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On one hand the advice to always close objects is so common that I would feel foolish to ignore it (e.g. VBScript Out Of Memory Error).
However it would be equally foolish to ignore the wisdom of Eric Lippert, who appears to disagree: http://blogs.msdn.com/b/ericlippert/archive/2004/04/28/when-are-you-required-to-set-objects-to-nothing.aspx
I've worked to fix a number of web apps with OOM errors in classic asp. My first (time consuming) task is always to search the code for unclosed objects, and objects not set to nothing.
But I've never been 100% convinced that this has helped. (That said, I have found it hard to pinpoint exactly what DOES help...)
This post by Eric is talking about standalone VBScript files, not classic ASP written in VBScript. See the comments, then Eric's own comment:
Re: ASP -- excellent point, and one that I had not considered. In ASP it is sometimes very difficult to know where you are and what scope you're in.
So from this I can say that everything he wrote isn't relevant for classic ASP i.e. you should always Set everything to Nothing.
As for memory issues, I think that assigning objects (or arrays) to global scope like Session or Application is the main reason for such problems. That's the first thing I would look for and rewrite to hold only single identifider in Session then use database to manage the data.
Basically by setting a COM object to Nothing, you are forcing its terminator to run deterministically, which gives you the opportunity to handle any errors it may raise.
If you don't do it, you can get into a situation like the following:
Your code raises an error
The error isn't handled in your code and therefore ...
other objects instantiated in your code go out of scope, and their terminators run
one of the terminators raises an error
and the error that is propagated is the one from the terminator going out of scope, masking the original error.
I do remember from the dark and distant past that it was specifically recommended to close ADO objects. I'm not sure if this was because of a bug in ADO objects, or simply for the above reason (which applies more generally to any objects that can raise errors in their terminators).
And this recommendation is often repeated, though often without any credible reason. ("While ASP should automatically close and free up all object instantiations, it is always a good idea to explicitly close and free up object references yourself").
It's worth noting that in the article, he's not saying you should never worry about setting objects to nothing - just that it should not be the default behaviour for every object in every script.
Though I do suspect he's a little too quick to dismiss the "I saw this elsewhere" method of coding behaviour, I'm willing to bet that there is a reason Eric didn't consider that has caused this to be passed along as a hard 'n' fast rule - dealing with junior programmers.
When you start looking more closely at the Dreyfus model of skill acquisition, you see that at the beginning levels of acquiring a new skill, learners need simple to follow recipes. They do not yet have the knowledge or ability to make the judgement calls Eric qualifies the recommendation with later on.
Think back to when you first started programming. Could you readily judge if you were "set[tting an] expensive objects to Nothing when you are done with them if you are done with them well before they go out of scope"? Did you really know which objects were expensive or when they truly went out of scope?
Thus, most entry level programmers are simply told "always set every object to Nothing when you are done with it" because it is within their grasp to understand and follow. Unfortunately, not many programmers take the time to self-educate, learn, and grow into the higher-level Dreyfus stages where you can use the more nuanced situational approach.
And then we come back to my earlier statement - even the best of us started out at that earlier stage, where we reflexively closed all objects because that was the best we were capable of. We left large bodies of code that people look at now, and project our current competence backwards to the earlier work and assume we did that for reasons we don't understand.
I've got to get going, but I hope to expand this a little futher...
I came across something strange the other day in php, but this question is more general than that.
What I wonder is, is it possible to have a function that returns something which as far as I know means that the return value/function is on the stack and pass the result to a function which takes a reference?
With my limited knowledge I would say no, but thats just a feeling that I have, and I've learned to never trust my feelings when it comes to programming.
I'm not too familiar with references in PHP, but you can do this in C/C++ and some cases in C#.
The thing is that you have to be careful. A value on the stack is only valid as long as the function whos stack frame it belongs to hasn't returned (ignoring stuff like the red zone). C and C++ expect you to be smart enough to not use a dangling reference like this.
Anyway, it doesn't matter in PHP because it's a high level language and hides details of allocation like that from you. Most likely, all your objects are allocated on the heap.
I'm building a interpreter/compiler for a school project (well now its turning into a hobby project) and an instructor warned me not to allow mutation of the 'this' binding (he said it was gross and made a huge deal about it) but I never learned why this is so... dangerous or bad. I'm very curious about why this is so bad. I figured this sort of feature could be useful in some way or another.
I'm wondering if anyone familiar with building languages can tell me what sort of problems mutation on the 'this' binding can cause, and if they know of any cool or useful tricks that one could do if it actually was allowed.
Do any languages that you're aware of allow mutation of 'this'?
Thanks,
I can think of several reasons why it would be a bad idea.
1) 'this' is a pointer to the object instance on which the method call is invoked. Allowing changes to it could lead to memory access validations in the worst case.
2) The caller expects the method to work on the instance on which it was invoked. Changing 'this' to something else would produce unexpected results.
3) I can't think of anything that mutation of 'this' would allow that can't be achieved in a more standard, familiar way.
I'm surprised your instructor got so het up. It's just a change in the language definition. I don't think being able to change the meaning of this is useful or good language design, but I think as long as you stick to reference semantics it's a valid experiment.
The main reason not to allow this to refer to anything other than the receiver of the current method is that you will confound the expectations of anyone who has ever read or written an object-oriented program. That's not good design.
Question How can I make sure my application is thread-safe? Are their any common practices, testing methods, things to avoid, things to look for?
Background I'm currently developing a server application that performs a number of background tasks in different threads and communicates with clients using Indy (using another bunch of automatically generated threads for the communication). Since the application should be highly availabe, a program crash is a very bad thing and I want to make sure that the application is thread-safe. No matter what, from time to time I discover a piece of code that throws an exception that never occured before and in most cases I realize that it is some kind of synchronization bug, where I forgot to synchronize my objects properly. Hence my question concerning best practices, testing of thread-safety and things like that.
mghie: Thanks for the answer! I should perhaps be a little bit more precise. Just to be clear, I know about the principles of multithreading, I use synchronization (monitors) throughout my program and I know how to differentiate threading problems from other implementation problems. But nevertheless, I keep forgetting to add proper synchronization from time to time. Just to give an example, I used the RTL sort function in my code. Looked something like
FKeyList.Sort (CompareKeysFunc);
Turns out, that I had to synchronize FKeyList while sorting. It just don't came to my mind when initially writing that simple line of code. It's these thins I wanna talk about. What are the places where one easily forgets to add synchronization code? How do YOU make sure that you added sync code in all important places?
You can't really test for thread-safeness. All you can do is show that your code isn't thread-safe, but if you know how to do that you already know what to do in your program to fix that particular bug. It's the bugs you don't know that are the problem, and how would you write tests for those? Apart from that threading problems are much harder to find than other problems, as the act of debugging can already alter the behaviour of the program. Things will differ from one program run to the next, from one machine to the other. Number of CPUs and CPU cores, number and kind of programs running in parallel, exact order and timing of stuff happening in the program - all of this and much more will have influence on the program behaviour. [I actually wanted to add the phase of the moon and stuff like that to this list, but you get my meaning.]
My advice is to stop seeing this as an implementation problem, and start to look at this as a program design problem. You need to learn and read all that you can find about multi-threading, whether it is written for Delphi or not. In the end you need to understand the underlying principles and apply them properly in your programming. Primitives like critical sections, mutexes, conditions and threads are something the OS provides, and most languages only wrap them in their libraries (this ignores things like green threads as provided by for example Erlang, but it's a good point of view to start out from).
I'd say start with the Wikipedia article on threads and work your way through the linked articles. I have started with the book "Win32 Multithreaded Programming" by Aaron Cohen and Mike Woodring - it is out of print, but maybe you can find something similar.
Edit: Let me briefly follow up on your edited question. All access to data that is not read-only needs to be properly synchronized to be thread-safe, and sorting a list is not a read-only operation. So obviously one would need to add synchronization around all accesses to the list.
But with more and more cores in a system constant locking will limit the amount of work that can be done, so it is a good idea to look for a different way to design your program. One idea is to introduce as much read-only data as possible into your program - locking is no longer necessary, as all access is read-only.
I have found interfaces to be a very valuable aid in designing multi-threaded programs. Interfaces can be implemented to have only methods for read-only access to the internal data, and if you stick to them you can be quite sure that a lot of the potential programming errors do not occur. You can freely share them between threads, and the thread-safe reference counting will make sure that the implementing objects are properly freed when the last reference to them goes out of scope or is assigned another value.
What you do is create objects that descend from TInterfacedObject. They implement one or more interfaces which all provide only read-only access to the internals of the object, but they can also provide public methods that mutate the object state. When you create the object you keep both a variable of the object type and a interface pointer variable. That way lifetime management is easy, because the object will be deleted automatically when an exception occurs. You use the variable pointing to the object to call all methods necessary to properly set up the object. This mutates the internal state, but since this happens only in the active thread there is no potential for conflict. Once the object is properly set up you return the interface pointer to the calling code, and since there is no way to access the object afterwards except by going through the interface pointer you can be sure that only read-only access can be performed. By using this technique you can completely remove the locking inside of the object.
What if you need to change the state of the object? You don't, you create a new one by copying the data from the interface, and mutate the internal state of the new objects afterwards. Finally you return the reference pointer to the new object.
By using this you will only need locking where you get or set such interfaces. It can even be done without locking, by using the atomic interchange functions. See this blog post by Primoz Gabrijelcic for a similar use case where an interface pointer is set.
Simple: don't use shared data. Every time you access shared data you risk running into a problem (if you forget to synchronize access). Even worse, each time you access shared data you risk blocking other threads which will hurt your paralelization.
I know this advice is not always applicable. Still, it doesn't hurt if you try to follow it as much as possible.
EDIT: Longer response to Smasher's comment. Would not fit in a comment :(
You are totally correct. That's why I like to keep a shadow copy of the main data in a readonly thread. I add a versioning to the structure (one 4-aligned DWORD) and increment this version in the (lock-protected) data writer. Data reader would compare global and private version (which can be done without locking) and only if they differr it would lock the structure, duplicate it to a local storage, update the local version and unlock. Then it would access the local copy of the structure. Works great if reading is the primary way to access the structure.
I'll second mghie's advice: thread safety is designed in. Read about it anywhere you can.
For a really low level look at how it is implemented, look for a book on the internals of a real time operating system kernel. A good example is MicroC/OS-II: The Real Time Kernel by Jean J. Labrosse, which contains the complete annotated source code to a working kernel along with discussions of why things are done the way they are.
Edit: In light of the improved question focusing on using a RTL function...
Any object that can be seen by more than one thread is a potential synchronization issue. A thread-safe object would follow a consistent pattern in every method's implementation of locking "enough" of the object's state for the duration of the method, or perhaps, narrowed to just "long enough". It is certainly the case that any read-modify-write sequence to any part of an object's state must be done atomically with respect to other threads.
The art lies in figuring out how to get useful work done without either deadlocking or creating an execution bottleneck.
As for finding such problems, testing won't be any guarantee. A problem that shows up in testing can be fixed. But it is extremely difficult to write either unit tests or regression tests for thread safety... so faced with a body of existing code your likely recourse is constant code review until the practice of thread safety becomes second nature.
As folks have mentioned and I think you know, being certain, in general, that your code is thread safe is impossible (I believe provably impossible but I would have to track down the theorem). Naturally, you want to make things easier than that.
What I try to do is:
Use a known pattern of multithreaded design: A thread pool, the actor model paradigm, the command pattern or some such approach. This way, the syncronization process happens in the same way, in a uniform way, throughout the application.
Limit and concentrate the points of synchronization. Write your code so you need synchronization in as few places as possible and the keep the synchronization code in one or few places in the code.
Write the synchronization code so that the logical relation between the values is clear on both on entering and on exiting the guard. I use lots of asserts for this (your environment may limit this).
Don't ever access shared variables without guards/synchronization. Be very clear what your shared data is. (I've heard there are paradigms for guardless multithreaded programming but that would require even more research).
Write your code as cleanly, clearly and DRY-ly as possible.
My simple answer combined with those answer is:
Create your application/program using
thread safety manner
Avoid using public static variable in
all places
Therefore it usually fall into this habit/practice easily but it needs some time to get used to:
program your logic (not the UI) in functional programming language such as F# or even using Scheme or Haskell. Also functional programming promotes thread safety practice while it also warns us to always code towards purity in functional programming.
If you use F#, there's also clear distinction about using mutable or immutable objects such as variables.
Since method (or simply functions) is a first class citizen in F# and Haskell, then the code you write will also have more disciplined toward less mutable state.
Also using the lazy evaluation style that usually can be found in these functional languages, you can be sure that your program is safe fromside effects, and you'll also realize that if your code needs effects, you have to clearly define it. IF side effects are taken into considerations, then your code will be ready to take advantage of composability within components in your codes and the multicore programming.
I'm designing a language. First, I want to decide what code to generate. The language will have lexical closures and prototype based inheritance similar to javascript. But I'm not a fan of gc and try to avoid as much as possible. So the question: Is there an elegant way to implement closures without resorting to allocate the stack frame on the heap and leave it to garbage collector?
My first thoughts:
Use reference counting and garbage collect the cycles (I don't really like this)
Use spaghetti stack (looks very inefficient)
Limit forming of closures to some contexts such a way that, I can get away with a return address stack and a locals' stack.
I won't use a high level language or follow any call conventions, so I can smash the stack as much as I like.
(Edit: I know reference counting is a form of garbage collection but I am using gc in its more common meaning)
This would be a better question if you can explain what you're trying to avoid by not using GC. As I'm sure you're aware, most languages that provide lexical closures allocate them on the heap and allow them to retain references to variable bindings in the activation record that created them.
The only alternative to that approach that I'm aware of is what gcc uses for nested functions: create a trampoline for the function and allocate it on the stack. But as the gcc manual says:
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
Short version is, you have three main choices:
allocate closures on the stack, and don't allow their use after their containing function exits.
allocate closures on the heap, and use garbage collection of some kind.
do original research, maybe starting from the region stuff that ML, Cyclone, etc. have.
This thread might help, although some of the answers here reflect answers there already.
One poster makes a good point:
It seems that you want garbage collection for closures
"in the absence of true garbage collection". Note that
closures can be used to implement cons cells. So your question
seem to be about garbage collection "in the absence of true
garbage collection" -- there is rich related literature.
Restricting problem to closures does not really change it.
So the answer is: no, there is no elegant way to have closures and no real GC.
The best you can do is some hacking to restrict your closures to a particular type of closure. All this is needless if you have a proper GC.
So, my question reflects some of the other ones here - why do you not want to implement GC? A simple mark+sweep or stop+copy takes about 2-300 lines of (Scheme) code, and isn't really that bad in terms of programming effort. In terms of making your programs slower:
You can implement a more complex GC which has better performance.
Just think of all the memory leaks programs in your language won't suffer from.
Coding with a GC available is a blessing. (Think C#, Java, Python, Perl, etc... vs. C++ or C).
I understand that I'm very late, but I stumbled upon this question by accident.
I believe that full support of closures indeed requires GC, but in some special cases stack allocation is safe. Determining these special cases requires some escape analysis. I suggest that you take a look at the BitC language papers, such as Closure Implementation in BitC. (Although I doubt whether the papers reflect the current plans.) The designers of BitC had the same problem you do. They decided to implement a special non-collecting mode for the compiler, which denies all closures that might escape. If turned on, it will restrict the language significantly. However, the feature is not implemented yet.
I'd advise you to use a collector - it's the most elegant way. You should also consider that a well-built garbage collector allocates memory faster than malloc does. The BitC folks really do value performance and they still think that GC is fine even for the most parts of their operating system, Coyotos. You can migitate the downsides by simple means:
create only a minimal amount of garbage
let the programmer control the collector
optimize stack/heap use by escape analysis
use an incremental or concurrent collector
if somehow possible, divide the heap like Erlang does
Many fear garbage collectors because of their experiences with Java. Java has a fantastic collector, but applications written in Java have performance problems because of the sheer amount of garbage generated. In addition, a bloated runtime and fancy JIT compilation is not really a good idea for desktop applications because of the longer startup and response times.
The C++ 0x spec defines lambdas without garbage collection. In short, the spec allows non-deterministic behavior in cases where the lambda closure contains references which are no longer valid. For example (pseudo-syntax):
(int)=>int create_lambda(int a)
{
return { (int x) => x + a }
}
create_lambda(5)(4) // undefined result
The lambda in this example refers to a variable (a) which is allocated on the stack. However, that stack frame has been popped and is not necessarily available once the function returns. In this case, it would probably work and return 9 as a result (assuming sane compiler semantics), but there is no way to guarantee it.
If you are avoiding garbage collection, then I'm assuming that you also allow explicit heap vs. stack allocation and (probably) pointers. If that is the case, then you can do like C++ and just assume that developers using your language will be smart enough to spot the problem cases with lambdas and copy to the heap explicitly (just like you would if you were returning a value synthesized within a function).
Use reference counting and garbage collect the cycles (I don't really like this)
It's possible to design your language so there are no cycles: if you can only make new objects and not mutate old ones, and if making an object can't make a cycle, then cycles never appear. Erlang works essentially this way, though in practice it does use GC.
If you have the machinery for a precise copying GC, you could allocate on the stack initially and copy to the heap and update pointers if you discover at exit that a pointer to this stack frame has escaped. That way you only pay if you actually do capture a closure that includes this stack frame. Whether this helps or hurts depends on how often you use closures and how much they capture.
You might also look into C++0x's approach (N1968), though as one might expect from C++ it consists of counting on the programmer to specify what gets copied and what gets referenced, and if you get it wrong you just get invalid accesses.
Or just don't do GC at all. There can be situations where it's better to just forget the memory leak and let the process clean up after it when it's done.
Depending on your qualms about GC, you might be afraid of the periodic GC sweeps. In this case you could do a selective GC when an item falls out of scope or the pointer changes. I'm not sure how expensive this would be though.
#Allen
What good is a closure if you can't use them when the containing function exits? From what I understand that's the whole point of closures.
You could work with the assumption that all closures will be called eventually and exactly one time. Now, when the closure is called you can do the cleanup at the closure return.
How do you plan on dealing with returning objects? They have to be cleaned up at some point, which is the exact same problem with closures.
So the question: Is there an elegant way to implement closures without resorting to allocate the stack frame on the heap and leave it to garbage collector?
GC is the only solution for the general case.
Better late than never?
You might find this interesting: Differential Execution.
It's a little-known control stucture, and its primary use is in programming user interfaces, including ones that can change dynamically while in use. It is a significant alternative to the Model-View-Controller paradigm.
I mention it because one might think that such code would rely heavily on closures and garbage-collection, but a side effect of the control structure is that it eliminates both of those, at least in the UI code.
Create multiple stacks?
I've read that the last versions of ML use GC only sparingly
I guess if the process is very short, which means it cannot use much memory, then GC is unnecessary. The situation is analogous to worrying about stack overflow. Don't nest too deeply, and you cannot overflow; don't run too long, and you cannot need the GC. Cleaning up becomes a matter of simply reclaiming the large region that you pre-allocated. Even a longer process can be divided into smaller processes that have their own heaps pre-allocated. This would work well with event handlers, for example. It does not work well, if you are writing compiler; in that case, a GC is surely not much of a handicap.