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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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I have a pointer to a very large static array of objects that don't have any destructors nor they inherit from any class. This array is allocated at the beginning of the program and never allocated/relocated again (by design). This array needs to be destroyed only at the very end of the program. Do I really need to call delete for the array, or is it OK to let the OS (Windows) clean up? Deleting delays exiting of program by 5-10sec. Without calling delete, OS will do that for us and program exits immediately.
In general, it is a bad idea to let os to clean up.
I assume that nothing bad will happens, but in the future you may use your code in another program, and you will have to change things.
bottom line - free everything that you allocated.
You should free it; for the following reasons:
its easy, and takes minimal effort
its good practice. Taking shortcuts too often will bite you somewhere sometime
it sets an example for other people reading your code
it protects you against issues. Trusting to OS to clean it up means you trust your application to exit properly. If this does not happen, the memory release will be stalled
it protects you from yourself: in the future you might have memory in a hardware device reserved. This might not be freed by the os. So to be clear: not all memory is always be freed by the OS.
... these are just some reasons to clean up your mess ;-) ... besides... you might want to deside to put the stuff in an actual class. Then, the cleanup code is already there.
Also note: not releasing memory is one of the most common runtime issues around. Only for that reason its wise to be strict upon it.
No. You don't necessarily have to free the allocation at shutdown if you can assume that your program runs within a modern operating system. If it significantly reduces the time of shutting down the program, it may be an option worth to consider.
Not freeing the allocation does however have the downside that memory leak detection tools will detect this intentional leak making their use problematic. As a solution, I would suggest making the leak toggleable so that you can run a non leaking version with detection tools, and use the leaking version in production.
Yes, but not for the reason you think.
Although in general it goes somewhat against "good practice", it is perfectly "OK" to just exit and let the OS clean up, since there is nothing to be done. Objections such as "it's a bad idea, evil things may happen, resources oh the woes, whatever" are not well-founded insofar as what really happens is that your process ceases to exist, and so do the memory pages that the OS had allocated for it, and that's just it. So, as long as you are 100% positive that nothing needs to happen in your destructor, there is no reason to ever deallocate, except to make tools like valgrind (and auditors, if applicable) happy.
However, the fact that operator delete[] takes 5-10 seconds indicates that this is not the case at all. Something does happen there.
On my desktop, deleting millions of objects takes pretty much "zero" time as long as the destructors are not very very un-trivial. I'm saying such a funny thing as "very very un-trivial" since trivial (and non-trivial) destructor is a term which means something very specific and indeed a "not very very un-trivial" non-trivial destructor call is still very fast.
Wrote a quick 15-liner to test. Structure defintion with some data, default contructor zero-initializing data, and a trivial destructor. A main function which allocates 100 million objects, iterates over the array and updates each element with the value of argc (to prevent the compiler from optimizing the whole thing out), and finally deleting the array.
Runtime overall: 0.2 seconds give or take 50 millis. The same program but with a non-trivial destructor that conditionally updates a global counter takes 0.3 seconds total. Mind you, that's allocation, construction, and iteration included.
So, obviously you are doing something which is not at all trivial. Lots of nested virtual destructor calls on sub-objects? Deallocations from within the object's destructor? Zeroing out or otherwise touching gigabytes of memory? Closing file handles?
Impossible to tell what, but there must be something that is happening, or it wouldn't take so long.
So... no it's not OK to just quit. Because there's something that's happening, and it won't happen if you skip the delete.
Why must we use destructors to de-allocate memory in c++,
As we can use
delete or delete[]
Is it not true that all the memory used up by a program is released when the program terminates.
Very often, it is not enough to get the memory back only after your program terminates. Most programs designed for continuous running need to allocate temporary memory of variable size, without a specific fame for that memory's life time. It is obvious that if you request memory and do not return it for a significant amount of time, your program will run out of memory, and terminate when it requests additional memory.
With this said, you can go long way without using destructors in C++ by allocating everything that you can allocate in the automatic area. The only time when you really need to use dynamic memory is when the lifetime of the object must extend past its allocation scope, but even then C++ containers would take care of most allocations for you (of course the implementation of the standard containers relies heavily on the constructor/destructor infrastructure of the C++ language).
"we use destructors to de-allocate memory"
What you are actually writing about are deallocation functions operator delete and operator delete[].
"Is it not true that all the memory used up by a program is released when the program terminates?"
AFAIK this is OS specific, yet the point is not what happens after the program terminates. The point is about what happens during the execution. There are many applications that run for several hours or even weeks and memory leaks might have very unpleasant consequences in these (not that memory leaks wouldn't be unpleasant in terms of other programs).
When you program reaches the point, where the resources that you've allocated are not needed anymore, you should do your very best to free them using appropriate means. And once your program terminates: Relying on OS cleaning your mess doesn't seem to be much of a good practice ;)
Destructors are guaranteed to be called if RAII is used. It is not like we have to use it, but it is generally a good idea to benefit from RAII because it allows for automatic resource management. In other words, if you write your program right, it won't have resource or memory leaks, so you don't even have to worry about it.
This is true not only for C++, but for other languages that have support for automatic resource management such as C#, Java, and even C (through non-standard extensions).
Basically, you need to read some book on C++ to understand the concept, probably. I also wrote a little article that might be helpful, see here.
Hope it helps :)
First of all, delete and delete[] are not destructors, they just call the destructors of the instances that are deleted (given that those do have destructors).
To answer your bigger question: The destructor of a class can do more than just release memory. For example, it might signal another program that it is about to close a connection or something.
Also, some programs run "forever" - or at least for as long as possible. I, for example, develop programs that are running on servers and they (hopefully) run for month. I do need to release memory (or other resources) as soon as possible otherwise the process will grow and eventually crash when the machine runs out of memory.
After reading this answer about what is memory leak one can conclude that if object is supposed to exist during all run of program, and was creating via new, there is no need to call delete on such object.
So, for example, if I have some main window, and some other windows/widgets on it, that I create in main window constructor with new, I don't need to call delete in destructor, as main window destructor will be called on program quit, so it isn't a memory leak.
Until your program becomes a module in a larger program, e.g. a servlet, at which point you now have a leak.
Also it will obscure your code reviews and use of valgrind. valgrind will think it's a leak, why would you want to have to remember that "that one is fine" each time you try to track down a leak elsewhere?
'cos main window destructor will be called on program quit, so it isn't a memory leak
It is a memory leak, but you can rely on the fact that the operating system will release the resources (at least the memory) used by your process when the process terminates - even if the process itself does not take actions to actively do this.
That may not be the case, though, when other kinds of resources need to be released, like file handles or network connections. More generally, when other kinds of responsibility are not fulfilled by a program or module that has those responsibilities.
Not leaking memory is a responsibility of your program, and it is good to practice programming in a way that the modules you write will fulfill their responsibilities. The tools and idioms used for this purpose are general enough to justify practicing their usage whenever this makes sense. For instance, the RAII idiom (Resource Acquisition Is Initialization) is fundamental in this respect, and it can be applied to your example as well - use a smart pointer.
Although in the particular example you are mentioning your memory leak is not going to be a huge problem (memory consumption won't keep growing as your program is running, because we're talking about one object), there is no real reason for keeping it there.
When you start a program, a process will be created by the OS containing all of the information needed to run the program.
When the process is destroyed, for example by closing the main window, the OS will clean up (not immediately) the image of the process from Memory and your Memory leaks will disappear.
pIt becomes a Problem if you have a long running process and create memory leaks, so the process will either die or the OS starts to run slow.
It is better to get into the habit of deleteing memory yourself (or use smart pointers), because code changes over time, and who knows if the object will last the lifetime of the program in the future, when you (or the next developer) have long since forgotten that you didn't have a matching delete for your new.
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!