There're two functions: one called make_a_handle Escapes a handle with EscapableHandleScope, and another one called do_something has its own HandleScope. Assuming the do_something function calls the make_a_handle, and never assigns nor uses the Escaped handle; would the Escaped handle be a target of garbage collection immediately, or live until the destruction of the do_something's HandleScope?
The latter. An EscapableHandleScope's Escape function internally uses an "escape slot", which is essentially a handle in the outer HandleScope.
For clarity, I'd also like to point out that when a HandleScope is destructed, objects referred to by its handles (assuming that was the last reference to them) will not be freed immediately; they only become eligible for garbage collection, which means they will be freed in the next garbage collection cycle.
Related
I'm writing an app which should at some point get the value of a defglobal variable and change it. For this I do the following:
DATA_OBJECT cur_time_q;
if (!EnvGetDefglobalValue(CLIEnvironment, cur_timeq_kw, &cur_time_q)) return CUR_TIME_GLBVAR_MISSING;
uint64_t cur_time = t_left;
SetType(cur_time_q, INTEGER);
void* val = EnvAddLong(CLIEnvironment, cur_time);
SetValue(cur_time_q, val);
EnvSetDefglobalValue(CLIEnvironment, cur_timeq_kw, &cur_time_q);
I partly took this approach from "Advanced Programming Guide" and it works fine, but I have some questions:
Does EnvAddLong(...) add a value, which will retain in memory, until the environment is destroyed? May it consume memory and increase the execution time of other API-functions like EnvRun(...), if the function with this fragment of code is called for, say, several thousand iterations?
Isn't it overkill? Should I go for something like EnvEval("(bind ...)") instead?
There's information in the CLIPS Advanced Programming Guide on how CLIPS handles garbage collection. API calls like EnvAddLong which are used to create values to pass to other API functions don't trigger garbage collection. Generally, API calls which cause code to execute or deallocate data structures such as Run, Reset, Clear, and Eval, trigger garbage collection and will deallocate any transient data created by functions like EnvAddLong. So if your program design repeatedly assigns values to globals and then runs, any CLIPS data structures you allocate will eventually be freed once the data is confirmed to be garbage and is no longer referenced by any CLIPS data structures.
If you can easily construct a string to pass to the Eval function, it's often easier to do this rather than make multiple API calls to achieve the same result.
The API was overhauled in release 6.4, so many tasks such as assigning a value to a defglobal can be done with one step rather than several.
CLIPSValue rv;
Defglobal *global;
mainEnv = CreateEnvironment();
Build(mainEnv,"(defglobal ?*x* = 3.1)");
Eval(mainEnv,"?*x*",&rv);
printf("%lf\n",rv.floatValue->contents);
global = FindDefglobal(mainEnv,"x");
if (global != NULL)
{
DefglobalSetInteger(global,343433);
Eval(mainEnv,"(println ?*x*)",NULL);
DefglobalGetValue(global,&rv);
printf("%lf\n",rv.floatValue->contents);
}
I wrote some code like this:
shared_ptr<int> r = make_shared<int>();
int *ar = r.get();
delete ar; // report double free or corruption
// still some code
When the code ran up to delete ar;, the program crashed, and reported "double free or corruption", I'm confused why double free? The "r" is still in the scope, and not popped-off from stack. Do the delete operator do something magic?? Does it know the raw pointer is handled by a smart pointer currently? and then counter in "r" be decremented to zero automatically?
I know the operations is not recommended， but I want to know why?
You are deleting a pointer that didn't come from new, so you have undefined behavior (anything can happen).
From cppreference on delete:
For the first (non-array) form, expression must be a pointer to an object type or a class type contextually implicitly convertible to such pointer, and its value must be either null or pointer to a non-array object created by a new-expression, or a pointer to a base subobject of a non-array object created by a new-expression. If expression is anything else, including if it is a pointer obtained by the array form of new-expression, the behavior is undefined.
If the allocation is done by new, we can be sure that the pointer we have is something we can use delete on. But in the case of shared_ptr.get(), we cannot be sure if we can use delete because it might not be the actual pointer returned by new.
shared_ptr<int> r = make_shared<int>();
There is no guarantee that this will call new int (which isn't strictly observable by the user anyway) or more generally new T (which is observable with a user defined, class specific operator new); in practice, it won't (there is no guarantee that it won't).
The discussion that follows isn't just about shared_ptr, but about "smart pointers" with ownership semantics. For any owning smart pointer smart_owning:
The primary motivation for make_owning instead of smart_owning<T>(new T) is to avoid having a memory allocation without owner at any time; that was essential in C++ when order of evaluation of expressions didn't provide the guarantee that evaluation of the sub-expressions in the argument list was immediately before call of that function; historically in C++:
f (smart_owning<T>(new T), smart_owning<U>(new U));
could be evaluated as:
T *temp1 = new T;
U *temp2 = new U;
auto &&temp3 = smart_owning<T>(temp1);
auto &&temp4 = smart_owning<U>(temp2);
This way temp1 and temp2 are not managed by any owning object for a non trivial time:
obviously new U can throw an exception
constructing an owning smart pointer usually requires the allocation of (small) ressources and can throw
So either temp1 or temp2 could be leaked (but not both) if an exception was thrown, which was the exact problem we were trying to avoid in the first place. This means composite expressions involving construction of owning smart pointers was a bad idea; this is fine:
auto &&temp_t = smart_owning<T>(new T);
auto &&temp_u = smart_owning<U>(new U);
f (temp_t, temp_u);
Usually expression involving as many sub-expression with function calls as f (smart_owning<T>(new T), smart_owning<U>(new U)) are considered reasonable (it's a pretty simple expression in term of number of sub-expressions). Disallowing such expressions is quite annoying and very difficult to justify.
[This is one reason, and in my opinion the most compelling reason, why the non determinism of the order of evaluation was removed by the C++ standardisation committee so that such code is not safe. (This was an issue not just for memory allocated, but for any managed allocation, like file descriptors, database handles...)]
Because code frequently needed to do things such as smart_owning<T>(allocate_T()) in sub-expressions, and because telling programmers to decompose moderately complex expressions involving allocation in many simple lines wasn't appealing (more lines of code doesn't mean easier to read), the library writers provided a simple fix: a function to do the creation of an object with dynamic lifetime and the creation of its owning object together. That solved the order of evaluation problem (but was complicated at first because it needed perfect forwarding of the arguments of the constructor).
Giving two tasks to a function (allocate an instance of T and a instance of smart_owning) gives the freedom to do an interesting optimization: you can avoid one dynamic allocation by putting both the managed object and its owner next to each others.
But once again, that was not the primary purpose of functions like make_shared.
Because exclusive ownership smart pointers by definition don't need to keep a reference count, and by definition don't need to share the data needed for the deleter either between instances, and so can keep that data in the "smart pointer"(*), no additional allocation is needed for the construction of unique_ptr; yet a make_unique function template was added, to avoid the dangling pointer issue, not to optimize a non-thing (an allocation that isn't done in the fist place).
(*) which BTW means unique owner "smart pointers" do not have pointer semantic, as pointer semantic implies that you can makes copies of the "pointer", and you can't have two copies of a unique owner pointing to the same instance; "smart pointers" were never pointers anyway, the term is misleading.
Summary:
make_shared<T> does an optional optimization where there is no separate dynamic memory allocation for T: there is no operator new(sizeof (T)). There is obviously still the creation of an instance with dynamic lifetime with another operator new: placement new.
If you replace the explicit memory deallocation with an explicit destruction and add a pause immediately after that point:
class C {
public:
~C();
};
shared_ptr<C> r = make_shared<C>();
C *ar = r.get();
ar->~C();
pause(); // stops the program forever
The program will probably run fine; it is still illogical, indefensible, incorrect to explicitly destroy an object managed by a smart pointer. It isn't "your" resource. If pause() could exit with an exception, the owning smart pointer would try to destroy the managed object which doesn't even exist anymore.
It of course depends on how library implements make_shared, however most probable implementation is that:
std::make_shared allocates one block for two things:
shared pointer control block
contained object
std::make_shared() will invoke memory allocator once and then it will call placement new twice to initialize (call constructors) of mentioned two things.
| block requested from allocator |
| shared_ptr control block | X object |
#1 #2 #3
That means that memory allocator has provided one big block, which address is #1.
Shared pointer then uses it for control block (#1) and actual contained object (#2).
When you invoke delete with actual object kept by shred_ptr ( .get() ) you call delete(#2).
Because #2 is not known by allocator you get an corruption error.
See here. I quot:
std::shared_ptr is a smart pointer that retains shared ownership of an object through a pointer. Several shared_ptr objects may own the same object. The object is destroyed and its memory deallocated when either of the following happens:
the last remaining shared_ptr owning the object is destroyed;
the last remaining shared_ptr owning the object is assigned another pointer via operator= or reset().
The object is destroyed using delete-expression or a custom deleter that is supplied to shared_ptr during construction.
So the pointer is deleted by shared_ptr. You're not suppose to delete the stored pointer yourself
UPDATE:
I didn't realize that there were more statements and the pointer was not out of scope, I'm sorry.
I was reading more and the standard doesn't say much about the behavior of get() but here is a note, I quote:
A shared_ptr may share ownership of an object while storing a pointer to another object. get() returns the stored pointer, not the managed pointer.
So it looks that it is allowed that the pointer returned by get() is not necessarily the same pointer allocated by the shared_ptr (presumably using new). So delete that pointer is undefined behavior. I will be looking a little more into the details.
UPDATE 2:
The standard says at § 20.7.2.2.6 (about make_shared):
6 Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [ Note: This provides efficiency equivalent to an intrusive smart pointer. — end note ]
7 [ Note: These functions will typically allocate more memory than sizeof(T) to allow for internal bookkeeping structures such as the reference counts. — end note ]
So an specific implementation of make_shared could allocate a single chunk of memory (or more) and use part of that memory to initialize the stored pointer (but maybe not all the memory allocated). get() must return a pointer to the stored object, but there is no requirement by the standard, as previously said, that the pointer returned by get() has to be the one allocated by new. So delete that pointer is undefined behavior, you got a signal raised but anything can happen.
If I'm playing in the REPL and I've defined a few different methods for a function:
julia> methods(next)
# 3 methods for generic function "next":
next(i::BigInt) at none:1
next(i::Int64) at none:1
next(i) at none:1
Can I make Julia forget some or all of these?
In short, no.
Julia does not have an analog of MATLAB’s clear function; once a name
is defined in a Julia session (technically, in module Main), it is
always present.
If memory usage is your concern, you can always replace objects with
ones that consume less memory. For example, if A is a gigabyte-sized
array that you no longer need, you can free the memory with A = 0. The
memory will be released the next time the garbage collector runs; you
can force this to happen with gc().
(source)
I have a Go object whose address in memory I would like to keep constant. in C# one can pin an object's location in memory. Is there a way to do this in Go?
An object on which you keep a reference won't move. There is no handle or indirection, and the address you get is permanent.
From the documentation :
Note that, unlike in C, it's perfectly OK to return the address of a
local variable; the storage associated with the variable survives
after the function returns
When you set a variable, you can read this address using the & operator, and you can pass it.
tl;dr no - but it does not matter unless you're trying to do something unusual.
Worth noting that the accepted answer is partially incorrect.
There is no guarantee that objects are not moved - either on the stack or on the Go heap - but as long as you don't use unsafe this will not matter to you because the Go runtime will take care of transparently updating your pointers in case an object is moved.
If OTOH you use unsafe to obtain a uintptr, invoke raw syscalls, perform CGO calls, or otherwise expose the address (e.g. oldAddr := fmt.Sprintf("%p", &foo)), etc. you should be aware that addresses can change, and that nor compiler nor runtime will magically patch things for you.
While currently the standard Go compiler only moves objects on the stack (e.g. when a goroutine stack needs to be resized), there is nothing in the Go language specification that prevents a different implementation from moving objects on the Go heap.
While there is (yet) no explicit support for pinning objects in the stack or in the Go heap, there is a recommended workaround: allocate manually the memory outside of the Go heap (e.g. via mmap) and using finalizers to automatically free that allocation once all references to it are dropped. The benefit of this approach is that memory allocated manually outside of the Go heap will never be moved by the Go runtime, so its address will never change, but it will still be deallocated automatically when it's not needed anymore, so it can't leak.
I have the following (adapted from Apple Low Level File Management - Resolving Aliases)
NSString *resolvedPath = nil;
...
resolvedPath = (NSString*)CFURLCopyFileSystemPath(resolvedUrl, kCFURLPOSIXPathStyle);
...
return resolvedPath;
Build & Analyse generates the following warning:-
194:3 Potential leak (when using garbage collection) of an object allocated on line 187 and stored into 'resolvedPath'
The detail states:-
187:32 Call to function 'CFURLCopyFileSystemPath' returns a Core Foundation object with a +1 retain count (owning reference). Core Foundation objects are not automatically garbage collected
194:3 Object returned to caller as an owning reference (single retain count transferred to caller)
194:3 Object allocated on line 187 and stored into 'resolvedPath' and returned from method 'getTarget:' is potentially leaked when using garbage collection. Callers of this method do not expect a returned object with a +1 retain count since they expect the object to be managed by the garbage collector
Do I have a memory leak?
If so how do I fix it?
If not how do I prevent the warnings?
It is just alerting you that the object created that is assigned to resolvedPath is returned with a retain count of 1, therefore unless your method is starting with new alloc or contains copy, the caller has no way to know it is dealing with a retained object, and it will therefore never get released.
To fix it change your method name from getTarget to newTarget.
Do I have a memory leak?
you should assume it is a leak. in GC, the allocation of a cf type is not guaranteed to be registered with the collector. GC only covers a range of types (explicit objc objects). you should assume cf types which you create or copy are not registered with the collector. whether you are returned a cftype from a cf function that is or is not registered with the collector is not disclosed (it may be because an ns type is a cf type).
If so how do I fix it?
using CFMakeCollectable or NSMakeCollectable