多线程分段错误（用于循环）

Question

Why does the following code give me a segmentation fault:

#include<iostream>
#include<thread>
#include<vector>

using namespace std;


double f(double a, double b, double* c) {
    *c = a + b;
}

int main() {
   vector<double> a ={1,2,3,4,5,6,7,8};                     
   vector<double> b ={2,1,3,4,5,2,8,2};                    
   int size = a.size();
   vector<double> c(size);                               
   vector<thread*> threads(size);

   for (int i = 0; i < size; ++i) {
        thread* t = new thread(f, a[i], b[i], &c[i]);             
        threads.push_back(t);
   }

   for (vector<thread*>::iterator it = threads.begin(); it != threads.end();     it++) {
       (*it)->join();                                      
   }

   cout << "Vector c is: ";
   for (int i =0; i < size; ++i) {
       cout << c[i] << " ";                                 
   }
}

I know that the segmentation fault happens inside the for loop where the iterator is used, but I'm not sure why.

Answer 1

vector<thread*> threads(size);

Declaration creates a vector with size amount of default-initialized thread* objects which are nullptr .

Then with push_back you insert additional non-null objects but the null ones remain there, and you dereference them when iterating over vector at the end.

Answer 2

You should change the for loop to read as below:

for (int i = 0; i < size; ++i) {
  thread *t = new thread(f, a[i], b[i], &c[i]);
  threads[i] = t;
}

And before ending, you should delete your heap-allocated thread s.

for (auto thread : threads)
  delete thread;

Even better is to simply use:

vector<thread> threads(size);

for (int i = 0; i < size; ++i)
  threads[i] = thread(f, a[i], b[i], &c[i]);

for (auto& thread : threads)
  thread.join();

By the way, you should pay attention to the compiler warnings. Change

double f(double a, double b, double *c) { *c = a + b; }

to

void f(double a, double b, double *c) { *c = a + b; }