#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

#define MAX_THREAD_NUM 2
#define THREAD_NUM 5

pthread_mutex_t mutex;
pthread_cond_t  cond;
int thread_num = 0;

void *
thread_func(void *arg)
{
    long id = (long)arg;

    pthread_mutex_lock(&mutex);
    while (thread_num >= MAX_THREAD_NUM)
        pthread_cond_wait(&cond, &mutex);
    thread_num++;
    pthread_mutex_unlock(&mutex);

    printf("Thread %ld started.\n", id);
    sleep(1);
    printf("Thread %ld finished.\n", id);

    pthread_mutex_lock(&mutex);
    thread_num--;
    pthread_cond_signal(&cond);
    pthread_mutex_unlock(&mutex);

    return 0;
}

int
main()
{
    long i;
    pthread_t handle[THREAD_NUM];

    /* initialize */
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&cond, NULL);

    /* spawn thread a number of THREAD_NUM */
    for (i = 0; i < THREAD_NUM; ++i)
        pthread_create(&handle[i], NULL, &thread_func, (void*)i);

    /* wait for running all thread */
    for (i = 0; i < THREAD_NUM; ++i)
        pthread_join(handle[i], NULL);

    /* destroy mutex*/
    pthread_cond_destroy(&cond);

    return 0;
}