实现信号量

我在下面有以下代码。我一次只希望一半的线程进入线程函数。如何创建信号量来阻止其他进程？每当线程完成使用该函数时，我将如何解锁以前被阻止的进程？

#include <iostream>
#include <unistd.h>
#include <sys/wait.h>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 4
long int sharedcount;
pthread_mutex_t count_mutex;
//Function that will be run by multiple threads
//Needs to return a void pointer and if it takes arguments
//it needs to be a void pointer
void *ThreadedFunction(void *threadid) 
{
    int success;
    long id = (long)threadid;
    //Lock mutex preventing the other threads from ru nning
    success = pthread_mutex_lock( &count_mutex );
    cout << "Thread " << id << " beginning.n";
    for(int i = 0; i < 100000000; i++)
        sharedcount++;
    cout << "Thread " << id << " exiting.n";
    cout << sharedcount << endl;
    //Unlock the mutex after the thread has finished running
    pthread_mutex_unlock( &count_mutex );
    //Kill the thread
    pthread_exit(NULL);
}
int main () 
{
    //Initialize mutex
    pthread_mutex_init(&count_mutex, NULL);
    //Create an array of threads
    pthread_t threads[NUM_THREADS];
    int rc;
    int i;
    sharedcount = 0;
    for( i=0; i < NUM_THREADS; i++ )
    {
        cout << "main() : creating thread, " << i << endl;
        //Create thread by storing it in a location in the array.  Call the
        //function for the threads to run inside.  And pass the argument (if any).
        //If no arguments pass NULL
        rc = pthread_create(&threads[i], NULL, ThreadedFunction, (void *)i);
        if (rc)
        {
             cout << "Error:unable to create thread," << rc << endl;
             exit(-1);
        }
    }
    //Have main thread wait for all other threads to stop running.
    for(i = 0; i < NUM_THREADS; i++)
    pthread_join(threads[i], NULL);
    //cout << sharedcount << endl;
    pthread_exit(NULL);
}