered data, generates another set of data for the user, transferring to the user terminal node and terminates. On activating the attention key, it again revives to process another set of data. Because the way it behaves, technically again, the OS won't reclaim memory from the terminated CICS Programs, unless you recycle the CICS transaction server.
Like the others have said, most operating systems will reclaim allocated memory upon process termination (and probably other resources like network sockets, file handles, etc).
Having said that, the memory may not be the only thing you need to worry about when dealing with new/delete (instead of raw malloc/free). The memory that's allocated in new may get reclaimed, but things that may be done in the destructors of the objects will not happen. Perhaps the destructor of some class writes a sentinel value into a file upon destruction. If the process just terminates, the file handle may get flushed and the memory reclaimed, but that sentinel value wouldn't get written.
Moral of the story, always clean up after yourself. Don't let things dangle. Don't rely on the OS cleaning up after you. Clean up after yourself.
This is more likely to depend on operating system than language. Ultimately any program in any language will get it's memory from the operating system.
I've never heard of an operating system that doesn't recycle memory when a program exits/crashes. So if your program has an upper bound on the memory it needs to allocate, then just allocating and never freeing is perfectly reasonable.
If the program is ever turned into a dynamic component ("plugin") that is loaded into another program's address space, it will be troublesome, even on an operating system with tidy memory management. We don't even have to think about the code being ported to less capable systems.
On the other hand, releasing all memory can impact the performance of a program's cleanup.
One program I was working on, a certain test case required 30 seconds or more for the program to exit, because it was recursing through the graph of all dynamic memory and releasing it piece by piece.
A reasonable solution is to have the capability there and cover it with test cases, but turn it off in production code so the application quits fast.
All operating systems deserving the title will clean up the mess your process made after termination. But there are always unforeseen events, what if it was denied access somehow and some poor programmer did not foresee the possibility and so it doesn't try again a bit later?
Always safer to just clean up yourself IF memory leaks are mission critical - otherwise not really worth the effort IMO if that effort is costly.
Edit:
You do need to clean up memory leaks if they are in place where they will accumulate, like in loops. The memory leaks I speak of are ones that build up in constant time throughout the course of the program, if you have a leak of any other sort it will most likely be a serious problem sooner or later.
In technical terms if your leaks are of memory 'complexity' O(1) they are fine in most cases, O(logn) already unpleasant (and in some cases fatal) and O(N)+ intolerable.
Shared memory on POSIX compliant systems persists until shm_unlink is called or the system is rebooted.
If you have interprocess communication, this can lead to other processes never completing and consuming resources depending on the protocol.
To give an example, I was once experimenting with printing to a PDF printer in Java when I terminated the JVM in the middle of a printer job, the PDF spooling process remained active, and I had to kill it in the task manager before I could retry printing.
I want to know, suppose I am debugging a code and at any point I allocated some memory and break point hits for example:
1: Statement to allocate 1 MB memory in **C**.
2: Any other statement where **BREAKPOINT HIT**.
Now my question is:
If I will kill my IDE (like visual studio) directly using task manager then allocated memory and resources will free or not.
If I will stop debugging then allocated memory and resources will free or not.
If yes then How can I confirm it whether memory and resources have freed.
On modern operating systems, all the memory for your program is returned to the system when the program is terminated, which will happen in either of those cases. This might not happen on some embedded systems, but you wouldn't be running an IDE on those.
For other resources than memory, e.g., open files, devices, etc., the OS will generally reclaim all resources (unless they're still in use by other processes), but for some systems and some resources, under some conditions, resources can be lost or locked (which should be considered a bug in the OS or the device driver).
As far as determining that the system actually freed the memory, it can be quite difficult because the system allocates memory to buffers and swap areas and doesn't necessarily have a count of free space that you can check. For other resources ... if you can't acquire them then they weren't released.
I've run into memory leaks many times. Usually when I'm malloc-ing like there's no tomorrow, or dangling FILE *s like dirty laundry. I generally assume (read: hope desperately) that all memory is cleaned up at least when the program terminates. Are there any situations where leaked memory won't be collected when the program terminates, or crashes?
If the answer varies widely from language-to-language, then let's focus on C(++).
Please note hyperbolic usage of the phrase, 'like there's no tomorrow', and 'dangling ... like dirty laundry'. Unsafe* malloc*ing can hurt the ones you love. Also, please use caution with dirty laundry.
No. Operating systems free all resources held by processes when they exit.
This applies to all resources the operating system maintains: memory, open files, network connections, window handles...
That said, if the program is running on an embedded system without an operating system, or with a very simple or buggy operating system, the memory might be unusable until a reboot. But if you were in that situation you probably wouldn't be asking this question.
The operating system may take a long time to free certain resources. For example the TCP port that a network server uses to accept connections may take minutes to become free, even if properly closed by the program. A networked program may also hold remote resources such as database objects. The remote system should free those resources when the network connection is lost, but it may take even longer than the local operating system.
The C Standard does not specify that memory allocated by malloc is released when the program terminates. This done by the operating system and not all OSes (usually these are in the embedded world) release the memory when the program terminates.
As all the answers have covered most aspects of your question w.r.t. modern OSes, but historically, there is one that is worth mentioning if you have ever programmed in the DOS world. Terminant and Stay Resident (TSR) programs would usually return control to the system but would reside in memory which could be revived by a software / hardware interrupt. It was normal to see messages like "out of memory! try unloading some of your TSRs" when working on these OSes.
So technically the program terminates, but because it still resides on memory, any memory leak would not be released unless you unload the program.
So you can consider this to be another case apart from OSes not reclaiming memory either because it's buggy or because the embedded OS is designed to do so.
I remember one more example. Customer Information Control System (CICS), a transaction server which runs primarily on IBM mainframes is pseudo-conversational. When executed, it processes the user entered data, generates another set of data for the user, transferring to the user terminal node and terminates. On activating the attention key, it again revives to process another set of data. Because the way it behaves, technically again, the OS won't reclaim memory from the terminated CICS Programs, unless you recycle the CICS transaction server.
Like the others have said, most operating systems will reclaim allocated memory upon process termination (and probably other resources like network sockets, file handles, etc).
Having said that, the memory may not be the only thing you need to worry about when dealing with new/delete (instead of raw malloc/free). The memory that's allocated in new may get reclaimed, but things that may be done in the destructors of the objects will not happen. Perhaps the destructor of some class writes a sentinel value into a file upon destruction. If the process just terminates, the file handle may get flushed and the memory reclaimed, but that sentinel value wouldn't get written.
Moral of the story, always clean up after yourself. Don't let things dangle. Don't rely on the OS cleaning up after you. Clean up after yourself.
This is more likely to depend on operating system than language. Ultimately any program in any language will get it's memory from the operating system.
I've never heard of an operating system that doesn't recycle memory when a program exits/crashes. So if your program has an upper bound on the memory it needs to allocate, then just allocating and never freeing is perfectly reasonable.
If the program is ever turned into a dynamic component ("plugin") that is loaded into another program's address space, it will be troublesome, even on an operating system with tidy memory management. We don't even have to think about the code being ported to less capable systems.
On the other hand, releasing all memory can impact the performance of a program's cleanup.
One program I was working on, a certain test case required 30 seconds or more for the program to exit, because it was recursing through the graph of all dynamic memory and releasing it piece by piece.
A reasonable solution is to have the capability there and cover it with test cases, but turn it off in production code so the application quits fast.
All operating systems deserving the title will clean up the mess your process made after termination. But there are always unforeseen events, what if it was denied access somehow and some poor programmer did not foresee the possibility and so it doesn't try again a bit later?
Always safer to just clean up yourself IF memory leaks are mission critical - otherwise not really worth the effort IMO if that effort is costly.
Edit:
You do need to clean up memory leaks if they are in place where they will accumulate, like in loops. The memory leaks I speak of are ones that build up in constant time throughout the course of the program, if you have a leak of any other sort it will most likely be a serious problem sooner or later.
In technical terms if your leaks are of memory 'complexity' O(1) they are fine in most cases, O(logn) already unpleasant (and in some cases fatal) and O(N)+ intolerable.
Shared memory on POSIX compliant systems persists until shm_unlink is called or the system is rebooted.
If you have interprocess communication, this can lead to other processes never completing and consuming resources depending on the protocol.
To give an example, I was once experimenting with printing to a PDF printer in Java when I terminated the JVM in the middle of a printer job, the PDF spooling process remained active, and I had to kill it in the task manager before I could retry printing.
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.
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.