Threaded train simulation

Question

Edit: I think I am doing something wrong, because when I compile and run my binary twice, I get different outputs.

I am trying to understand flows with pthread, so I made a little code to simulate the passage of a train on a bridge (which can only handle 2 trains at a time)

I managed to do this so that only one train crosses the bridge at a time with this code:

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

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void    *train()
{
  int   km;
  static int    t = 0;

  km = 1;
  while (km != 10)
    {
      printf("I'm at %02d km\n", km++);
      sleep(1);
      if (km == 2)
        pthread_mutex_lock(&mutex);
      if (km == 4)
        pthread_mutex_unlock(&mutex);
    }
}

int     main()
{
  pthread_t     train1;
  pthread_t     train2;
  pthread_t     train3;

  pthread_create(&train1, NULL, train, NULL);
  pthread_create(&train2, NULL, train, NULL);
  pthread_create(&train3, NULL, train, NULL);

  pthread_join(train1, NULL);
  pthread_join(train2, NULL);
  pthread_join(train3, NULL);
}

This worked fine (or maybe I did something wrong, if so, please tell me)

And then I tried for the case when 2 trains can simultaneously pass over the bridge.

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

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void    *train()
{
  int   km;
  static int    t = 0;

  km = 1;
  while (km != 30)
    {
      printf("I'm at %02d km\n", km++);
      sleep(1);
      if (km == 2 && t <= 2)
        {
          ++t;
          pthread_mutex_lock(&mutex);
        }
      if (km == 4)
        {
          t--;
          pthread_mutex_unlock(&mutex);
        }
    }
}

int     main()
{
  pthread_t     train1;
  pthread_t     train2;
  pthread_t     train3;
  pthread_t     train4;

  pthread_create(&train1, NULL, train, NULL);
  pthread_create(&train2, NULL, train, NULL);
  pthread_create(&train3, NULL, train, NULL);
  pthread_create(&train4, NULL, train, NULL);

  pthread_join(train1, NULL);
  pthread_join(train2, NULL);
  pthread_join(train3, NULL);
  pthread_join(train4, NULL);
}

So, I used a static int to “pop up” my train to 2 on the bridge, but it actually doesn't work, and I don't understand why ..

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Answer 1

, pthread_mutex_lock(&mutex); , mutex , . , t.

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

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void    *train()
{
    int   km;
    static int t = 0;

    km = 1;
    while (km != 30)
    {
        printf("I'm at %02d km\n", km);

        // If we've reached the bridge, wait until less than two trains are on it.
        if (km == 2)
        {
            pthread_mutex_lock(&mutex);
            while (t == 2) { // To be read as: "While two trains are on the bridge, wait."
                pthread_cond_wait(&cond, &mutex);
            }
            ++t; // Put this train onto the bridge.
            pthread_mutex_unlock(&mutex);
        }

        // Leave the bridge.
        if (km == 4)
        {
            pthread_mutex_lock(&mutex);
            --t; // Take this train off the bridge.
            pthread_cond_signal(&cond); // Signal another train to enter.
            pthread_mutex_unlock(&mutex);
        }

        // Move forward 1 km.
        sleep(1);
        ++km;
    }
}

int     main()
{
    pthread_t     train1;
    pthread_t     train2;
    pthread_t     train3;
    pthread_t     train4;

    pthread_create(&train1, NULL, train, NULL);
    pthread_create(&train2, NULL, train, NULL);
    pthread_create(&train3, NULL, train, NULL);
    pthread_create(&train4, NULL, train, NULL);

    pthread_join(train1, NULL);
    pthread_join(train2, NULL);
    pthread_join(train3, NULL);
    pthread_join(train4, NULL);
}

if (km == 4) , , , / t . ( , , !)

Answer 2

static int t - , : , , .