为什么在使用pthread_join时出现分段错误？

Question

Here is the code I have, it compiles and runs using g++ but I get a segmentation fault. I know it happens around the pthread_join statement but I cant figure out why.

#include <iostream>
#include <stdio.h>
#include <fstream>
#include <pthread.h>
#include <stdlib.h>
#include <sstream>

using namespace std;

struct data{
    string filename;
    int x;
    int y;
};

void *threadFunction(void *input){
    data *file = (data *) input;
    string filename = file->filename;
    ifstream myFile;
    int xCount = 0;
    int yCount = 0;
    myFile.open(filename.c_str());
    string line;
    while(myFile >> line){
        if(line == "X"){
            xCount++;
        }else if(line == "Y"){
            yCount++;
        }
    }
    file->x = xCount;
    file->y = yCount;
    return (void *) file;
}

int main(){
    pthread_t myThreads[20];
    data *myData = new data[20];

    for(int i = 0; i < 20; i++){
        ostringstream names;
        names << "/filepath/input" << i+1 << ".txt";
        myData[i].filename = names.str();
        myData[i].x = 0;
        myData[i].y = 0;
    }
    for(int i = 0; i < 20; i++){
        int check = pthread_create(&myThreads[i], NULL, threadFunction, (void *) &myData[i]);
        if(check != 0){
            cout << "Error Creating Thread\n";
            exit(-1);
        }
    }



    int xCount = 0;
    int yCount = 0;

    for(int i = 0; i < 20; i++){
        data* returnedItem;
        pthread_join(myThreads[i], (void**) returnedItem);
        xCount += returnedItem->x;
        yCount += returnedItem->y;
    }

    cout << "Total X: " << xCount << "\n";
    cout << "Total Y: " << yCount << "\n";

}

Am I not calling return properly from my threadFunction? I've been trying a bunch of different things and I still don't know what's going on...any help would be greatly appreciated! (the text file I open contain either an X or Y per line. My goal is to count the total number of Xs and Ys in 20 text files)

Answer 1

pthread_join(myThreads[i], (void**) returnedItem);

should be

pthread_join(myThreads[i], (void**) &returnedItem);

You're asking join to set the value of returnedItem to be whatever void* your thread function returned ... so you need to give the address of returnedItem .

Answer 2

The second argument to pthread_join() is a void** into which the result is to be stored. You are passing a random value, however. This could should rather look something like this:

void* result;
pthread_join(myThread[i], &result);
data* returnedItem = static_cast<data*>(result);

Of course, this assumes that a data* was, indeed, returned.

Answer 3

Second argument of pthread_join will be used to return, return value of the thread, so somewhere inside pthread_join we have a code that call *secondArgument = thread_return_value , but lets look at what you are doing here:

// You are not initializing returnedItem, so it contain some garbage value
// For example 0x12345678
data* returnedItem;
// Now you cast that garbage to (void**), and pthread_join will call
// *(0x12345678) = thread_return_value that will cause segmentation fault
pthread_join(myThreads[i], (void**) returnedItem);

But you want return value to be copied to returnedItem , am I right? in case that you answer is yes you should pass address of returnedItem to pthread_join so it can copy it there. So change your call to:

pthread_join(myThreads[i], (void**) &returnedItem);