Mutex for two threads in C++ - c++

Question

I've created two threads that use a variable declared in main via pthread_create's last argument. I want thread2 to use modify the value of that variable after thread1 is done with a set of particular instructions. I know how we can use mutex in one thread but how about two or more threads?
Refer to the following code:
#include<iostream>
#include<unistd.h>
#include<stdlib.h>
#include<pthread.h>
#define NUM 6
using namespace std;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
void *even(void *arg)
{
int sum = 0;
int count = 0;
for (int i = 1; count < NUM/2; i++)
{
if( !(i % 2) )
{
count++;
sum += i;
}
}
cout << "In Even Thread: " << sum << endl;
pthread_mutex_lock(&mutex);
*((int*)arg) = sum;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond);
}
void *odd(void *arg)
{
int sum = 0;
int count = 0;
for (int i = 1; count < NUM/2; i++)
{
if( i % 2 )
{
count++;
sum += i;
}
}
cout << "In Odd Thread: " << sum << endl;
pthread_cond_wait(&cond, &mutex);
*((int*)arg) = *((int*)arg) + sum;
}
int main()
{
int mainSum = 0;
pthread_t tidO, tidE;
pthread_create(&tidO, NULL, odd, (void *)&mainSum);
pthread_create(&tidE, NULL, even, (void *)&mainSum);
pthread_join(tidO, NULL);
pthread_join(tidE, NULL);
cout << "Sum of first " << NUM << " Natural Numbers: " << mainSum << endl;
return 0;
}

Answer 1

You should lock mutex before accessing the variable and unlock after you finish you finish you job with the variable. Use lock_guard to keep mutex locked and use brackets for lock_guard lifetime. http://www.cplusplus.com/reference/mutex/lock_guard/
std::mutex mtx; //global mutex
int v; //global variable;
int k;
...
//repeat this code in each thread to access the variable
{
std::lock_guard<std::mutex> lck (mtx);
//access the variable;
k = v; //read global variable or
v = k; //write
}

Answer 2

Your use case needs a condition variable combined with a mutex.
http://en.cppreference.com/w/cpp/thread/condition_variable

Answer 3

You should create mutex in same place as shared variable. In your case it is main function.
Then you need to share this mutex in all threads which are accessing shared variable.
You can share this mutex over global variable or better in same attribute as shared variable. But you will have to define structure where you place variable and mutex together.
Update
You should lock mutex pthread_mutex_lock(&mutex); before pthread_cond_wait(&cond, &mutex);
The pthread_cond_signal() and pthread_cond_broadcast() functions have no effect if there are no threads currently blocked on cond.