c++ pthread_create returning value

Question

I am trying out multithreading of inorder traversal of binary trees. I am getting errors due to pthread_create.

    #include <iostream>
    #include <pthread.h>
    #include <list>

    using namespace std;


    static pthread_t threads[9];
    static int count=0;


    struct node
    {
    int value=0;
    node *left=NULL;
    node *right=NULL;
    };

    list<int> inordertraversal(node* n)
    {

    list<int> l1,l2;

    if(n->left!=NULL)
    list<int> l1=pthread_create(&threads[count++],NULL,inordertraversal,n->left);

    if(n->right!=NULL)
    list<int> l2=pthread_create(&threads[count++],NULL,inordertraversal,n->right);

    list<int> l;
    l.insert(l.begin(),l1.begin(),l1.end());
    l.push_back(n->value);
    l.insert(l.end()--,l2.begin(),l2.end());

    return l;

    }


    struct node* newNode(int data)
    {

    node* node;
    node->value=data;
    node->left=NULL;
    node->right=NULL;
    return node;

    }




    int main()
    {

     struct node *root=newNode(7);
     root->left=newNode(9);
     root->right=newNode(5);
     root->left->left=newNode(13);
     root->left->right=newNode(17);
     root->right->left=newNode(56);
     root->right->right=newNode(21);
     root->left->left->left=newNode(45);
     root->right->left->right=newNode(45);
     root->left->left->right=newNode(67);

    list<int> l=inordertraversal(root);

    for(list<int>::iterator it=l.begin();it!=l.end();it++)
    {
    cout<<*it<<" ";
    }


    }

I would like to return list elements from the function passed on to the thread using pthread_create. The error is as follows:-

/usr/include/pthread.h|244|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)’ [-fpermissive]|

/home/dinu94/dummyspace/interview_prep/ThreadedBinaryTree/main.cpp|25|error: conversion from ‘int’ to non-scalar type ‘std::list’ requested|

/home/dinu94/dummyspace/interview_prep/ThreadedBinaryTree/main.cpp|28|error: invalid conversion from ‘std::list ()(node)’ to ‘void* ()(void)’ [-fpermissive]|

/usr/include/pthread.h|244|error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)’ [-fpermissive]|

/home/dinu94/dummyspace/interview_prep/ThreadedBinaryTree/main.cpp|28|error: conversion from ‘int’ to non-scalar type ‘std::list’ requested

I am not sure how to proceed.

edit: what are the alternative ways of returning value if pthread_create is the wrong way to do?

Thanks

basav basav · Accepted Answer · 2015-10-04T04:24:50

A pthread thread function has to conform to function definition specified by library:

int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)

so, your function list<int> inordertraversal(node* n) does not conform to void* ()(void)

you will have to change your function signature to a function which conforms to above signature and then call that recursively.

void* inorder(void)
{
    pthread_create(&threads[count++],NULL,wrapper,(void *)n->left);

}

Have list<int> l; as a class member, so that you dont need to pass it around with every function call.

c++ pthread_create returning value

3 Answers