CODEWITHSUNDEEP
c++multithreadingvisual-studio-2010visual-c++
Sumanth
Sumanth

Reputation: 605

Multithreading on Class object

I am getting unhandled exception and access violation reading location error. But sometimes it is executing perfectly. Am i passing vector and vector iterators properly? What might be the possible cause of error. 

struct DataStructure
{
    MapSmoother *m1;
    std::vector<Vertex *> v1;
    std::vector<Vertex *>::iterator vit_f;
    std::vector<Vertex *>::iterator vit_e;
    DataStructure() {
        m1 = NULL;
        v1;
        vit_f;
        vit_e;
    }
};
DWORD WINAPI thread_fun(void* p)
{
        DataStructure *input = (DataStructure*)p;
        while ( input->vit_f != input->vit_e ) {
            Vertex *v = *((input->vit_f)++);
            (*(input->m1)).relax(v);
        }
        return 0;
}
int main()
{
    //Read and encode color to Mesh
    // msmoother is an object
    DataStructure* input = new DataStructure();
    input->m1 = &msmoother;
    for(int i = 0; i < 7; ++i) {
        for(int color = 1; color <= k; color++) {
            std::vector<Vertex *> verList;
          //all the vertices with the same color index will be stored in verList vector
            input->v1 = verList;    //Copied to structure
            std::vector<Vertex *>::iterator vit1 = verList.begin();
            std::vector<Vertex *>::iterator vit2 = verList.end();
            input->vit_f = vit1;
            input->vit_e = vit2;
            HANDLE hThread[50];
            cout << "     Processing for color: " << color << endl;
            for(int j = 0; j < 50; ++j){
                hThread[j] = CreateThread(0,0,(LPTHREAD_START_ROUTINE)&thread_fun,input,0,NULL);
            }
            WaitForMultipleObjects(THREAD_COUNT,hThread,TRUE,INFINITE);
            //clear verList vector
            //Close Handles to all threads
        }
    }
}

Error screenshot

Upvotes: 1

Views: 134

Answers (2)

WhozCraig
WhozCraig

Reputation: 66254

No, you're not passing them in correctly.

This code: 

input->v1 = verList;    //Copied to structure

Makes a copy of the vector, which is intentional by the looks of it. Since the vector contains mere pointers to actual data, you're just making a copy of a bunch of pointers. However, the followup code:

std::vector<Vertex *>::iterator vit1 = verList.begin();
std::vector<Vertex *>::iterator vit2 = verList.end();
input->vit_f = vit1;
input->vit_e = vit2;

is where the problem lays. The iterators you're setting in your input object are, in fact, from the local verList; not from your input->v1 list.

Try:

input->vit_f = input->v1.begin();
input->vit_e = input->v1.end();

Addendum

Updated for OP to avoid race condition and uneeded vector copy.

First, redefine DataStructure as:

struct DataStructure
{
    MapSmoother *m1;
    std::vector<Vertex *>::iterator first, last
    DataStructure() : m1(NULL) {}
};

Next, define your thread function as:

DWORD WINAPI thread_fun(void* p)
{
    DataStructure *input = static_cast<DataStructure*>(p);
    if (input && input->m1)
    {
        for (std::vector<Vertex *>::iterator it = input->first
             it != input->last; ++it) 
        {
             input->m1->relax(*it);
        }
    }
    delete input;
    return 0;
}

And finally, invoke this as: (inside your loops)

// all the vertices with the same color index will be stored in verList vector
std::vector<Vertex *> verList;
// ... store vector data ...

cout << "     Processing for color: " << color << endl;
HANDLE hThread[50] = {NULL};
for(int j = 0; j < _countof(hThread); ++j)
{
    DataStructure *input= new DataStructure;
    input->first = verList.begin();
    input->last = verList.end();
    input->m1 = &msmoother
    hThread[j] = CreateThread(0,0,(LPTHREAD_START_ROUTINE)&thread_fun,input,0,NULL);
}
WaitForMultipleObjects(THREAD_COUNT,hThread,TRUE,INFINITE);

Or something like that. I just hammered this online so I've no idea if it even compiles, but should hopefully give a decent enough idea to you. Every thread gets its own pair of iterators into the verList array, and as such concurrent access. note: they cannot modify the vector; only use it. The structure they get the iterators in is allocated by the creator-thread, but owned by the thread itself, which is ultimately responsible for destroying it.

Upvotes: 2

dreamlax
dreamlax

Reputation: 95385

I'm a C++ noob, so I may be wrong but I think the problem may be here:

std::vector<Vertex *>::iterator vit1 = verList.begin();
std::vector<Vertex *>::iterator vit2 = verList.end();
input->vit_f = vit1;
input->vit_e = vit2;

You are passing around iterators to verList rather than to the vector that has been copied to the DataStructure. I think what you want here is:

input->v1 = verList; // copy to structure
input->vit_f = input->v1.begin();
input->vit_e = input->v1.end();

Upvotes: 2

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