We are all taught that you MUST free every pointer that is allocated. I'm a bit curious, though, about the real cost of not freeing memory. In some obvious cases, like when malloc() is called inside a loop or part of a thread execution, it's very important to free so there are no memory leaks. But consider the following two examples:
First, if I have code that's something like this:
int main()
{
char *a = malloc(1024);
/* Do some arbitrary stuff with 'a' (no alloc functions) */
return 0;
}
What's the real result here? My thinking is that the process dies and then the heap space is gone anyway so there's no harm in missing the call to free (however, I do recognize the importance of having it anyway for closure, maintainability, and good practice). Am I right in this thinking?
Second, let's say I have a program that acts a bit like a shell. Users can declare variables like aaa = 123 and those are stored in some dynamic data structure for later use. Clearly, it seems obvious that you'd use some solution that will calls some *alloc function (hashmap, linked list, something like that). For this kind of program, it doesn't make sense to ever free after calling malloc because these variables must be present at all times during the program's execution and there's no good way (that I can see) to implement this with statically allocated space. Is it bad design to have a bunch of memory that's allocated but only freed as part of the process ending? If so, what's the alternative?
Just about every modern operating system will recover all the allocated memory space after a program exits. The only exception I can think of might be something like Palm OS where the program's static storage and runtime memory are pretty much the same thing, so not freeing might cause the program to take up more storage. (I'm only speculating here.)
So generally, there's no harm in it, except the runtime cost of having more storage than you need. Certainly in the example you give, you want to keep the memory for a variable that might be used until it's cleared.
However, it's considered good style to free memory as soon as you don't need it any more, and to free anything you still have around on program exit. It's more of an exercise in knowing what memory you're using, and thinking about whether you still need it. If you don't keep track, you might have memory leaks.
On the other hand, the similar admonition to close your files on exit has a much more concrete result - if you don't, the data you wrote to them might not get flushed, or if they're a temp file, they might not get deleted when you're done. Also, database handles should have their transactions committed and then closed when you're done with them. Similarly, if you're using an object oriented language like C++ or Objective C, not freeing an object when you're done with it will mean the destructor will never get called, and any resources the class is responsible might not get cleaned up.
Yes you are right, your example doesn't do any harm (at least not on most modern operating systems). All the memory allocated by your process will be recovered by the operating system once the process exits.
Source: Allocation and GC Myths (PostScript alert!)
Allocation Myth 4: Non-garbage-collected programs
should always deallocate all memory
they allocate.
The Truth: Omitted
deallocations in frequently executed
code cause growing leaks. They are
rarely acceptable. but Programs that
retain most allocated memory until
program exit often perform better
without any intervening deallocation.
Malloc is much easier to implement if
there is no free.
In most cases, deallocating memory
just before program exit is pointless.
The OS will reclaim it anyway. Free
will touch and page in the dead
objects; the OS won't.
Consequence: Be careful with "leak
detectors" that count allocations.
Some "leaks" are good!
That said, you should really try to avoid all memory leaks!
Second question: your design is ok. If you need to store something until your application exits then its ok to do this with dynamic memory allocation. If you don't know the required size upfront, you can't use statically allocated memory.
=== What about future proofing and code reuse? ===
If you don't write the code to free the objects, then you are limiting the code to only being safe to use when you can depend on the memory being free'd by the process being closed ... i.e. small one-time use projects or "throw-away"[1] projects)... where you know when the process will end.
If you do write the code that free()s all your dynamically allocated memory, then you are future proofing the code and letting others use it in a larger project.
[1] regarding "throw-away" projects. Code used in "Throw-away" projects has a way of not being thrown away. Next thing you know ten years have passed and your "throw-away" code is still being used).
I heard a story about some guy who wrote some code just for fun to make his hardware work better. He said "just a hobby, won't be big and professional". Years later lots of people are using his "hobby" code.
You are correct, no harm is done and it's faster to just exit
There are various reasons for this:
All desktop and server environments simply release the entire memory space on exit(). They are unaware of program-internal data structures such as heaps.
Almost all free() implementations do not ever return memory to the operating system anyway.
More importantly, it's a waste of time when done right before exit(). At exit, memory pages and swap space are simply released. By contrast, a series of free() calls will burn CPU time and can result in disk paging operations, cache misses, and cache evictions.
Regarding the possiblility of future code reuse justifing the certainty of pointless ops: that's a consideration but it's arguably not the Agile way. YAGNI!
I completely disagree with everyone who says OP is correct or there is no harm.
Everyone is talking about a modern and/or legacy OS's.
But what if I'm in an environment where I simply have no OS?
Where there isn't anything?
Imagine now you are using thread styled interrupts and allocate memory.
In the C standard ISO/IEC:9899 is the lifetime of memory stated as:
7.20.3 Memory management functions
1 The order and contiguity of storage allocated by successive calls to the calloc,
malloc, and realloc functions is unspecified. The pointer returned if the allocation
succeeds is suitably aligned so that it may be assigned to a pointer to any type of object
and then used to access such an object or an array of such objects in the space allocated
(until the space is explicitly deallocated). The lifetime of an allocated object extends
from the allocation until the deallocation.[...]
So it has not to be given that the environment is doing the freeing job for you.
Otherwise it would be added to the last sentence: "Or until the program terminates."
So in other words:
Not freeing memory is not just bad practice. It produces non portable and not C conform code.
Which can at least be seen as 'correct, if the following: [...], is supported by environment'.
But in cases where you have no OS at all, no one is doing the job for you
(I know generally you don't allocate and reallocate memory on embedded systems,
but there are cases where you may want to.)
So speaking in general plain C (as which the OP is tagged),
this is simply producing erroneous and non portable code.
I typically free every allocated block once I'm sure that I'm done with it. Today, my program's entry point might be main(int argc, char *argv[]) , but tomorrow it might be foo_entry_point(char **args, struct foo *f) and typed as a function pointer.
So, if that happens, I now have a leak.
Regarding your second question, if my program took input like a=5, I would allocate space for a, or re-allocate the same space on a subsequent a="foo". This would remain allocated until:
The user typed 'unset a'
My cleanup function was entered, either servicing a signal or the user typed 'quit'
I can not think of any modern OS that does not reclaim memory after a process exits. Then again, free() is cheap, why not clean up? As others have said, tools like valgrind are great for spotting leaks that you really do need to worry about. Even though the blocks you example would be labeled as 'still reachable' , its just extra noise in the output when you're trying to ensure you have no leaks.
Another myth is "If its in main(), I don't have to free it", this is incorrect. Consider the following:
char *t;
for (i=0; i < 255; i++) {
t = strdup(foo->name);
let_strtok_eat_away_at(t);
}
If that came prior to forking / daemonizing (and in theory running forever), your program has just leaked an undetermined size of t 255 times.
A good, well written program should always clean up after itself. Free all memory, flush all files, close all descriptors, unlink all temporary files, etc. This cleanup function should be reached upon normal termination, or upon receiving various kinds of fatal signals, unless you want to leave some files laying around so you can detect a crash and resume.
Really, be kind to the poor soul who has to maintain your stuff when you move on to other things .. hand it to them 'valgrind clean' :)
It is completely fine to leave memory unfreed when you exit; malloc() allocates the memory from the memory area called "the heap", and the complete heap of a process is freed when the process exits.
That being said, one reason why people still insist that it is good to free everything before exiting is that memory debuggers (e.g. valgrind on Linux) detect the unfreed blocks as memory leaks, and if you have also "real" memory leaks, it becomes more difficult to spot them if you also get "fake" results at the end.
This code will usually work alright, but consider the problem of code reuse.
You may have written some code snippet which doesn't free allocated memory, it is run in such a way that memory is then automatically reclaimed. Seems allright.
Then someone else copies your snippet into his project in such a way that it is executed one thousand times per second. That person now has a huge memory leak in his program. Not very good in general, usually fatal for a server application.
Code reuse is typical in enterprises. Usually the company owns all the code its employees produce and every department may reuse whatever the company owns. So by writing such "innocently-looking" code you cause potential headache to other people. This may get you fired.
What's the real result here?
Your program leaked the memory. Depending on your OS, it may have been recovered.
Most modern desktop operating systems do recover leaked memory at process termination, making it sadly common to ignore the problem (as can be seen by many other answers here.)
But you are relying on a safety feature not being part of the language, one you should not rely upon. Your code might run on a system where this behaviour does result in a "hard" memory leak, next time.
Your code might end up running in kernel mode, or on vintage / embedded operating systems which do not employ memory protection as a tradeoff. (MMUs take up die space, memory protection costs additional CPU cycles, and it is not too much to ask from a programmer to clean up after himself).
You can use and re-use memory (and other resources) any way you like, but make sure you deallocated all resources before exiting.
If you're using the memory you've allocated, then you're not doing anything wrong. It becomes a problem when you write functions (other than main) that allocate memory without freeing it, and without making it available to the rest of your program. Then your program continues running with that memory allocated to it, but no way of using it. Your program and other running programs are deprived of that memory.
Edit: It's not 100% accurate to say that other running programs are deprived of that memory. The operating system can always let them use it at the expense of swapping your program out to virtual memory (</handwaving>). The point is, though, that if your program frees memory that it isn't using then a virtual memory swap is less likely to be necessary.
There's actually a section in the OSTEP online textbook for an undergraduate course in operating systems which discusses exactly your question.
The relevant section is "Forgetting To Free Memory" in the Memory API chapter on page 6 which gives the following explanation:
In some cases, it may seem like not calling free() is reasonable. For
example, your program is short-lived, and will soon exit; in this case,
when the process dies, the OS will clean up all of its allocated pages and
thus no memory leak will take place per se. While this certainly “works”
(see the aside on page 7), it is probably a bad habit to develop, so be wary
of choosing such a strategy
This excerpt is in the context of introducing the concept of virtual memory. Basically at this point in the book, the authors explain that one of the goals of an operating system is to "virtualize memory," that is, to let every program believe that it has access to a very large memory address space.
Behind the scenes, the operating system will translate "virtual addresses" the user sees to actual addresses pointing to physical memory.
However, sharing resources such as physical memory requires the operating system to keep track of what processes are using it. So if a process terminates, then it is within the capabilities and the design goals of the operating system to reclaim the process's memory so that it can redistribute and share the memory with other processes.
EDIT: The aside mentioned in the excerpt is copied below.
ASIDE: WHY NO MEMORY IS LEAKED ONCE YOUR PROCESS EXITS
When you write a short-lived program, you might allocate some space
using malloc(). The program runs and is about to complete: is there
need to call free() a bunch of times just before exiting? While it seems
wrong not to, no memory will be "lost" in any real sense. The reason is
simple: there are really two levels of memory management in the system.
The first level of memory management is performed by the OS, which
hands out memory to processes when they run, and takes it back when
processes exit (or otherwise die). The second level of management
is within each process, for example within the heap when you call
malloc() and free(). Even if you fail to call free() (and thus leak
memory in the heap), the operating system will reclaim all the memory of
the process (including those pages for code, stack, and, as relevant here,
heap) when the program is finished running. No matter what the state
of your heap in your address space, the OS takes back all of those pages
when the process dies, thus ensuring that no memory is lost despite the
fact that you didn’t free it.
Thus, for short-lived programs, leaking memory often does not cause any
operational problems (though it may be considered poor form). When
you write a long-running server (such as a web server or database management
system, which never exit), leaked memory is a much bigger issue,
and will eventually lead to a crash when the application runs out of
memory. And of course, leaking memory is an even larger issue inside
one particular program: the operating system itself. Showing us once
again: those who write the kernel code have the toughest job of all...
from Page 7 of Memory API chapter of
Operating Systems: Three Easy Pieces
Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau
Arpaci-Dusseau Books
March, 2015 (Version 0.90)
There's no real danger in not freeing your variables, but if you assign a pointer to a block of memory to a different block of memory without freeing the first block, the first block is no longer accessible but still takes up space. This is what's called a memory leak, and if you do this with regularity then your process will start to consume more and more memory, taking away system resources from other processes.
If the process is short-lived you can often get away with doing this as all allocated memory is reclaimed by the operating system when the process completes, but I would advise getting in the habit of freeing all memory you have no further use for.
You are correct, memory is automatically freed when the process exits. Some people strive not to do extensive cleanup when the process is terminated, since it will all be relinquished to the operating system. However, while your program is running you should free unused memory. If you don't, you may eventually run out or cause excessive paging if your working set gets too big.
You are absolutely correct in that respect. In small trivial programs where a variable must exist until the death of the program, there is no real benefit to deallocating the memory.
In fact, I had once been involved in a project where each execution of the program was very complex but relatively short-lived, and the decision was to just keep memory allocated and not destabilize the project by making mistakes deallocating it.
That being said, in most programs this is not really an option, or it can lead you to run out of memory.
It depends on the scope of the project that you're working on. In the context of your question, and I mean just your question, then it doesn't matter.
For a further explanation (optional), some scenarios I have noticed from this whole discussion is as follow:
(1) - If you're working in an embedded environment where you cannot rely on the main OS' to reclaim the memory for you, then you should free them since memory leaks can really crash the program if done unnoticed.
(2) - If you're working on a personal project where you won't disclose it to anyone else, then you can skip it (assuming you're using it on the main OS') or include it for "best practices" sake.
(3) - If you're working on a project and plan to have it open source, then you need to do more research into your audience and figure out if freeing the memory would be the better choice.
(4) - If you have a large library and your audience consisted of only the main OS', then you don't need to free it as their OS' will help them to do so. In the meantime, by not freeing, your libraries/program may help to make the overall performance snappier since the program does not have to close every data structure, prolonging the shutdown time (imagine a very slow excruciating wait to shut down your computer before leaving the house...)
I can go on and on specifying which course to take, but it ultimately depends on what you want to achieve with your program. Freeing memory is considered good practice in some cases and not so much in some so it ultimately depends on the specific situation you're in and asking the right questions at the right time. Good luck!
If you're developing an application from scratch, you can make some educated choices about when to call free. Your example program is fine: it allocates memory, maybe you have it work for a few seconds, and then closes, freeing all the resources it claimed.
If you're writing anything else, though -- a server/long-running application, or a library to be used by someone else, you should expect to call free on everything you malloc.
Ignoring the pragmatic side for a second, it's much safer to follow the stricter approach, and force yourself to free everything you malloc. If you're not in the habit of watching for memory leaks whenever you code, you could easily spring a few leaks. So in other words, yes -- you can get away without it; please be careful, though.
If a program forgets to free a few Megabytes before it exits the operating system will free them. But if your program runs for weeks at a time and a loop inside the program forgets to free a few bytes in each iteration you will have a mighty memory leak that will eat up all the available memory in your computer unless you reboot it on a regular basis => even small memory leaks might be bad if the program is used for a seriously big task even if it originally wasn't designed for one.
It depends on the OS environment the program is running in, as others have already noted, and for long running processes, freeing memory and avoiding even very slow leaks is important always. But if the operating system deals with stuff, as Unix has done for example since probably forever, then you don't need to free memory, nor close files (the kernel closes all open file descriptors when a process exits.)
If your program allocates a lot of memory, it may even be beneficial to exit without "hesitation". I find that when I quit Firefox, it spends several !minutes ! paging in gigabytes of memory in many processes. I guess this is due to having to call destructors on C++ objects. This is actually terrible. Some might argue, that this is necessary to save state consistently, but in my opinion, long-running interactive programs like browsers, editors and design programs, just to mention a few, should ensure that any state information, preferences, open windows/pages, documents etc is frequently written to permanent storage, to avoid loss of work in case of a crash. Then this state-saving can be performed again quickly when the user elects to quit, and when completed, the processes should just exit immediately.
All memory allocated for this process will be marked unused by OS then reused, because the memory allocation is done by user space functions.
Imagine OS is a god, and the memories is the materials for creating a wolrd of process, god use some of materials creat a world (or to say OS reserved some of memory and create a process in it). No matter what the creatures in this world have done the materials not belong to this world won't be affected. After this world expired, OS the god, can recycle materials allocated for this world.
Modern OS may have different details on releasing user space memory, but that has to be a basic duty of OS.
I think that your two examples are actually only one: the free() should occur only at the end of the process, which as you point out is useless since the process is terminating.
In you second example though, the only difference is that you allow an undefined number of malloc(), which could lead to running out of memory. The only way to handle the situation is to check the return code of malloc() and act accordingly.
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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.
No.
As professionals, the question we should not be asking ourselves is, "Is it ever OK to do this?" but rather "Is there ever a good reason to do this?" And "hunting down that memory leak is a pain" isn't a good reason.
I like to keep things simple. And the simple rule is that my program should have no memory leaks.
That makes my life simple, too. If I detect a memory leak, I eliminate it, rather than run through some elaborate decision tree structure to determine whether it's an "acceptable" memory leak.
It's similar to compiler warnings – will the warning be fatal to my particular application? Maybe not.
But it's ultimately a matter of professional discipline. Tolerating compiler warnings and tolerating memory leaks is a bad habit that will ultimately bite me in the rear.
To take things to an extreme, would it ever be acceptable for a surgeon to leave some piece of operating equipment inside a patient?
Although it is possible that a circumstance could arise where the cost/risk of removing that piece of equipment exceeds the cost/risk of leaving it in, and there could be circumstances where it was harmless, if I saw this question posted on SurgeonOverflow.com and saw any answer other than "no," it would seriously undermine my confidence in the medical profession.
–
If a third party library forced this situation on me, it would lead me to seriously suspect the overall quality of the library in question. It would be as if I test drove a car and found a couple loose washers and nuts in one of the cupholders – it may not be a big deal in itself, but it portrays a lack of commitment to quality, so I would consider alternatives.
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.
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.
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?
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.
There is nothing conceptually wrong with having the os clean up after the application is run.
It really depends on the application and how it will be run. Continually occurring leaks in an application that needs to run for weeks has to be taken care of, but a small tool that calculates a result without too high of a memory need should not be a problem.
There is a reason why many scripting language do not garbage collect cyclical references… for their usage patterns, it's not an actual problem and would thus be as much of a waste of resources as the wasted memory.
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.
I'm going to give the unpopular but practical answer that it's always wrong to free memory unless doing so will reduce the memory usage of your program. For instance a program that makes a single allocation or series of allocations to load the dataset it will use for its entire lifetime has no need to free anything. In the more common case of a large program with very dynamic memory requirements (think of a web browser), you should obviously free memory you're no longer using as soon as you can (for instance closing a tab/document/etc.), but there's no reason to free anything when the user selects clicks "exit", and doing so is actually harmful to the user experience.
Why? Freeing memory requires touching memory. Even if your system's malloc implementation happens not to store metadata adjacent to the allocated memory blocks, you're likely going to be walking recursive structures just to find all the pointers you need to free.
Now, suppose your program has worked with a large volume of data, but hasn't touched most of it for a while (again, web browser is a great example). If the user is running a lot of apps, a good portion of that data has likely been swapped to disk. If you just exit(0) or return from main, it exits instantly. Great user experience. If you go to the trouble of trying to free everything, you may spend 5 seconds or more swapping all the data back in, only to throw it away immediately after that. Waste of user's time. Waste of laptop's battery life. Waste of wear on the hard disk.
This is not just theoretical. Whenever I find myself with too many apps loaded and the disk starts thrashing, I don't even consider clicking "exit". I get to a terminal as fast as I can and type killall -9 ... because I know "exit" will just make it worse.
I think in your situation the answer may be that it's okay. But you definitely need to document that the memory leak is a conscious decision. You don't want a maintenance programmer to come along, slap your code inside a function, and call it a million times. So if you make the decision that a leak is okay you need to document it (IN BIG LETTERS) for whoever may have to work on the program in the future.
If this is a third party library you may be trapped. But definitely document that this leak occurs.
But basically if the memory leak is a known quantity like a 512 KB buffer or something then it is a non issue. If the memory leak keeps growing like every time you call a library call your memory increases by 512KB and is not freed, then you may have a problem. If you document it and control the number of times the call is executed it may be manageable. But then you really need documentation because while 512 isn't much, 512 over a million calls is a lot.
Also you need to check your operating system documentation. If this was an embedded device there may be operating systems that don't free all the memory from a program that exits. I'm not sure, maybe this isn't true. But it is worth looking into.
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.
If you allocate memory and use it until the last line of your program, that's not a leak. If you allocate memory and forget about it, even if the amount of memory isn't growing, that's a problem. That allocated but unused memory can cause other programs to run slower or not at all.
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.
If you allocate a bunch of heap at the beginning of your program, and you don't free it when you exit, that is not a memory leak per se. A memory leak is when your program loops over a section of code, and that code allocates heap and then "loses track" of it without freeing it.
In fact, there is no need to make calls to free() or delete right before you exit. When the process exits, all of its memory is reclaimed by the OS (this is certainly the case with POSIX. On other OSes – particularly embedded ones – YMMV).
The only caution I'd have with not freeing the memory at exit time is that if you ever refactor your program so that it, for example, becomes a service that waits for input, does whatever your program does, then loops around to wait for another service call, then what you've coded can turn into a memory leak.
In this sort of question context is everything. Personally I can't stand leaks, and in my code I go to great lengths to fix them if they crop up, but it is not always worth it to fix a leak, and when people are paying me by the hour I have on occasion told them it was not worth my fee for me to fix a leak in their code. Let me give you an example:
I was triaging a project, doing some perf work and fixing a lot of bugs. There was a leak during the applications initialization that I tracked down, and fully understood. Fixing it properly would have required a day or so refactoring a piece of otherwise functional code. I could have done something hacky (like stuffing the value into a global and grabbing it some point I know it was no longer in use to free), but that would have just caused more confusion to the next guy who had to touch the code.
Personally I would not have written the code that way in the first place, but most of us don't get to always work on pristine well designed codebases, and sometimes you have to look at these things pragmatically. The amount of time it would have taken me to fix that 150 byte leak could instead be spent making algorithmic improvements that shaved off megabytes of ram.
Ultimately, I decided that leaking 150 bytes for an app that used around a gig of ram and ran on a dedicated machine was not worth fixing it, so I wrote a comment saying that it was leaked, what needed to be changed in order to fix it, and why it was not worth it at the time.
this is so domain-specific that its hardly worth answering. use your freaking head.
space shuttle operating system: nope, no memory leaks allowed
rapid development proof-of-concept code: fixing all those memory leaks is a waste of time.
and there is a spectrum of intermediate situations.
the opportunity cost ($$$) of delaying a product release to fix all but the worst memory leaks is usually dwarfs any feelings of being "sloppy or unprofessional". Your boss pays you to make him money, not to get a warm, fuzzy feelings.
You have to first realize that there's a big difference between a perceived memory leak and an actual memory leak. Very frequently analysis tools will report many red herrings, and label something as having been leaked (memory or resources such as handles etc) where it actually isn't. Often times this is due to the analysis tool's architecture. For example, certain analysis tools will report run time objects as memory leaks because it never sees those object freed. But the deallocation occurs in the runtime's shutdown code, which the analysis tool might not be able to see.
With that said, there will still be times when you will have actual memory leaks that are either very difficult to find or very difficult to fix. So now the question becomes is it ever OK to leave them in the code?
The ideal answer is, "no, never." A more pragmatic answer may be "no, almost never." Very often in real life you have limited number of resources and time to resolve and endless list of tasks. When one of the tasks is eliminating memory leaks, the law of diminishing returns very often comes in to play. You could eliminate say 98% of all memory leaks in an application in a week, but the remaining 2% might take months. In some cases it might even be impossible to eliminate certain leaks because of the application's architecture without a major refactoring of code. You have to weigh the costs and benefits of eliminating the remaining 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.
Even if you are sure that your 'known' memory leak will not cause havoc, don't do it. At best, it will pave a way for you to make a similar and probably more critical mistake at a different time and place.
For me, asking this is like questioning "Can I break the red light at 3 AM in the morning when no one is around?". Well sure, it may not cause any trouble at that time but it will provide a lever for you to do the same in rush hour!
No, you should not have leaks that the OS will clean for you. The reason (not mentioned in the answers above as far as I could check) is that you never know when your main() will be re-used as a function/module in another program. If your main() gets to be a frequently-called function in another persons' software - this software will have a memory leak that eats memory over time.
KIV
I agree with vfilby – it depends. In Windows, we treat memory leaks as relatively serous bugs. But, it very much depends on the component.
For example, memory leaks are not very serious for components that run rarely, and for limited periods of time. These components run, do theire work, then exit. When they exit all their memory is freed implicitly.
However, memory leaks in services or other long run components (like the shell) are very serious. The reason is that these bugs 'steal' memory over time. The only way to recover this is to restart the components. Most people don't know how to restart a service or the shell – so if their system performance suffers, they just reboot.
So, if you have a leak – evaluate its impact two ways
To your software and your user's experience.
To the system (and the user) in terms of being frugal with system resources.
Impact of the fix on maintenance and reliability.
Likelihood of causing a regression somewhere else.
Foredecker
I'm surprised to see so many incorrect definitions of what a memory leak actually is. Without a concrete definition, a discussion on whether it's a bad thing or not will go nowhere.
As some commentors have rightly pointed out, a memory leak only happens when memory allocated by a process goes out of scope to the extent that the process is no longer able to reference or delete it.
A process which is grabbing more and more memory is not necessarily leaking. So long as it is able to reference and deallocate that memory, then it remains under the explicit control of the process and has not leaked. The process may well be badly designed, especially in the context of a system where memory is limited, but this is not the same as a leak. Conversely, losing scope of, say, a 32 byte buffer is still a leak, even though the amount of memory leaked is small. If you think this is insignificant, wait until someone wraps an algorithm around your library call and calls it 10,000 times.
I see no reason whatsoever to allow leaks in your own code, however small. Modern programming languages such as C and C++ go to great lengths to help programmers prevent such leaks and there is rarely a good argument not to adopt good programming techniques - especially when coupled with specific language facilities - to prevent leaks.
As regards existing or third party code, where your control over quality or ability to make a change may be highly limited, depending on the severity of the leak, you may be forced to accept or take mitigating action such as restarting your process regularly to reduce the effect of the leak.
It may not be possible to change or replace the existing (leaking) code, and therefore you may be bound to accept it. However, this is not the same as declaring that it's OK.
I guess it's fine if you're writing a program meant to leak memory (i.e. to test the impact of memory leaks on system performance).
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.
I think you've answered your own question. The biggest drawback is how they interfere with the memory leak detection tools, but I think that drawback is a HUGE drawback for certain types of applications.
I work with legacy server applications that are supposed to be rock solid but they have leaks and the globals DO get in the way of the memory detection tools. It's a big deal.
In the book "Collapse" by Jared Diamond, the author wonders about what the guy was thinking who cut down the last tree on Easter Island, the tree he would have needed in order to build a canoe to get off the island. I wonder about the day many years ago when that first global was added to our codebase. THAT was the day it should have been caught.
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.
I'll answer no.
In theory, the operating system will clean up after you if you leave a mess (now that's just rude, but since computers don't have feelings it might be acceptable). But you can't anticipate every possible situation that might occur when your program is run. Therefore (unless you are able to conduct a formal proof of some behaviour), creating memory leaks is just irresponsible and sloppy from a professional point of view.
If a third-party component leaks memory, this is a very strong argument against using it, not only because of the imminent effect but also because it shows that the programmers work sloppily and that this might also impact other metrics. Now, when considering legacy systems this is difficult (consider web browsing components: to my knowledge, they all leak memory) but it should be the norm.
Historically, it did matter on some operating systems under some edge cases. These edge cases could exist in the future.
Here's an example, on SunOS in the Sun 3 era, there was an issue if a process used exec (or more traditionally fork and then exec), the subsequent new process would inherit the same memory footprint as the parent and it could not be shrunk. If a parent process allocated 1/2 gig of memory and didn't free it before calling exec, the child process would start using that same 1/2 gig (even though it wasn't allocated). This behavior was best exhibited by SunTools (their default windowing system), which was a memory hog. Every app that it spawned was created via fork/exec and inherited SunTools footprint, quickly filling up swap space.
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?
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.
As a general rule, if you've got memory leaks that you feel you can't avoid, then you need to think harder about object ownership.
But to your question, my answer in a nutshell is In production code, yes. During development, no. This might seem backwards, but here's my reasoning:
In the situation you describe, where the memory is held until the end of the program, it's perfectly okay to not release it. Once your process exits, the OS will clean up anyway. In fact, it might make the user's experience better: In a game I've worked on, the programmers thought it would be cleaner to free all the memory before exiting, causing the shutdown of the program to take up to half a minute! A quick change that just called exit() instead made the process disappear immediately, and put the user back to the desktop where he wanted to be.
However, you're right about the debugging tools: They'll throw a fit, and all the false positives might make finding your real memory leaks a pain. And because of that, always write debugging code that frees the memory, and disable it when you ship.
Suppose I had a program like this:
int main(void)
{
int* arr = new int[x];
//processing; neglect to call delete[]
return 0;
}
In a trivial example such as this, I assume there is little actual harm in neglecting to free the memory allocated for arr, since it should be released by the OS when the program is finished running. For any non-trivial program, however, this is considered to be bad practice and will lead to memory leaks.
My question is, what are the consequences of memory leaks in a non-trivial program? I realize that memory leaks are bad practice, but I do not understand why they are bad and what trouble they cause.
A memory leak can diminish the performance of the computer by reducing the amount of available memory. Eventually, in the worst case, too much of the available memory may become allocated and all or part of the system or device stops working correctly, the application fails, or the system slows down unacceptably due to thrashing.
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.
Much more serious leaks include those:
where the program runs for an extended time and consumes additional memory over time, such as background tasks on servers, but especially in embedded devices which may be left running for many years
where new memory is allocated frequently for one-time tasks, such as when rendering the frames of a computer game or animated video
where the program can request memory — such as shared memory — that is not released, even when the program terminates
where memory is very limited, such as in an embedded system or portable device
where the leak occurs within the operating system or memory manager
when a system device driver causes the leak
running on an operating system that does not automatically release memory on program termination. Often on such machines if memory is lost, it can only be reclaimed by a reboot, an example of such a system being AmigaOS.
Check out here for more info.
There is an underlying assumption to your question:
The role of delete and delete[] is solely to release memory.
... and it is erroneous.
For better or worse, delete and delete[] have a dual role:
Run destructors
Free memory (by calling the right overload of operator delete)
With the corrected assumption, we can now ask the corrected question:
What is the risk in not calling delete/delete[] to end the lifetime of dynamically allocated variables ?
As mentioned, an obvious risk is leaking memory (and ultimately crashing). However this is the least of your worries. The much bigger risk is undefined behavior, which means that:
compiler may inadvertently not produce executable code that behaves as expected: Garbage in, Garbage out.
in pragmatic terms, the most likely output is that destructors are not run...
The latter is extremely worrisome:
Mutexes: Forget to release a lock and you get a deadlock...
File Descriptors: Some platforms (such as FreeBSD I believe) have a notoriously low default limit on the number of file descriptors a process may open; fail to close your file descriptors and you will not be able to open any new file or socket!
Sockets: on top of being a file descriptor, there is a limited range of ports associated to an IP (which with the latest version of Linux is no longer global, yeah!). The absolute maximum is 65,536 (u16...) but the ephemeral port range is usually much smaller (half of it). If you forget to release connections in a timely fashion you can easily end up in a situation where even though you have plenty of bandwidth available, your server stops accepting new connections because there is no ephemeral port available.
...
The problem with the attitude of well, I got enough memory anyway is that memory is probably the least of your worries simply because memory is probably the least scarce resource you manipulate.
Of course you could say: Okay, I'll concentrate on other resources leak, but tools nowadays report them as memory leaks (and it's sufficient) so isolating that leak among hundreds/thousands is like seeking a needle in a haystack...
Note: did I mention that you can still run out of memory ? Whether on lower-end machines/systems or on a restricted processes/virtual-machines memory can be quite tight for the task at hand.
Note: if you find yourself calling delete, you are doing it wrong. Learn to use the Standard Library std::unique_ptr and its containers std::vector. In C++, automatic memory management is easy, the real challenge is to avoid dangling pointers...
Let's say we have this program running:
while(true)
{
int* arr = new int;
}
The short term problem is that your computer will eventually run out of memory and the program will crash.
Instead, we could have this program that would run forever because there is no memory leak:
while(true)
{
int* arr = new int;
delete arr;
}
When a simple program like this crashes there is no long term consequences because the operating system will free the memory after the crash.
But you can imagine more critical systems where a system crash will have catastrophic consequences such as:
while(true)
{
int* arr = new int;
generateOxygenForAstronauts();
}
Think about the astronauts and free your memory!
A tool that runs for a short period of time and then exits can often get away with having memory leaks, as your example indicates. But a program that is expected to run without failure for long periods of time must be completely free of memory leaks. As others have mentioned, the whole system will bog down first. Additionally, code that leaks memory often is very bad at handling allocation failures - the result of a failed allocation is usually a crash and loss of data. From the user's perspective, this crash usually happens at exactly the worst possible moment (e.g. during file save, when file buffers get allocated).
Well, it is a strange question, since the immediate answer is straightforward: as you lose memory to memory leaks, you can/will eventually run out of memory. How big a problem that represents to a specific program depends on how big each leak is, how often these leaks occur and for how long. That's all there is to it.
A program that allocates relatively low amount of memory and/or is not run continuously might not suffer any problems from memory leaks at all. But a program that is run continuously will eventually run out of memory, even if it leaks it very slowly.
Now, if one decided to look at it closer, every block of memory has two sides to it: it occupies a region of addresses in the address space of the process and it occupies a portion of the actual physical storage.
On a platform without virtual memory, both sides work against you. Once the memory block is leaked, you lose the address space and you lose the storage.
On a platform with virtual memory the actual storage is a practically unlimited resource. You can leak as much memory as you want, you will never run out of the actual storage (within practically reasonable limits, of course). A leaked memory block will eventually be pushed out to external storage and forgotten for good, so it will not directly affect the program in any negative way. However, it will still hold its region of address space. And the address space still remains a limited resource, which you can run out of.
One can say, that if we take an imaginary virtual memory platform with address space that is overwhelmingly larger than anything ever consumable by our process (say, 2048-bit platform and a typical text editor), then memory leaks will have no consequence for our program. But in real life memory leaks typically constitute a serious problem.
Nowadays compilers do some optimization on your code before generating the binary. And so single newing without deleting it wouldn't have much of harm.
But in general by doing any "Newing" you should "delete" that portion of memory you've reserved in your program.
And also be aware that simple deleting doesn't guarantee not running out of memory.
There are different aspects from O.S. and the compiler side to control this feature.
This link may help you a little
And this one too
Lets say I have a function like this:
int main()
{
char* str = new char[10];
for(int i=0;i<5;i++)
{
//Do stuff with str
}
delete[] str;
return 0;
}
Why would I need to delete str if I am going to end the program anyways?
I wouldn't care if that memory goes to a land full of unicorns if I am just going to exit, right?
Is it just good practice?
Does it have deeper consequences?
If in fact your question really is "I have this trivial program, is it OK that I don't free a few bytes before it exits?" the answer is yes, that's fine. On any modern operating system that's going to be just fine. And the program is trivial; it's not like you're going to be putting it into a pacemaker or running the braking systems of a Toyota Camry with this thing. If the only customer is you then the only person you can possibly impact by being sloppy is you.
The problem then comes in when you start to generalize to non-trivial cases from the answer to this question asked about a trivial case.
So let's instead ask two questions about some non-trivial cases.
I have a long-running service that allocates and deallocates memory in complex ways, perhaps involving multiple allocators hitting multiple heaps. Shutting down my service in the normal mode is a complicated and time-consuming process that involves ensuring that external state -- files, databases, etc -- are consistently shut down. Should I ensure that every byte of memory that I allocated is deallocated before I shut down?
Yes, and I'll tell you why. One of the worst things that can happen to a long-running service is if it accidentally leaks memory. Even tiny leaks can add up to huge leaks over time. A standard technique for finding and fixing memory leaks is to instrument the allocation heaps so that at shutdown time they log all the resources that were ever allocated without being freed. Unless you like chasing down a lot of false positives and spending a lot of time in the debugger, always free your memory even if doing so is not strictly speaking necessary.
The user is already expecting that shutting the service down might take billions of nanoseconds so who cares if you cause a little extra pressure on the virtual allocator making sure that everything is cleaned up? This is just the price you pay for big complicated software. And it's not like you're shutting down the service all the time, so again, who cares if its a few milliseconds slower than it could be?
I have that same long-running service. If I detect that one of my internal data structures is corrupt I wish to "fail fast". The program is in an undefined state, it is likely running with elevated privileges, and I am going to assume that if I detect corrupted state, it is because my service is actively being attacked by hostile parties. The safest thing to do is to shut down the service immediately. I would rather allow the attackers to deny service to the clients than to risk the service staying up and compromising my users' data further. In this emergency shutdown scenario should I make sure that every byte of memory I allocated is freed?
Of course not. The operating system is going to take care of that for you. If your heap is corrupt, the attackers may be hoping that you free memory as part of their exploit. Every millisecond counts. And why would you bother polishing the doorknobs and mopping the kitchen before you drop a tactical nuke on the building?
So the answer to the question "should I free memory before my program exits?" is "it depends on what your program does".
Yes it is good practice. You should NEVER assume that your OS will take care of your memory deallocation, if you get into this habit, it will screw you later on.
To answer your question, however, upon exiting from the main, the OS frees all memory held by that process, so that includes any threads that you may have spawned or variables allocated. The OS will take care of freeing up that memory for others to use.
Important note : delete's freeing of memory is almost just a side-effect. The important thing it does is to destruct the object. With RAII designs, this could mean anything from closing files, freeing OS handles, terminating threads, or deleting temporary files.
Some of these actions would be handled by the OS automatically when your process exits, but not all.
In your example, there's no reason NOT to call delete. but there's no reason to call new either, so you can sidestep the issue this way.
char str[10];
Or, you can sidestep the delete (and the exception safety issues involved) by using smart pointers...
So, generally you should always be making sure your object's lifetime is properly managed.
But it's not always easy: Workarounds for the static initialization order fiasco often mean that you have no choice but to rely on the OS cleaning up a handful of singleton-type objects for you.
Contrary answer: No, it is a waste of time. A program with a vast amount of allocated data would have to touch nearly every page in order to return all of the allocations to the free list. This wastes CPU time, creates memory pressure for uninteresting data, and possibly even causes the process to swap pages back in from disk. Simply exiting releases all of the memory back to the OS without any further action.
(not that I disagree with the reasons in "Yes", I just think there are arguments both ways)
Your Operating System should take care of the memory and clean it up when you exit your program, but it is in general good practice to free up any memory you have reserved. I think personally it is best to get into the correct mindset of doing so, as while you are doing simple programs, you are most likely doing so to learn.
Either way, the only way to guaranteed that the memory is freed up is by doing so yourself.
new and delete are reserved keyword brothers. They should cooperate with each other through a code block or through the parent object's lifecyle. Whenever the younger brother commits a fault (new), the older brother will want to to clean (delete) it up. Then the mother (your program) will be happy and proud of them.
I cannot agree more to Eric Lippert's excellent advice:
So the answer to the question "should I free memory before my program exits?" is "it depends on what your program does".
Other answers here have provided arguments for and against both, but the real crux of the matter is what your program does. Consider a more non-trivial example wherein the type instance being dynamically allocated is an custom class and the class destructor performs some actions which produces side effect. In such a situation the argument of memory leaks or not is trivial the more important problem is that failing to call delete on such a class instance will result in Undefined behavior.
[basic.life] 3.8 Object lifetime
Para 4:
A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the object occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined
behavior.
So the answer to your question is as Eric says "depends on what your program does"
It's a fair question, and there are a few things to consider when answering:
some objects have more complex destructors which don't just release memory when they're deleted. They may have other side effects, which you don't want to skip.
It is not guaranteed by the C++ standard that your memory will be released when the process terminates. (Of course on a modern OS it will be freed, but if you were on some weird OS which didn't do that, you'd have to free your memory properly
on the other hand, running destructors at program exit can actually take up quite a lot of time, and if all the do is release memory (which would be released anyway), then yes, it makes a lot of sense to just short-circuit that and exit immediately instead.
Most operating systems will reclaim memory upon process exit. Exceptions may include certain RTOS's, old mobile devices etc.
In an absolute sense your app won't leak memory; however it's good practice to clean up memory you allocate even if you know it won't cause a real leak. This issue is leaks are much, much harder to fix than not having them to begin with. Let's say you decide that you want to move the functionality in your main() to another function. You'll may end up with a real leak.
It's also bad aesthetics, many developers will look at the unfreed 'str' and feel slight nausea :(
Why would I need to delete str if I am going to end the program anyways?
Because you don't want to be lazy ...
I wouldn't care if that memory goes to a land full of unicorns if I am just going to exit, right?
Nope, I don't care about the land of unicorns either. The Land of Arwen is a different matter, Then we could cut their horns off and put them to good use(I've heard its a good aphrodisiac).
Is it just good practice?
It is justly a good practice.
Does it have deeper consequences?
Someone else has to clean up after you. Maybe you like that, I moved out from under my parents' roof many years ago.
Place a while(1) loop construct around your code without delete. The code-complexity does not matter. Memory leaks are related to process time.
From the perspective of debug, not releasing system resources(file handles, etc) can cause more significant and hard to find bugs. Memory-leaks while important are typically much easier to diagnose(why can't I write to this file?). Bad style will become more of a problem if you start working with threads.
int main()
{
while(1)
{
char* str = new char[10];
for(int i=0;i<5;i++)
{
//Do stuff with str
}
}
delete[] str;
return 0;
}
Another reason that I haven't see mentioned yet is to keep the output of static and dynamic analyzer tools (e.g. valgrind or Coverity) cleaner and quieter. Clean output with zero memory leaks or zero reported issues means that when a new one pops up it is easier to detect and fix.
You never know how your simple example will be used or evolved. Its better to start as clean and crisp as possible.
Not to mention that if you are going to apply for a job as a C++ programmer there is a very good chance that you won't get past the interview because of the missing delete. First - programmers don't like any leaks usually (and the guy at the interview will be surely one of them) and second - most companies (all I worked in at least) have the "no-leak" policy. Generally, the software you write is supposed to run for quite a while, creating and destroying objects on the go. In such an environment leaks can lead to disasters...
You got a lot of professional experience answers. Here I'm telling a naive but an answer I considered as the fact.
Summary
3. Does it have deeper consequences?
A: Will answer in some detail.
2. Is it just good practice?
A: It is considered a good practice. Release resources/memory you've retrieved when you're sure about it no longer used.
Why would I need to delete str if I am going to end the program anyways?
I wouldn't care if that memory goes to a land full of unicorns if I am just going to exit, right?
A: You need or need not, in fact, you tell why. There're some explanation follows.
I think it depends. Here are some assumed questions; the term program may mean either an application or a function.
Q: Does it depend on what the program does?
A: If universe destroyed was acceptable, then no. However, the program might not work correctly as expected, and even be a program that doesn't complete what it supposed to. You might want to seriously think about why you build a program like this?
Q: Does it depend on how the program is complicated?
A: No. See Explanation.
Q: Does it depend on what the stability of the program is expected?
A: Closely.
And I consider it depends on
What's the universe of the program?
How's the expectation of the program that it done its work?
How much does the program care about others, and the universe where it is?
About the term universe, see Explanation.
For summary, it depends on what do you care about.
Explanation
Important: If we define the term program as a function, then its universe is application. There are many details omitted; as an idea for understanding, it's long enough, though.
We may ever seen this kind of diagram which illustrates the relationship between application software and system software.
But for being aware the scope of which one covers, I'd suggest a reversed layout. Since we are talking about software only, the hardware layer is omitted in following diagram.
With this diagram, we realize that the OS covers the biggest scope, which is current the universe, sometimes we say the environment. You may imagine that the whole achitecture consists of a lot of disks like the diagram, to be either a cylinder or torus(a ball is fine but difficult to imagine). Here I should mention that the outmost of OS is in fact a unibody, the runtime may be either single or multiple by different implemention.
It's important that the runtime is responsible to both OS and applications, but the latter is more critical. Runtime is the universe of applications, if it's destroyed all applications run under it are gone.
Unlike human on the Earth, we live here, but we don't consist of Earth; we will still live in other suitable environment if the time when the Earth are destroying and we weren't there.
However, we can no longer exist when the universe is destroyed, because we are not only live in the universe, but also consist of the universe.
As mentioned above, the runtime is also responsible to the OS. The left circle in the following diagram is what it may looks like.
This is mostly like a C program in the OS. When the relationship between an application and OS match this, is just the same situation as runtime in the OS above. In this diagram, the OS is the universe of applications. The reason of the applications here should be responsible to the OS, is that OS might not virtualize the code of them, or allowed to be crashed. If OS are always prevent they to do so, then it's self-responsible, no matter what applications do. But think about the drivers, it's one of the scenarios that OS must allowed to be crashed, since this kind of applications are treated as part of OS.
Finally, let's see the right circle in the diagram above. In this case, the application itself be the universe. Sometimes, we call this kind of application operating system. If an OS never allowed custom code to be loaded and run, then it does all the things itself. Even it allowed, after itself is terminated, the memory goes nowhere but hardware. All the deallocation that may be necessary, is before it was terminated.
So, how much does your program care about the others? How much does it care about its universe? And how's the expectation of the program that it done its work? It depends on what do you care about.
TECHNICALLY, a programmer shouldn't rely on the OS to do any thing.
The OS isn't required to reclaim lost memory in this fashion.
If you do write the code that deletes all your dynamically allocated memory, then you are future proofing the code and letting others use it in a larger project.
Source: Allocation and GC Myths(PostScript alert!)
Allocation Myth 4: Non-garbage-collected programs should always
deallocate all memory they allocate.
The Truth: Omitted deallocations in frequently executed code cause
growing leaks. They are rarely acceptable. but Programs that retain
most allocated memory until program exit often perform better without
any intervening deallocation. Malloc is much easier to implement if
there is no free.
In most cases, deallocating memory just before program exit is
pointless. The OS will reclaim it anyway. Free will touch and page in
the dead objects; the OS won't.
Consequence: Be careful with "leak detectors" that count allocations.
Some "leaks" are good!
I think it's a very poor practice to use malloc/new without calling free/delete.
If the memory's going to get reclaimed anyway, what harm can there be from explicitly deallocating when you need to?
Maybe if the OS "reclaims" memory faster than free does then you'll see increased performance; this technique won't help you with any program that must remain running for any long period of time.
Having said that, so I'd recommend you use free/delete.
If you get into this habit, who's to say that you won't one day accidentally apply this approach somewhere it matters?
One should always deallocate resources after one is done with them, be it file handles/memory/mutexs. By having that habit, one will not make that sort of mistake when building servers. Some servers are expected to run 24x7. In those cases, any leak of any sort means that your server will eventually run out of that resource and hang/crash in some way. A short utility program, ya a leak isn't that bad. Any server, any leak is death. Do yourself a favor. Clean up after yourself. It's a good habit.
Think about your class 'A' having to deconstruct. If you don't call
'delete' on 'a', that destructor won't get called. Usually, that won't
really matter if the process ends anyway. But what if the destructor
has to release e.g. objects in a database? Flush a cache to a logfile?
Write a memory cache back to disk? **You see, it's not just 'good
practice' to delete objects, in some situations it is required**.
Instead of talking about this specific example i will talk about general cases, so generally it is important to explicitly call delete to de-allocate memory because (in case of C++) you may have some code in the destructor that you want to execute. Like maybe writing some data to a log file or sending shutting down signal to some other process etc. If you let the OS free your memory for you, your code in your destructor will not be executed.
On the other hand most operating systems will deallocate the memory when your program ends. But it is good practice to deallocate it yourself and like I gave destructor example above the OS won't call your destructor, which can create undesirable behavior in certain cases!
I personally consider it bad practice to rely on OS to free your memory (even though it will do) the reason is if later on you have to integrate your code with a larger program you will spend hours to track down and fix memory leaks!
So clean your room before leaving!
I'm writing an caching app that consumes large amounts of memory.
Hopefully, I'll manage my memory well enough, but I'm just thinking about what
to do if I do run out of memory.
If a call to allocate even a simple object fails, is it likely that even a syslog call
will also fail?
EDIT: Ok perhaps I should clarify the question. If malloc or new returns a NULL or 0L value then it essentially means the call failed and it can't give you the memory for some reason. So, what would be the sensible thing to do in that case?
EDIT2: I've just realised that a call to "new" can throw an exception. This could be caught at a higher level so I can perhaps gracefully exit further up. At that point, it may even be possible to recover depending on how much memory is freed. In the least I should by that point hopefully be able to log something. So while I have seen code that checks the value of a pointer after new, it is unnecessary. While in C, you should check the return value for malloc.
Well, if you are in a case where there is a failure to allocate memory, you're going to get a std::bad_alloc exception. The exception causes the stack of your program to be unwound. In all likelihood, the inner loops of your application logic are not going to be handling out of memory conditions, only higher levels of your application should be doing that. Because the stack is getting unwound, a significant chunk of memory is going to be free'd -- which in fact should be almost all the memory used by your program.
The one exception to this is when you ask for a very large (several hundred MB, for example) chunk of memory which cannot be satisfied. When this happens though, there's usually enough smaller chunks of memory remaining which will allow you to gracefully handle the failure.
Stack unwinding is your friend ;)
EDIT: Just realized that the question was also tagged with C -- if that is the case, then you should be having your functions free their internal structures manually when out of memory conditions are found; not to do so is a memory leak.
EDIT2: Example:
#include <iostream>
#include <vector>
void DoStuff()
{
std::vector<int> data;
//insert a whole crapload of stuff into data here.
//Assume std::vector::push_back does the actual throwing
//i.e. data.resize(SOME_LARGE_VALUE_HERE);
}
int main()
{
try
{
DoStuff();
return 0;
}
catch (const std::bad_alloc& ex)
{ //Observe that the local variable `data` no longer exists here.
std::cerr << "Oops. Looks like you need to use a 64 bit system (or "
"get a bigger hard disk) for that calculation!";
return -1;
}
}
EDIT3: Okay, according to commenters there are systems out there which do not follow the standard in this regard. On the other hand, on such systems, you're going to be SOL in any case, so I don't see why they merit discussion. But if you are on such a platform, it is something to keep in mind.
Doesn't this question make assumptions regarding overcommitted memory?
I.e., an out of memory situation might not be recoverable! Even if you have no memory left, calls to malloc and other allocators may still succeed until the program attempts to use the memory. Then, BAM!, some process gets killed by the kernel in order to satisfy memory load.
I don't have any specific experience on Linux, but I spent a lot of time working in video games on games consoles, where running out of memory is verboten, and on Windows-based tools.
On a modern OS, you're most likely to run out of address space. Running out of memory, as such, is basically impossible. So just allocate a large buffer, or buffers, on startup, in order to hold all the data you'll ever need, whilst leaving a small amount for the OS. Writing random junk to these regions would probably be a good idea in order to force the OS to actually assign the memory to your process. If your process survives this attempt to use every byte it's asked for, there's some kind of backing now reserved for all of this stuff, so now you're golden.
Write/steal your own memory manager, and direct it to allocate from these buffers. Then use it, consistently, in your app, or take advantage of gcc's --wrap option to forward calls from malloc and friends appropriately. If you use any libraries that can't be directed to call into your memory manager, junk them, because they'll just get in your way. Lack of overridable memory management calls is evidence of deeper-seated issues; you're better of without this particular component. (Note: even if you're using --wrap, trust me, this is still evidence of a problem! Life is too short to use those libraries that don't let you overload their memory management!)
Once you run out of memory, OK, you're screwed, but you've still got that space you left free before, so if freeing up some of the memory you've asked for is too difficult you can (with care) call system calls to write a message to the system log and then terminate, or whatever. Just make sure to avoid calls to the C library, because they'll probably try to allocate some memory when you least expect it -- programmers who work with systems that have virtualised address spaces are notorious for this kind of thing -- and that's the very thing that has caused the problem in the first place.
This approach might sound like a pain in the arse. Well... it is. But it's straightforward, and it's worth putting in a bit of effort for that. I think there's a Kernighan-and/or-Ritche quote about this.
If your application is likely to allocate large blocks of memory and risks hitting the per-process or VM limits, waiting until an allocation actually fails is a difficult situation from which to recover. By the time malloc returns NULL or new throws std::bad_alloc, things may be too far gone to reliably recover. Depending on your recovery strategy, many operations may still require heap allocations themselves, so you have to be extremely careful on which routines you can rely.
Another strategy you may wish to consider is to query the OS and monitor the available memory, proactively managing your allocations. This way you can avoid allocating a large block if you know it is likely to fail, and will thus have a better chance of recovery.
Also, depending on your memory usage patterns, using a custom allocator may give you better results than the standard built-in malloc. For example, certain allocation patterns can actually lead to memory fragmentation over time, so even though you have free memory, the available blocks in the heap arena may not have an available block of the right size. A good example of this is Firefox, which switched to dmalloc and saw a great increase in memory efficiency.
I don't think that capturing the failure of malloc or new will gain you much in your situation. linux allocates large chunks of virtual pages in malloc by means of mmap. By this you may find yourself in a situation where you allocate much more virtual memory than you have (real + swap).
The program then will only fail much later with a segfault (SIGSEGV) when you write to the first page for which there isn't any place in swap. In theory you could test for such situations by writing a signal handler and then dirtying all pages that you allocate.
But usually this will not help much either, since your application will be in a very bad state long before that: constantly swapping, computing mechanically with your harddisk...
It's possible for writes to the syslog to fail in low memory conditions: there's no way to know that for every platform without looking at the source for the relevant functions. They could need dynamic memory to format strings that are passed in, for instance.
Long before you run out of memory, however, you'll start paging stuff to disk. And when that happens, you can forget any performance advantages from caching.
Personally, I'm convinced by the design behind Varnish: the operating system offers services to solve a lot of the relevant problems, and it makes sense to use those services (minor editing):
So what happens with Squid's elaborate memory management is that it gets into fights with the kernel's elaborate memory management ...
Squid creates a HTTP object in RAM and it gets used some times rapidly after creation. Then after some time it get no more hits and the kernel notices this. Then somebody tries to get memory from the kernel for something and the kernel decides to push those unused pages of memory out to swap space and use the (cache-RAM) more sensibly for some data which is actually used by a program. This however, is done without Squid knowing about it. Squid still thinks that these http objects are in RAM, and they will be, the very second it tries to access them, but until then, the RAM is used for something productive. ...
After some time, Squid will also notice that these objects are unused, and it decides to move them to disk so the RAM can be used for more busy data. So Squid goes out, creates a file and then it writes the http objects to the file.
Here we switch to the high-speed camera: Squid calls write(2), the address it gives is a "virtual address" and the kernel has it marked as "not at home". ...
The kernel tries to find a free page, if there are none, it will take a little used page from somewhere, likely another little used Squid object, write it to the paging ... space on the disk (the "swap area") when that write completes, it will read from another place in the paging pool the data it "paged out" into the now unused RAM page, fix up the paging tables, and retry the instruction which failed. ...
So now Squid has the object in a page in RAM and written to the disk two places: one copy in the operating system's paging space and one copy in the filesystem. ...
Here is how Varnish does it:
Varnish allocate some virtual memory, it tells the operating system to back this memory with space from a disk file. When it needs to send the object to a client, it simply refers to that piece of virtual memory and leaves the rest to the kernel.
If/when the kernel decides it needs to use RAM for something else, the page will get written to the backing file and the RAM page reused elsewhere.
When Varnish next time refers to the virtual memory, the operating system will find a RAM page, possibly freeing one, and read the contents in from the backing file.
And that's it. Varnish doesn't really try to control what is cached in RAM and what is not, the kernel has code and hardware support to do a good job at that, and it does a good job.
You may not need to write caching code at all.
As has been stated, exhausting memory means that all bets are off. IMHO the best method of handling this situation is to fail gracefully (as opposed to simply crashing!). Your cache could allocate a reasonable amount of memory on instantiation. The size of this memory would equate to an amount that, when freed, will allow the program to terminate reasonably. When your cache detects that memory is becoming low then it should release this memory and instigate a graceful shutdown.
I'm writing an caching app that consumes large amounts of memory.
Hopefully, I'll manage my memory well enough, but I'm just thinking about what to do if I do run out of memory.
If you are writing deamon which should run 24/7/365, then you should not use dynamic memory management: preallocate all the memory in advance and manage it using some slab allocator/memory pool mechanism. That will also protect you again the heap fragmentation.
If a call to allocate even a simple object fails, is it likely that even a syslog call will also fail?
Should not. This is partially reason why syslog exists as a syscall: that application can report an error independent of its internal state.
If malloc or new returns a NULL or 0L value then it essentially means the call failed and it can't give you the memory for some reason. So, what would be the sensible thing to do in that case?
I generally try in the situations to properly handle the error condition, applying the general error handling rules. If error happens during initialization - terminate with error, probably configuration error. If error happens during request processing - fail the request with out-of-memory error.
For plain heap memory, malloc() returning 0 generally means:
that you have exhausted the heap and unless your application free some memory, further malloc()s wouldn't succeed.
the wrong allocation size: it is quite common coding error to mix signed and unsigned types when calculating block size. If the size ends up mistakenly negative, passed to malloc() where size_t is expected, it becomes very large number.
So in some sense it is also not wrong to abort() to produce the core file which can be analyzed later to see why the malloc() returned 0. Though I prefer to (1) include the attempted allocation size in the error message and (2) try to proceed further. If application would crash due to other memory problem down the road (*), it would produce core file anyway.
(*) From my experience of making software with dynamic memory management resilient to malloc() errors I see that often malloc() returns 0 not reliably. First attempts returning 0 are followed by a successful malloc() returning valid pointer. But first access to the pointed memory would crash the application. This is my experience on both Linux and HP-UX - and I have seen similar pattern on Solaris 10 too. The behavior isn't unique to Linux. To my knowledge the only way to make an application 100% resilient to memory problems is to preallocate all memory in advance. And that is mandatory for mission critical, safety, life support and carrier grade applications - they are not allowed dynamic memory management past initialization phase.
I don't know why many of the sensible answers are voted down. In most server environments, running out of memory means that you have a leak somewhere, and that it makes little sense to 'free some memory and try to go on'. The nature of C++ and especially the standard library is that it requires allocations all the time. If you are lucky, you might be able to free some memory and execute a clean shutdown, or at least emit a warning.
It is however far more likely that you won't be able to do a thing, unless the allocation that failed was a huge one, and there is still memory available for 'normal' things.
Dan Bernstein is one of the very few guys I know that can implement server software that operates in memory constrained situations.
For most of the rest of us, we should probably design our software that it leaves things in a useful state when it bails out because of an out of memory error.
Unless you are some kind of brain surgeon, there isn't a lot else to do.
Also, very often you won't even get an std::bad_alloc or something like that, you'll just get a pointer in return to your malloc/new, and only die when you actually try to touch all of that memory. This can be prevented by turning off overcommit in the operating system, but still.
Don't count on being able to deal with the SIGSEGV when you touch memory that the kernel hoped you wouldn't be.. I'm not quite sure how this works on the windows side of things, but I bet they do overcommit too.
All in all, this is not one of C++'s strong spots.
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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.
No.
As professionals, the question we should not be asking ourselves is, "Is it ever OK to do this?" but rather "Is there ever a good reason to do this?" And "hunting down that memory leak is a pain" isn't a good reason.
I like to keep things simple. And the simple rule is that my program should have no memory leaks.
That makes my life simple, too. If I detect a memory leak, I eliminate it, rather than run through some elaborate decision tree structure to determine whether it's an "acceptable" memory leak.
It's similar to compiler warnings – will the warning be fatal to my particular application? Maybe not.
But it's ultimately a matter of professional discipline. Tolerating compiler warnings and tolerating memory leaks is a bad habit that will ultimately bite me in the rear.
To take things to an extreme, would it ever be acceptable for a surgeon to leave some piece of operating equipment inside a patient?
Although it is possible that a circumstance could arise where the cost/risk of removing that piece of equipment exceeds the cost/risk of leaving it in, and there could be circumstances where it was harmless, if I saw this question posted on SurgeonOverflow.com and saw any answer other than "no," it would seriously undermine my confidence in the medical profession.
–
If a third party library forced this situation on me, it would lead me to seriously suspect the overall quality of the library in question. It would be as if I test drove a car and found a couple loose washers and nuts in one of the cupholders – it may not be a big deal in itself, but it portrays a lack of commitment to quality, so I would consider alternatives.
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.
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.
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?
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.
There is nothing conceptually wrong with having the os clean up after the application is run.
It really depends on the application and how it will be run. Continually occurring leaks in an application that needs to run for weeks has to be taken care of, but a small tool that calculates a result without too high of a memory need should not be a problem.
There is a reason why many scripting language do not garbage collect cyclical references… for their usage patterns, it's not an actual problem and would thus be as much of a waste of resources as the wasted memory.
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.
I'm going to give the unpopular but practical answer that it's always wrong to free memory unless doing so will reduce the memory usage of your program. For instance a program that makes a single allocation or series of allocations to load the dataset it will use for its entire lifetime has no need to free anything. In the more common case of a large program with very dynamic memory requirements (think of a web browser), you should obviously free memory you're no longer using as soon as you can (for instance closing a tab/document/etc.), but there's no reason to free anything when the user selects clicks "exit", and doing so is actually harmful to the user experience.
Why? Freeing memory requires touching memory. Even if your system's malloc implementation happens not to store metadata adjacent to the allocated memory blocks, you're likely going to be walking recursive structures just to find all the pointers you need to free.
Now, suppose your program has worked with a large volume of data, but hasn't touched most of it for a while (again, web browser is a great example). If the user is running a lot of apps, a good portion of that data has likely been swapped to disk. If you just exit(0) or return from main, it exits instantly. Great user experience. If you go to the trouble of trying to free everything, you may spend 5 seconds or more swapping all the data back in, only to throw it away immediately after that. Waste of user's time. Waste of laptop's battery life. Waste of wear on the hard disk.
This is not just theoretical. Whenever I find myself with too many apps loaded and the disk starts thrashing, I don't even consider clicking "exit". I get to a terminal as fast as I can and type killall -9 ... because I know "exit" will just make it worse.
I think in your situation the answer may be that it's okay. But you definitely need to document that the memory leak is a conscious decision. You don't want a maintenance programmer to come along, slap your code inside a function, and call it a million times. So if you make the decision that a leak is okay you need to document it (IN BIG LETTERS) for whoever may have to work on the program in the future.
If this is a third party library you may be trapped. But definitely document that this leak occurs.
But basically if the memory leak is a known quantity like a 512 KB buffer or something then it is a non issue. If the memory leak keeps growing like every time you call a library call your memory increases by 512KB and is not freed, then you may have a problem. If you document it and control the number of times the call is executed it may be manageable. But then you really need documentation because while 512 isn't much, 512 over a million calls is a lot.
Also you need to check your operating system documentation. If this was an embedded device there may be operating systems that don't free all the memory from a program that exits. I'm not sure, maybe this isn't true. But it is worth looking into.
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.
If you allocate memory and use it until the last line of your program, that's not a leak. If you allocate memory and forget about it, even if the amount of memory isn't growing, that's a problem. That allocated but unused memory can cause other programs to run slower or not at all.
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.
If you allocate a bunch of heap at the beginning of your program, and you don't free it when you exit, that is not a memory leak per se. A memory leak is when your program loops over a section of code, and that code allocates heap and then "loses track" of it without freeing it.
In fact, there is no need to make calls to free() or delete right before you exit. When the process exits, all of its memory is reclaimed by the OS (this is certainly the case with POSIX. On other OSes – particularly embedded ones – YMMV).
The only caution I'd have with not freeing the memory at exit time is that if you ever refactor your program so that it, for example, becomes a service that waits for input, does whatever your program does, then loops around to wait for another service call, then what you've coded can turn into a memory leak.
In this sort of question context is everything. Personally I can't stand leaks, and in my code I go to great lengths to fix them if they crop up, but it is not always worth it to fix a leak, and when people are paying me by the hour I have on occasion told them it was not worth my fee for me to fix a leak in their code. Let me give you an example:
I was triaging a project, doing some perf work and fixing a lot of bugs. There was a leak during the applications initialization that I tracked down, and fully understood. Fixing it properly would have required a day or so refactoring a piece of otherwise functional code. I could have done something hacky (like stuffing the value into a global and grabbing it some point I know it was no longer in use to free), but that would have just caused more confusion to the next guy who had to touch the code.
Personally I would not have written the code that way in the first place, but most of us don't get to always work on pristine well designed codebases, and sometimes you have to look at these things pragmatically. The amount of time it would have taken me to fix that 150 byte leak could instead be spent making algorithmic improvements that shaved off megabytes of ram.
Ultimately, I decided that leaking 150 bytes for an app that used around a gig of ram and ran on a dedicated machine was not worth fixing it, so I wrote a comment saying that it was leaked, what needed to be changed in order to fix it, and why it was not worth it at the time.
this is so domain-specific that its hardly worth answering. use your freaking head.
space shuttle operating system: nope, no memory leaks allowed
rapid development proof-of-concept code: fixing all those memory leaks is a waste of time.
and there is a spectrum of intermediate situations.
the opportunity cost ($$$) of delaying a product release to fix all but the worst memory leaks is usually dwarfs any feelings of being "sloppy or unprofessional". Your boss pays you to make him money, not to get a warm, fuzzy feelings.
You have to first realize that there's a big difference between a perceived memory leak and an actual memory leak. Very frequently analysis tools will report many red herrings, and label something as having been leaked (memory or resources such as handles etc) where it actually isn't. Often times this is due to the analysis tool's architecture. For example, certain analysis tools will report run time objects as memory leaks because it never sees those object freed. But the deallocation occurs in the runtime's shutdown code, which the analysis tool might not be able to see.
With that said, there will still be times when you will have actual memory leaks that are either very difficult to find or very difficult to fix. So now the question becomes is it ever OK to leave them in the code?
The ideal answer is, "no, never." A more pragmatic answer may be "no, almost never." Very often in real life you have limited number of resources and time to resolve and endless list of tasks. When one of the tasks is eliminating memory leaks, the law of diminishing returns very often comes in to play. You could eliminate say 98% of all memory leaks in an application in a week, but the remaining 2% might take months. In some cases it might even be impossible to eliminate certain leaks because of the application's architecture without a major refactoring of code. You have to weigh the costs and benefits of eliminating the remaining 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.
Even if you are sure that your 'known' memory leak will not cause havoc, don't do it. At best, it will pave a way for you to make a similar and probably more critical mistake at a different time and place.
For me, asking this is like questioning "Can I break the red light at 3 AM in the morning when no one is around?". Well sure, it may not cause any trouble at that time but it will provide a lever for you to do the same in rush hour!
No, you should not have leaks that the OS will clean for you. The reason (not mentioned in the answers above as far as I could check) is that you never know when your main() will be re-used as a function/module in another program. If your main() gets to be a frequently-called function in another persons' software - this software will have a memory leak that eats memory over time.
KIV
I agree with vfilby – it depends. In Windows, we treat memory leaks as relatively serous bugs. But, it very much depends on the component.
For example, memory leaks are not very serious for components that run rarely, and for limited periods of time. These components run, do theire work, then exit. When they exit all their memory is freed implicitly.
However, memory leaks in services or other long run components (like the shell) are very serious. The reason is that these bugs 'steal' memory over time. The only way to recover this is to restart the components. Most people don't know how to restart a service or the shell – so if their system performance suffers, they just reboot.
So, if you have a leak – evaluate its impact two ways
To your software and your user's experience.
To the system (and the user) in terms of being frugal with system resources.
Impact of the fix on maintenance and reliability.
Likelihood of causing a regression somewhere else.
Foredecker
I'm surprised to see so many incorrect definitions of what a memory leak actually is. Without a concrete definition, a discussion on whether it's a bad thing or not will go nowhere.
As some commentors have rightly pointed out, a memory leak only happens when memory allocated by a process goes out of scope to the extent that the process is no longer able to reference or delete it.
A process which is grabbing more and more memory is not necessarily leaking. So long as it is able to reference and deallocate that memory, then it remains under the explicit control of the process and has not leaked. The process may well be badly designed, especially in the context of a system where memory is limited, but this is not the same as a leak. Conversely, losing scope of, say, a 32 byte buffer is still a leak, even though the amount of memory leaked is small. If you think this is insignificant, wait until someone wraps an algorithm around your library call and calls it 10,000 times.
I see no reason whatsoever to allow leaks in your own code, however small. Modern programming languages such as C and C++ go to great lengths to help programmers prevent such leaks and there is rarely a good argument not to adopt good programming techniques - especially when coupled with specific language facilities - to prevent leaks.
As regards existing or third party code, where your control over quality or ability to make a change may be highly limited, depending on the severity of the leak, you may be forced to accept or take mitigating action such as restarting your process regularly to reduce the effect of the leak.
It may not be possible to change or replace the existing (leaking) code, and therefore you may be bound to accept it. However, this is not the same as declaring that it's OK.
I guess it's fine if you're writing a program meant to leak memory (i.e. to test the impact of memory leaks on system performance).
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.
I think you've answered your own question. The biggest drawback is how they interfere with the memory leak detection tools, but I think that drawback is a HUGE drawback for certain types of applications.
I work with legacy server applications that are supposed to be rock solid but they have leaks and the globals DO get in the way of the memory detection tools. It's a big deal.
In the book "Collapse" by Jared Diamond, the author wonders about what the guy was thinking who cut down the last tree on Easter Island, the tree he would have needed in order to build a canoe to get off the island. I wonder about the day many years ago when that first global was added to our codebase. THAT was the day it should have been caught.
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.
I'll answer no.
In theory, the operating system will clean up after you if you leave a mess (now that's just rude, but since computers don't have feelings it might be acceptable). But you can't anticipate every possible situation that might occur when your program is run. Therefore (unless you are able to conduct a formal proof of some behaviour), creating memory leaks is just irresponsible and sloppy from a professional point of view.
If a third-party component leaks memory, this is a very strong argument against using it, not only because of the imminent effect but also because it shows that the programmers work sloppily and that this might also impact other metrics. Now, when considering legacy systems this is difficult (consider web browsing components: to my knowledge, they all leak memory) but it should be the norm.
Historically, it did matter on some operating systems under some edge cases. These edge cases could exist in the future.
Here's an example, on SunOS in the Sun 3 era, there was an issue if a process used exec (or more traditionally fork and then exec), the subsequent new process would inherit the same memory footprint as the parent and it could not be shrunk. If a parent process allocated 1/2 gig of memory and didn't free it before calling exec, the child process would start using that same 1/2 gig (even though it wasn't allocated). This behavior was best exhibited by SunTools (their default windowing system), which was a memory hog. Every app that it spawned was created via fork/exec and inherited SunTools footprint, quickly filling up swap space.
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?
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.
As a general rule, if you've got memory leaks that you feel you can't avoid, then you need to think harder about object ownership.
But to your question, my answer in a nutshell is In production code, yes. During development, no. This might seem backwards, but here's my reasoning:
In the situation you describe, where the memory is held until the end of the program, it's perfectly okay to not release it. Once your process exits, the OS will clean up anyway. In fact, it might make the user's experience better: In a game I've worked on, the programmers thought it would be cleaner to free all the memory before exiting, causing the shutdown of the program to take up to half a minute! A quick change that just called exit() instead made the process disappear immediately, and put the user back to the desktop where he wanted to be.
However, you're right about the debugging tools: They'll throw a fit, and all the false positives might make finding your real memory leaks a pain. And because of that, always write debugging code that frees the memory, and disable it when you ship.