Thread sync challenge

thread_sync.c

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

sem_t sem;
pthread_mutex_t mut;

void thread_fct()
{
    int towait;

    while (42)
    {
        towait = rand();
        printf("start %d\n", towait % 500);
        usleep(towait % 500);
        printf("end\n");

        pthread_mutex_trylock(&mut);
        
        if (!sem_trywait(&sem))
        {
            pthread_mutex_lock(&mut);
            pthread_mutex_unlock(&mut);
            sem_post(&sem);
        }
        else
        {
            pthread_mutex_unlock(&mut);
        }
    }
}

int main(int argc, char const *argv[])
{
    int sem_nb;
    pthread_t *thread;

    sem_nb = atoi(argv[1]);
    sem_init(&sem, 0, sem_nb - 1);
    thread = malloc((sem_nb + 2) * sizeof(pthread_t));
    for (int i = 0; i < sem_nb; ++i)
        pthread_create(&thread[i], NULL, thread_fct, NULL);
    for (int i = 0; i < sem_nb; ++i)
        pthread_join(thread[i], NULL);
    return 0;
}