Why am I getting a Seg Fault when creating threads? - c++

Question

I am confused as to why I am getting a segmentation fault when creating and firing off threads here. It happens in the t[j] = thread(getMax, A); line and I am very confused as to why this is happening. threadMax[] is the max of each thread. getMax() returns the maximum value of an array.
#include <iostream>
#include <stdlib.h>
#include <sys/time.h>
#include <thread>
#define size 10
#define numThreads 10
using namespace std;
int threadMax[numThreads] = {0};
int num =0;
void getMax(double *A){
num += 1;
double max = A[0];
double min = A[0];
for (int i =0; i<size; i++){
if(A[i] > max){
max = A[i];
}
}
threadMax[num] = max;
}
int main(){
int max =0;
double S,E;
double *A = new double[size];
srand(time(NULL));
thread t[numThreads];
//Assign random values to array
for(int i = 0; i<size; i++){
A[i] = (double(rand()%100));
}
//create Threads
for(int j =0; j <numThreads; j++){
cout << A[j] << " " << j << "\n";
t[j] = thread(getMax, A);
}
//join threads
for(int i =0; i< numThreads; i++){
t[i].join();
}
//Find Max from all threads
for(int i =0; i < numThreads; i++){
if(threadMax[i] > max){
max = threadMax[i];
}
}
cout <<max;
delete [] A;
return 0;
}

Answer 1

The behavior of this code is undefined:
void getMax(double *A){
num += 1;
double max = A[0];
double min = A[0];
for (int i =0; i<size; i++){
if(A[i] > max){
max = A[i];
}
}
threadMax[num] = max;
}
The num += 1 can allow multiple threads to attempt to modify num at the same time. Worse, when num is read in the threadMax[num] = max;, threads may see values of num modified by other threads while they were running.
You need to assign each thread a number in some safe way.
Here are three ways it can fail:
Two threads do num += 1; at exactly the same time and as a result, num only increments once.
Every thread does num += 1; before any thread does threadMax[num] = max;. All threads overwrite the same entry in the array. (Which, actually, is out of bounds!)
The code crashes because its behavior is undefined.

Answer 2

As others have stated, your num variable is not protected from race conditions inside of getMax(), which can lead to it being corrupted, thus causing getMax() to access the threadMax[] array out of bounds.
You can avoid that by simply getting rid of that num variable altogether and pass the array index as an input parameter to std::thread instead.
Try something more like this:
#include <iostream>
#include <vector>
#include <array>
#include <thread>
#include <algorithm>
#include <cstdlib>
#include <ctime>
using namespace std;
const size_t size = 10;
const size_t numThreads = 10;
double threadMax[numThreads] = {};
void getMax(int idx, double *A){
threadMax[idx] = *max_element(A, A + size);
}
int main(){
srand(time(nullptr));
vector<double> A(size);
array<thread, numThreads> t;
//Assign random values to array
generate_n(A.begin(), size, [](){ return double(rand() % 100); });
/* or:
for(double &d : A){
d = double(rand() % 100);
}
*/
//create Threads
for(int j = 0; j < numThreads; ++j){
cout << A[j] << " " << j << "\n";
t[j] = thread(getMax, j, A.data());
}
//join threads
for(thread &thd : t){
thd.join();
}
//Find Max from all threads
double max = *max_element(threadMax.begin(), threadMax.end());
cout << max;
return 0;
}