issues with pthreads and quicksort

Question

I am trying to turn a simple quicksort implementation to start using threads for performance, simply just to learn some stuff and for fun. I am having a bit of trouble compiling because of the arguments I am supplying to pthread_create. I am not quite sure what I am doing. the way i am implementing threads atm was just a proof of concept then I was going to try and make it better any suggestions on how to better acheive this would be greatly appreciated :D here is my code..

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

#define MAX_THREADS 1 
int no_threads = 0;
std::mutex mtx;

struct arg_struct
{
    int * arr;
    int left;
    int right;
};

void *quicksort(arg_struct* arguments)
{
    int l = arguments->left, r = arguments->right;
    int tmp;
    int pivot = arguments->arr[(arguments->left + arguments->right)/2];

    /* partition */
    while(l <= r)
    {   
        //increment l untill arr[l] is larger than pivot 
        while (arguments->arr[l] < pivot)
            l++;
        //increment r untill arr[r] is less than pivot 
        while(arguments->arr[r] > pivot)
            r--;

        if(l <= r)
        {
            tmp = arguments->arr[l];
            arguments->arr[l] = arguments->arr[r];
            arguments->arr[r] = tmp;
            l++;
            r--;
        }
    }
    auto args = new arg_struct;
    args->left = arguments->left;
    args->right = r;
    args->arr = arguments->arr;

    if (arguments->left < r)

        mtx.lock(); 
        if(no_threads == 0)
        {
            pthread_t thread;
            int rc = pthread_create(&thread, NULL, &quicksort, args );   
            if(rc)
            {

            } 
        } 
        mtx.unlock();      
        if(no_threads == 1)
        {


            quicksort(args);

        } 
    if (l < arguments->right)
        quicksort(args);
}

int main(int argc, char ** argv)
{

    int arr[] = {55,5,6,2,4,6,2,4,534,5,4,2,1,1,3,5,634,7,87};

    arg_struct * arguments = new arg_struct; 
    arguments->arr = arr;
    arguments->left = 0;
    arguments->right =  sizeof(arr) / sizeof(arr[0]);
    quicksort(arguments);

    for(auto i = 0; i <= (int)sizeof(arr)/sizeof(arr[0])-1; i++)
    {
        std::cout << arr[i] << std::endl;
    } 

} 

and the errors

threading.cpp:52:22: error: no matching function for call to 'pthread_create'
            int rc = pthread_create(&thread, NULL, &quicksort, args );   
                     ^~~~~~~~~~~~~~
/usr/include/pthread.h:234:12: note: candidate function not viable: no known conversion from 'void *(*)(arg_struct *)' to 'void *(*)(void *)' for 3rd argument
extern int pthread_create (pthread_t *__restrict __newthread,
           ^
1 error generated.

Answer 1

pthread_create() expects a void* -> void* function as third argument, but you provide quicksort which has signature arg_struct* -> void* . This is what the error message says:

 no known conversion from 'void *(*)(arg_struct *)' to 'void *(*)(void *)' for 3rd argument 

Simply change quicksort 's signature:

void* quicksort(void* arg)
{
    arg_struct* arguments = reinterpret_cast<arg_struct*>(arg);
    // ...

---

Now, if you use a decently recent compiler, C++11 provides a more portable and easier way to manage thread: std::thread .