42 Exam 06 📢 📌

Most students encounter the Dining Philosophers problem during the common core project. 42 Exam 06 simplifies this: you do not implement the full project. Instead, you typically have to code a smaller version, often referred to as the "One Philosopher" or "Basic Simulation."

The prompt usually reads something like:

Write a program that takes a number_of_philosophers and a time_to_die as arguments. Each philosopher is a process. They must eat, sleep, and think. If a philosopher doesn’t start eating before time_to_die milliseconds after their last meal, they die and the simulation stops.

The 42 exam environment provides no GDB (usually). Your only debug tool is write to stderr.

A minimal viable structure:

typedef struct s_minishell
char    **env;
    int     last_exit;
 t_minishell;
int main(void)
t_minishell shell;
char *line;
shell.env = init_env(environ);
shell.last_exit = 0;
while (1)
line = readline("minishell$ ");
    if (!line) // ctrl-D
        break;
    if (*line)
        add_history(line);
    parse_and_execute(line, &shell);
    free(line);
return (0);

Parsing must handle:

You cannot use pthread_join because you have no threads. You must use waitpid(-1, &status, WNOHANG) in a loop to check which child (philosopher) has died without blocking the monitor.

Here’s a reconstructed problem from a 2023 42 Exam 06 session:

Subject: Write a program signal_ping that takes no arguments. It creates a pipe, then forks twice (Child A and Child B). Child A sends SIGUSR1 to Child B every second. Child B counts how many signals it receives and prints the count every 5 seconds. The parent must terminate both children cleanly when it receives SIGTERM.

The hidden challenges:

Why cadets fail: They try to use a global variable across processes (impossible without shared memory). They forget that fork copies memory, not shares it.

The correct approach: Use the pipe as a control channel. Parent writes "EXIT" to the pipe, children read from it.

Do not fear sem_open. Memorize this incantation:

sem_t *forks;
forks = sem_open("/forks", O_CREAT, 0644, number_of_philosophers);
// ... later
sem_wait(forks);
// eat
sem_post(forks);
// finally
sem_close(forks);
sem_unlink("/forks");

Warning: The Moulinette resets /dev/shm/. Use unique names like /sem_philo_<pid> to avoid conflicts. 42 Exam 06

Let’s be honest. Most 42 students fail Exam 06 at least once. The reasons are predictable: