I have an object that is called from two different threads and after it was called by both it destroys itself by "delete this".
How do I implement this thread-safe? Thread-safe means that the object never destroys itself exactly one time (it must destroys itself after the second callback).
I created some example code:
class IThreadCallBack
{
virtual void CallBack(int) = 0;
};
class M: public IThreadCallBack
{
private:
bool t1_finished, t2_finished;
public:
M(): t1_finished(false), t2_finished(false)
{
startMyThread(this, 1);
startMyThread(this, 2);
}
void CallBack(int id)
{
if (id == 1)
{
t1_finished = true;
}
else
{
t2_finished = true;
}
if (t1_finished && t2_finished)
{
delete this;
}
}
};
int main(int argc, char **argv) {
M* MObj = new M();
while(true);
}
Obviously I can't use a Mutex as member of the object and lock the delete, because this would also delete the Mutex. On the other hand, if I set a "toBeDeleted"-flag inside a mutex-protected area, where the finised-flag is set, I feel unsure if there are situations possible where the object isnt deleted at all.
Note that the thread-implementation makes sure that the callback method is called exactly one time per thread in any case.
Edit / Update:
What if I change Callback(..) to:
void CallBack(int id)
{
mMutex.Obtain()
if (id == 1)
{
t1_finished = true;
}
else
{
t2_finished = true;
}
bool both_finished = (t1_finished && t2_finished);
mMutex.Release();
if (both_finished)
{
delete this;
}
}
Can this considered to be safe? (with mMutex being a member of the m class?)
I think it is, if I don't access any member after releasing the mutex?!
Use Boost's Smart Pointer. It handles this automatically; your object won't have to delete itself, and it is thread safe.
Edit:
From the code you've posted above, I can't really say, need more info. But you could do it like this: each thread has a shared_ptr object and when the callback is called, you call shared_ptr::reset(). The last reset will delete M. Each shared_ptr could be stored with thread local storeage in each thread. So in essence, each thread is responsible for its own shared_ptr.
Instead of using two separate flags, you could consider setting a counter to the number of threads that you're waiting on and then using interlocked decrement.
Then you can be 100% sure that when the thread counter reaches 0, you're done and should clean up.
For more info on interlocked decrement on Windows, on Linux, and on Mac.
I once implemented something like this that avoided the ickiness and confusion of delete this entirely, by operating in the following way:
Start a thread that is responsible for deleting these sorts of shared objects, which waits on a condition
When the shared object is no longer being used, instead of deleting itself, have it insert itself into a thread-safe queue and signal the condition that the deleter thread is waiting on
When the deleter thread wakes up, it deletes everything in the queue
If your program has an event loop, you can avoid the creation of a separate thread for this by creating an event type that means "delete unused shared objects" and have some persistent object respond to this event in the same way that the deleter thread would in the above example.
I can't imagine that this is possible, especially within the class itself. The problem is two fold:
1) There's no way to notify the outside world not to call the object so the outside world has to be responsible for setting the pointer to 0 after calling "CallBack" iff the pointer was deleted.
2) Once two threads enter this function you are, and forgive my french, absolutely fucked. Calling a function on a deleted object is UB, just imagine what deleting an object while someone is in it results in.
I've never seen "delete this" as anything but an abomination. Doesn't mean it isn't sometimes, on VERY rare conditions, necessary. Problem is that people do it way too much and don't think about the consequences of such a design.
I don't think "to be deleted" is going to work well. It might work for two threads, but what about three? You can't protect the part of code that calls delete because you're deleting the protection (as you state) and because of the UB you'll inevitably cause. So the first goes through, sets the flag and aborts....which of the rest is going to call delete on the way out?
The more robust implementation would be to implement reference counting. For each thread you start, increase a counter; for each callback call decrease the counter and if the counter has reached zero, delete the object. You can lock the counter access, or you could use the Interlocked class to protect the counter access, though in that case you need to be careful with potential race between the first thread finishing and the second starting.
Update: And of course, I completely ignored the fact that this is C++. :-) You should use InterlockExchange to update the counter instead of the C# Interlocked class.
Related
I'm implementing a Signal/Slot framework, and got to the point that I want it to be thread-safe. I already had a lot of support from the Boost mailing-list, but since this is not really boost-related, I'll ask my pending question here.
When is a signal/slot implementation (or any framework that calls functions outside itself, specified in some way by the user) considered thread-safe? Should it be safe w.r.t. its own data, i.e. the data associated to its implementation details? Or should it also take into account the user's data, which might or might not be modified whatever functions are passed to the framework?
This is an example given on the mailing-list (Edit: this is an example use-case --i.e. user code--. My code is behind the calls to the Emitter object):
int * somePtr = nullptr;
Emitter<Event> em; // just an object that can emit the 'Event' signal
void mainThread()
{
em.connect<Event>(someFunction);
// now, somehow, 2 threads are created which, at some point
// execute the thread1() and thread2() functions below
}
void someFunction()
{
// can somePtr change after the check but before the set?
if (somePtr)
*somePtr = 17;
}
void cleanupPtr()
{
// this looks safe, but compilers and CPUs can reorder this code:
int *tmp = somePtr;
somePtr = null;
delete tmp;
}
void thread1()
{
em.emit<Event>();
}
void thread2()
{
em.disconnect<Event>(someFunction);
// now safe to cleanup (?)
cleanupPtr();
}
In the above code, it might happen that Event is emitted, causing someFunction to be executed. If somePtr is non-null, but becomes null just after the if, but before the assignment, we're in trouble. From the point of view of thread2, this is not obvious because it is disconnecting someFunction before calling cleanupPtr.
I can see why this could potentially lead to trouble, but who's responsibility is this? Should my library protect the user from using it in every irresponsible but imaginable way?
I suspect there is no clearly good answer, but clarity will come from documenting the guarantees you wish to make about concurrent access to an Emitter object.
One level of guarantee, which to me is what is implied by a promise of thread safety, is that:
Concurrent operations on the object are guaranteed to leave the object in a consistent state (at least, from the point of view of the accessing threads.)
Non-commutative operations will be performed as if they were scheduled serially in some (unknown) order.
Then the question is, what does the emit method promise semantically: passing control to the connected routine, or evaluation of the function? If the former, then your work sounds like it is already done; if the latter, then the 'as-if ordered' requirement would mean that you need to enforce some level of synchronisation.
Users of the library can work with either, provided it is clear what is being promised.
Firstly the simplest possibility: If you don't claim your library to be thread-safe, you don't have to bother about this.
(But even) if you do:
In your example the user would have to take care about thread-safety, since both functions could be dangerous, even without using your event-system (IMHO, this is a pretty good way to determine who should take care about those kind of problems). A possible way for him to do this in C++11 could be:
#include <mutex>
// A mutex is used to control thread-acess to a shared resource
std::mutex _somePtr_mutex;
int* somePtr = nullptr;
void someFunction()
{
/*
Create a 'lock_guard' to manage your mutex.
Is the mutex '_somePtr_mutex' already locked?
Yes: Wait until it's unlocked.
No: Lock it and continue execution.
*/
std::lock_guard<std::mutex> lock(_somePtr_mutex);
if(somePtr)
*somePtr = 17;
// End of scope: 'lock' gets destroyed and hence unlocks '_somePtr_mutex'
}
void cleanupPtr()
{
/*
Create a 'lock_guard' to manage your mutex.
Is the mutex '_somePtr_mutex' already locked?
Yes: Wait until it's unlocked.
No: Lock it and continue execution.
*/
std::lock_guard<std::mutex> lock(_somePtr_mutex);
int *tmp = somePtr;
somePtr = null;
delete tmp;
// End of scope: 'lock' gets destroyed and hence unlocks '_somePtr_mutex'
}
The last question is easy. If you say your library is threadsafe, it should threadsafe. It makes no sense to say it is partly threadsafe or, it is only threadsafe if you do not abuse it. In that case you have to explain what exactly is not threadsafe.
Now to your first question regarded someFunction:
The operation is non atomic. Which means the CPU can interrupt between the if and the assigment. And that will happen, I know that :-) The other thread can erase the pointer anytime. Even between two short and fast looking statements.
Now to cleanupPtr:
I am not a compiler expert, but if you want to be shure that your assigment take place in the same moment you wrote it in code you should write the keyword volatile in front of the declaration of somePtr. The compiler will now know that you use that attribute in a multithreaded situation and will not buffer the value in a register of the CPU.
If you have a thread situation with a reader thread and a writer thread, the keyword volatile can (IMHO) be enough to sync them. As long as the attributes you use to exchange information between threads are generic.
For other situations you can use mutex or atomics. I will give you an example for mutex. I use C++11 for that, but it works similar with previous versions of C++ using boost.
Using mutex:
int * somePtr = nullptr;
Emitter<Event> em; // just an object that can emit the 'Event' signal
std::recursive_mutex g_mutex;
void mainThread()
{
em.connect<Event>(someFunction);
// now, somehow, 2 threads are created which, at some point
// execute the thread1() and thread2() functions below
}
void someFunction()
{
std::lock_guard<std::recursive_mutex> lock(g_mutex);
// can somePtr change after the check but before the set?
if (somePtr)
*somePtr = 17;
}
void cleanupPtr()
{
std::lock_guard<std::recursive_mutex> lock(g_mutex);
// this looks safe, but compilers and CPUs can reorder this code:
int *tmp = somePtr;
somePtr = null;
delete tmp;
}
void thread1()
{
em.emit<Event>();
}
void thread2()
{
em.disconnect<Event>(someFunction);
// now safe to cleanup (?)
cleanupPtr();
}
I only added a recursive mutex here without changing any other code of the sample, even if it's now cargo code.
There are two kinds of mutex in the std. A utterly useless std::mutex and the std::recursive_mutex which work like you expect a mutex should work. The std::mutex exclude the access of any further call even from the same thread. Which can happen if a method which needs mutex protection calls a public method which use the same mutex. std::recursive_mutex is reentrant for the same thread.
Atomics (or interlocks in win32) are another way, but only to exchange values between threads or access them concurrently. Your example is missing such values, but in your case, I would look a little deeper in them (std::atomic).
UPDATE
If your are the user of a library which is not explicit declared as threadsafe by the developer, take it as non threadsafe and shield every call to it with a mutex lock.
To stick with the example. If you cannot change someFunction the you have to wrap the function like:
void threadsafeSomeFunction()
{
std::lock_guard<std::recursive_mutex> lock(g_mutex);
someFunction();
}
I want to pass some data around threads but want to refrain from using global variables if I can manage it. The way I wrote my thread routine has the user passing in a separate function for each "phase" of a thread's life cycle: For instance this would be a typical usage of spawning a thread:
void init_thread(void *arg) {
graphics_init();
}
void process_msg_thread(message *msg, void *arg) {
if (msg->ID == MESSAGE_DRAW) {
graphics_draw();
}
}
void cleanup_thread(void *arg) {
graphics_cleanup();
}
int main () {
threadCreator factory;
factory.createThread(init_thread, 0, process_msg_thread, 0, cleanup_thread, 0);
// even indexed arguments are the args to be passed into their respective functions
// this is why each of those functions must have a fixed function signature is so they can be passed in this way to the factory
}
// Behind the scenes: in the newly spawned thread, the first argument given to
// createThread() is called, then a message pumping loop which will call the third
// argument is entered. Upon receiving a special exit message via another function
// of threadCreator, the fifth argument is called.
The most straightforward way to do it is using globals. I'd like to avoid doing that though because it is bad programming practice because it generates clutter.
A certain problem arises when I try to refine my example slightly:
void init_thread(void *arg) {
GLuint tex_handle[50]; // suppose I've got 50 textures to deal with.
graphics_init(&tex_handle); // fill up the array with them during graphics init which loads my textures
}
void process_msg_thread(message *msg, void *arg) {
if (msg->ID == MESSAGE_DRAW) { // this message indicates which texture my thread was told to draw
graphics_draw_this_texture(tex_handle[msg->texturehandleindex]); // send back the handle so it knows what to draw
}
}
void cleanup_thread(void *arg) {
graphics_cleanup();
}
I am greatly simplifying the interaction with the graphics system here but you get the point. In this example code tex_handle is an automatic variable, and all its values are lost when init_thread completes, so will not be available when process_msg_thread needs to reference it.
I can fix this by using globals but that means I can't have (for instance) two of these threads simultaneously since they would trample on each other's texture handle list since they use the same one.
I can use thread-local globals but is that a good idea?
I came up with one last idea. I can allocate storage on the heap in my parent thread, and send a pointer to in to the children to mess with. So I can just free it when parent thread leaves away since I intend for it to clean up its children threads before it exits anyway. So, something like this:
void init_thread(void *arg) {
GLuint *tex_handle = (GLuint*)arg; // my storage space passed as arg
graphics_init(tex_handle);
}
void process_msg_thread(message *msg, void *arg) {
GLuint *tex_handle = (GLuint*)arg; // same thing here
if (msg->ID == MESSAGE_DRAW) {
graphics_draw_this_texture(tex_handle[msg->texturehandleindex]);
}
}
int main () {
threadCreator factory;
GLuint *tex_handle = new GLuint[50];
factory.createThread(init_thread, tex_handle, process_msg_thread, tex_handle, cleanup_thread, 0);
// do stuff, wait etc
...
delete[] tex_handle;
}
This looks more or less safe because my values go on the heap, my main thread allocates it then lets children mess with it as they wish. The children can use the storage freely since the pointer was given to all the functions that need access.
So this got me thinking why not just have it be an automatic variable:
int main () {
threadCreator factory;
GLuint tex_handle[50];
factory.createThread(init_thread, &tex_handle, process_msg_thread, &tex_handle, cleanup_thread, 0);
// do stuff, wait etc
...
} // tex_handle automatically cleaned up at this point
This means children thread directly access parent's stack. I wonder if this is kosher.
I found this on the internets: http://software.intel.com/sites/products/documentation/hpc/inspectorxe/en-us/win/ug_docs/olh/common/Problem_Type__Potential_Privacy_Infringement.htm
it seems Intel Inspector XE detects this behavior. So maybe I shouldn't do it? Is it just simply a warning of potential privacy infringement as suggested by the the URL or are there other potential issues that may arise that I am not aware of?
P.S. After thinking through all this I realize that maybe this architecture of splitting a thread into a bunch of functions that get called independently wasn't such a great idea. My intention was to remove the complexity of requiring coding up a message handling loop for each thread that gets spawned. I had anticipated possible problems, and if I had a generalized thread implementation that always checked for messages (like my custom one that specifies the thread is to be terminated) then I could guarantee that some future user could not accidentally forget to check for that condition in each and every message loop of theirs.
The problem with my solution to that is that those individual functions are now separate and cannot communicate with each other. They may do so only via globals and thread local globals. I guess thread local globals may be my best option.
P.P.S. This got me thinking about RAII and how the concept of the thread at least as I have ended up representing it has a certain similarity with that of a resource. Maybe I could build an object that represents a thread more naturally than traditional ways... somehow. I think I will go sleep on it.
Put your thread functions into a class. Then they can communicate using instance variables. This requires your thread factory to be changed, but is the cleanest way to solve your problem.
Your idea of using automatic variables will work too as long as you can guarantee that the function whose stack frame contains the data will never return before your child threads exit. This is not really easy to achieve, even after main() returns child threads can still run.
My code have one thread continuosly handling objects queued by other threads. Queued objects are created using "new" in a function that will have finished when the object will be handled. I have no problem with this but deleting the object.
Should I just not delete the object? Maybe change the way of passing/creating this objects?
Object* myQueue[10];
function() {
Object* myobject = new Object();
queueObject(myobject);
}
queueObject(Object* object) {
myQueue[index_x] = object;
sem_post(&mySemaphore);
}
//// Thread 1
function();
...
//// Thread 2
handleObjects() {
while(true) {
sem_wait(&mySemaphore);
// handle myQueue[index_x]
delete myQueue[index_x]; ---> this produces Segmentation Fault
}
}
(the treatment of index_x is not posted to abbreviate)
I'm guessing you have a race condition. What is the synchronization mechanism you're using to prevent index_x from being modified by both threads?
Typically a worker thread should call sem_wait, modify the critical data, and then call sem_post. I can't provide 100% accurate example code without seeing how you're using index_x, but it will look something like the following:
queueObject(Object* object) {
sem_wait(&mySemaphore);
myQueue[index_x++] = object;
sem_post(&mySemaphore);
}
handleObjects() {
while(true) {
sem_wait(&mySemaphore);
// handle myQueue[index_x]
delete myQueue[--index_x]
sem_post(&mySemaphore);
}
}
Currently it looks like you have nothing to prevent index_x from being modified by both threads, this can cause index_x to do whacky things (fail to increment or decrement being the most common whacky thing). Here is a wikipedia article explaining exactly what can go wrong.
Add some checks around the delete
if ( myQueue[index] != 0 ) {
delete myQueue[index];
myQueue[index] = 0;
} else {
for diagnosis print large warning here - something is confused
}
this catches double deletion via the same index. However there are several other ways a crash could occur. Catching those would need other actions.
Consider:
Is there any possibility of a race condition? Could two threads attempt to delete at the same index? Do you need to add any synchronization?
Is it possible for the same object to be added to the array twice, with different indexes? In extremis I might add code to verify that the item isn't already in the array before adding it.
Things seem to be working but I'm unsure if this is the best way to go about it.
Basically I have an object which does asynchronous retrieval of data. This object has a vector of pointers which are allocated and de-allocated on the main thread. Using boost functions a process results callback is bound with one of the pointers in this vector. When it fires it will be running on some arbitrary thread and modify the data of the pointer.
Now I have critical sections around the parts that are pushing into the vector and erasing in case the asynch retrieval object is receives more requests but I'm wondering if I need some kind of guard in the callback that is modifying the pointer data as well.
Hopefully this slimmed down pseudo code makes things more clear:
class CAsyncRetriever
{
// typedefs of boost functions
class DataObject
{
// methods and members
};
public:
// Start single asynch retrieve with completion callback
void Start(SomeArgs)
{
SetupRetrieve(SomeArgs);
LaunchRetrieves();
}
protected:
void SetupRetrieve(SomeArgs)
{
// ...
{ // scope for data lock
boost::lock_guard<boost::mutex> lock(m_dataMutex);
m_inProgress.push_back(SmartPtr<DataObject>(new DataObject)));
m_callback = boost::bind(&CAsyncRetriever::ProcessResults, this, _1, m_inProgress.back());
}
// ...
}
void ProcessResults(DataObject* data)
{
// CALLED ON ANOTHER THREAD ... IS THIS SAFE?
data->m_SomeMember.SomeMethod();
data->m_SomeOtherMember = SomeStuff;
}
void Cleanup()
{
// ...
{ // scope for data lock
boost::lock_guard<boost::mutex> lock(m_dataMutex);
while(!m_inProgress.empty() && m_inProgress.front()->IsComplete())
m_inProgress.erase(m_inProgress.begin());
}
// ...
}
private:
std::vector<SmartPtr<DataObject>> m_inProgress;
boost::mutex m_dataMutex;
// other members
};
Edit: This is the actual code for the ProccessResults callback (plus comments for your benefit)
void ProcessResults(CRetrieveResults* pRetrieveResults, CRetData* data)
{
// pRetrieveResults is delayed binding that server passes in when invoking callback in thread pool
// data is raw pointer to ref counted object in vector of main thread (the DataObject* in question)
// if there was an error set the code on the atomic int in object
data->m_nErrorCode.Store_Release(pRetrieveResults->GetErrorCode());
// generic iterator of results bindings for generic sotrage class item
TPackedDataIterator<GenItem::CBind> dataItr(&pRetrieveResults->m_DataIter);
// namespace function which will iterate results and initialize generic storage
GenericStorage::InitializeItems<GenItem>(&data->m_items, dataItr, pRetrieveResults->m_nTotalResultsFound); // this is potentially time consuming depending on the amount of results and amount of columns that were bound in storage class definition (i.e.about 8 seconds for a million equipment items in release)
// atomic uint32_t that is incremented when kicking off async retrieve
m_nStarted.Decrement(); // this one is done processing
// boost function completion callback bound to interface that requested results
data->m_complete(data->m_items);
}
As it stands, it appears that the Cleanup code can destroy an object for which a callback to ProcessResults is in flight. That's going to cause problems when you deref the pointer in the callback.
My suggestion would be that you extend the semantics of your m_dataMutex to encompass the callback, though if the callback is long-running, or can happen inline within SetupRetrieve (sometimes this does happen - though here you state the callback is on a different thread, in which case you are OK) then things are more complex. Currently m_dataMutex is a bit confused about whether it controls access to the vector, or its contents, or both. With its scope clarified, ProcessResults could then be enhanced to verify validity of the payload within the lock.
No, it isn't safe.
ProcessResults operates on the data structure passed to it through DataObject. It indicates that you have shared state between different threads, and if both threads operate on the data structure concurrently you might have some trouble coming your way.
Updating a pointer should be an atomic operation, but you can use InterlockedExchangePointer (in Windows) to be sure. Not sure what the Linux equivalent would be.
The only consideration then would be if one thread is using an obsolete pointer. Does the other thread delete the object pointed to by the original pointer? If so, you have a definite problem.
I have a multithreaded application that runs using a custom thread pool class. The threads all execute the same function, with different parameters.
These parameters are given to the threadpool class the following way:
// jobParams is a struct of int, double, etc...
jobParams* params = new jobParams;
params.value1 = 2;
params.value2 = 3;
int jobId = 0;
threadPool.addJob(jobId, params);
As soon as a thread has nothing to do, it gets the next parameters and runs the job function. I decided to take care of the deletion of the parameters in the threadpool class:
ThreadPool::~ThreadPool() {
for (int i = 0; i < this->jobs.size(); ++i) {
delete this->jobs[i].params;
}
}
However, when doing so, I sometimes get a heap corruption error:
Invalid Address specified to RtlFreeHeap
The strange thing is that in one case it works perfectly, but in another program it crashes with this error. I tried deleting the pointer at other places: in the thread after the execution of the job function (I get the same heap corruption error) or at the end of the job function itself (no error in this case).
I don't understand how deleting the same pointers (I checked, the addresses are the same) from different places changes anything. Does this have anything to do with the fact that it's multithreaded?
I do have a critical section that handles the access to the parameters. I don't think the problem is about synchronized access. Anyway, the destructor is called only once all threads are done, and I don't delete any pointer anywhere else. Can pointer be deleted automatically?
As for my code. The list of jobs is a queue of a structure, composed of the id of a job (used to be able to get the output of a specific job later) and the parameters.
getNextJob() is called by the threads (they have a pointer to the ThreadPool) each time they finished to execute their last job.
void ThreadPool::addJob(int jobId, void* params) {
jobData job; // jobData is a simple struct { int, void* }
job.ID = jobId;
job.params = params;
// insert parameters in the list
this->jobs.push(job);
}
jobData* ThreadPool::getNextJob() {
// get the data of the next job
jobData* job = NULL;
// we don't want to start a same job twice,
// so we make sure that we are only one at a time in this part
WaitForSingleObject(this->mutex, INFINITE);
if (!this->jobs.empty())
{
job = &(this->jobs.front());
this->jobs.pop();
}
// we're done with the exclusive part !
ReleaseMutex(this->mutex);
return job;
}
Let's turn this on its head: Why are you using pointers at all?
class Params
{
int value1, value2; // etc...
}
class ThreadJob
{
int jobID; // or whatever...
Params params;
}
class ThreadPool
{
std::list<ThreadJob> jobs;
void addJob(int job, const Params & p)
{
ThreadJob j(job, p);
jobs.push_back(j);
}
}
No new, delete or pointers... Obviously some of the implementation details may be cocked, but you get the overall picture.
Thanks for extra code. Now we can see a problem -
in getNextJob
if (!this->jobs.empty())
{
job = &(this->jobs.front());
this->jobs.pop();
After the "pop", the memory pointed to by 'job' is undefined. Don't use a reference, copy the actual data!
Try something like this (it's still generic, because JobData is generic):
jobData ThreadPool::getNextJob() // get the data of the next job
{
jobData job;
WaitForSingleObject(this->mutex, INFINITE);
if (!this->jobs.empty())
{
job = (this->jobs.front());
this->jobs.pop();
}
// we're done with the exclusive part !
ReleaseMutex(this->mutex);
return job;
}
Also, while you're adding jobs to the queue you must ALSO lock the mutex, to prevent list corruption. AFAIK std::lists are NOT inherently thread-safe...?
Using operator delete on pointer to void results in undefined behavior according to the specification.
Chapter 5.3.5 of the draft of the C++ specification. Paragraph 3.
In the first alternative (delete object), if the static type of the operand is different from its dynamic type, the static type shall be a base class of the operand’s dynamic type and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.73)
And corresponding footnote.
This implies that an object cannot be deleted using a pointer of type void* because there are no objects of type void
All access to the job queue must be synchronized, i.e. performed only from 1 thread at a time by locking the job queue prior to access. Do you already have a critical section or some similar pattern to guard the shared resource? Synchronization issues often lead to weird behaviour and bugs which are hard to reproduce.
It's hard to give a definitive answer with this amount of code. But generally speaking, multithreaded programming is all about synchronizing access to data that might be accessed from multiple threads. If there is no long or other synchronization primitive protecting access to the threadpool class itself, then you can potentially have multiple threads reaching your deletion loop at the same time, at which point you're pretty much guaranteed to be double-freeing memory.
The reason you're getting no crash when you delete a job's params at the end of the job function might be because access to a single job's params is already implicitly serialized by your work queue. Or you might just be getting lucky. In either case, it's best to think about locks and synchronization primitive as not being something that protects code, but as being something that protects data (I've always thought the term "critical section" was a bit misleading here, as it tends to lead people to think of a 'section of lines of code' rather than in terms of data access).. In this case, since you want to access your jobs data from multiple thread, you need to be protecting it via a lock or some other synchronization primitive.
If you try to delete an object twice, the second time will fail, because the heap is already freed. This is the normal behavior.
Now, since you are in a multithreading context... it might be that the deletions are done "almost" in parallel, which might avoid the error on the second deletion, because the first one is not yet finalized.
Use smart pointers or other RAII to handle your memory.
If you have access to boost or tr1 lib you can do something like this.
class ThreadPool
{
typedef pair<int, function<void (void)> > Job;
list< Job > jobList;
HANDLE mutex;
public:
void addJob(int jobid, const function<void (void)>& job) {
jobList.push_back( make_pair(jobid, job) );
}
Job getNextJob() {
struct MutexLocker {
HANDLE& mutex;
MutexLocker(HANDLE& mutex) : mutex(mutex){
WaitForSingleObject(mutex, INFINITE);
}
~MutexLocker() {
ReleaseMutex(mutex);
}
};
Job job = make_pair(-1, function<void (void)>());
const MutexLocker locker(this->mutex);
if (!this->jobList.empty()) {
job = this->jobList.front();
this->jobList.pop();
}
return job;
}
};
void workWithDouble( double value );
void workWithInt( int value );
void workWithValues( int, double);
void test() {
ThreadPool pool;
//...
pool.addJob( 0, bind(&workWithDouble, 0.1));
pool.addJob( 1, bind(&workWithInt, 1));
pool.addJob( 2, bind(&workWithValues, 1, 0.1));
}