I am new to openMP and mutli-threading. I need to do some summation work and I know that when writing to the shared variable, it need to use lock like omp_lock_t. But when I do so, the result still goes wrong.

The code is:

#include <omp.h>

#include <cstdio>
struct simu
{
public:
    simu() : data{ nullptr }
    {
        omp_init_lock(&lock);
    }
    ~simu()
    {
        omp_destroy_lock(&lock);
    }
    void calcluate()
    {
        omp_set_lock(&lock);
        (*data)  = 1;
        omp_unset_lock(&lock);
    }
public:
    omp_lock_t lock;
    int *data;
};

int main()
{
    printf("thread_num = %d\n", omp_get_num_procs());

    const int size = 2000;
    int a = 1;
    int b = 2;

    simu s[size];
    simu *ps[size];
    for (int i = 0; i < size;   i)
    {
        s[i].data = (0 == i % 2) ? &a : &b;
        ps[i]     = &s[i];
    }

    for (int k = 0; k < size;   k)
    {
        ps[k]->calcluate();
    }

    printf("a = %d, b = %d\n", a, b);

    a = 1;
    b = 2;

    #pragma omp parallel for default(shared) num_threads(4)
    for (int k = 0; k < size;   k)
    {
        ps[k]->calcluate();
    }

    printf("a = %d, b = %d\n", a, b);

    return 0;
}

And the result is

thread_num = 8
a = 1001, b = 1002
a = 676, b = 679

I run this code on Win10. Can anyone explain why the result is wrong?

CodePudding user response：

A lock protects the actual data item from simultaneous writes. Your lock is in the object that points at the item, so this is pointless. You need to let you data point to an object that contains a lock.