Effective wordcount multithreading in C++

Question

I am fairly new to C++ and multi-threading and need some help with creating a word count that effectively divides work between multiple threads.

Suppose, I have a function that counts words in a line (string):

count_words_in_line(line);

For one thread, the total words in line is the simple sum of this function output for each line, but how do I divide that into threads?

My idea was to use two threads - one to count even and one to count odd lines, but the code results in a Segmentation Fault.

What am I doing wrong? Is there a better approach?

I would like to not use a threadpool, and would ideally like to specify the number of threads in an argument in order to measure the performance of the multithreaded implementation.

Here is my relevant code:

bool odd = true;
auto thread_count_odd = [&counter, &infile, &odd, &line, &mutex]() {
    while (std::getline(infile, line)) {
        if (odd) {
            std::cout<<"Count odd"<<std::endl;
            mutex.lock();
            counter += count_words_in_line(line);
            mutex.unlock();
        }
        odd = !odd;
    }
};

bool even = false;
auto thread_count_even = [&counter, &infile, &even, &line, &mutex]() {

    while (std::getline(infile, line)) {
        if (even) {
            std::cout<<"Count even"<<std::endl;
            mutex.lock();
            counter += count_words_in_line(line);
            mutex.unlock();
        }
        even = !even;
    }
};

std::thread t1(thread_count_odd);
std::thread t2(thread_count_even);

t1.join();
t2.join();

Answer 1

I think the issue is that you must have a mutex around getline call. Both threads are accessing infile at the same time which might be causing issues.

I have this code which would work for your situation using conditional variables. Hope this helps

`

#include<iostream>
#include<thread>
#include<string>
#include<mutex>
#include<condition_variable>
#include<unistd.h>
#include <fstream>
#define MAX_THREADS 50
using namespace std;
thread *threads = new thread[MAX_THREADS];

condition_variable cv[MAX_THREADS];
mutex m1;
int counter=0;
int count_words_in_line(string line){
        /*write your code here*/
        return 1;
}

void printString(int tid, ifstream &inFile, int tcount)
{
    unique_lock<mutex> lock(m1);
    while(1)
    {
        string line;
        inFile >> line;
        string a = "";
        if(line==a)break;
        cv[(tid+1)%tcount].notify_one();
        cv[tid].wait(lock);
        counter += count_words_in_line(line);
    }
    cv[(tid+1)%tcount].notify_one();
}

int main(int argc, char** argv)
{
    int tcount, ccount, k;
    std::ifstream inFile;
    string name;
    inFile.open("input.txt");
    string str;
    tcount = 2;

    for(int i = 0; i < tcount; i++) {
        threads[i] = thread(printString, i, ref(inFile), tcount);
    }

    for (int i = 0; i < tcount; i++)
        threads[i].join();

    cout << counter << endl;
    return 0;
}

`