dining-philosophers-2.c

// This example code is from: https://rosettacode.org/wiki/Dining_philosophers#C
// Please let me know if this does not constitute fair use.

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

// When building, you must link with the external pthread library: for example, 'gcc dining-philosophers-2.c -lpthread'

// This solution makes use of functions like 'trylock' to avoid deadlock.

typedef struct philData {
    pthread_mutex_t *fork_lft, *fork_rgt;
    const char *name;
    pthread_t thread;
    int   fail;
} Philosopher;

int running = 1;

void *PhilPhunction(void *p) {
    Philosopher *phil = (Philosopher*)p;
    int failed;
    int tries_left;
    pthread_mutex_t *fork_lft, *fork_rgt, *fork_tmp;

    while (running) {
        printf("%s is sleeping --er thinking\n", phil->name);
        sleep( 1+ rand()%8);

        fork_lft = phil->fork_lft;
        fork_rgt = phil->fork_rgt;
        printf("%s is hungry\n", phil->name);
        tries_left = 2;   /* try twice before being forceful */
        do {
            failed = pthread_mutex_lock( fork_lft);
            failed = (tries_left>0)? pthread_mutex_trylock( fork_rgt )
                                   : pthread_mutex_lock(fork_rgt);
            if (failed) {
                pthread_mutex_unlock( fork_lft);
                fork_tmp = fork_lft;
                fork_lft = fork_rgt;
                fork_rgt = fork_tmp;
                tries_left -= 1;
            }
        } while(failed && running);

        if (!failed) {
            printf("%s is eating\n", phil->name);
            sleep( 1+ rand() % 8);
            pthread_mutex_unlock( fork_rgt);
            pthread_mutex_unlock( fork_lft);
        }
    }
    return NULL;
}

void Ponder()
{
    const char *nameList[] = { "Kant", "Guatma", "Russel", "Aristotle", "Bart" };
    pthread_mutex_t forks[5];
    Philosopher philosophers[5];
    Philosopher *phil;
    int i;
    int failed;

    for (i=0;i<5; i++) {
        failed = pthread_mutex_init(&forks[i], NULL);
        if (failed) {
            printf("Failed to initialize mutexes.");
            exit(1);
        }
    }

    for (i=0;i<5; i++) {
        phil = &philosophers[i];
        phil->name = nameList[i];
        phil->fork_lft = &forks[i];
        phil->fork_rgt = &forks[(i+1)%5];
        phil->fail = pthread_create( &phil->thread, NULL, PhilPhunction, phil);
    }

    sleep(40);
    running = 0;
    printf("cleanup time\n");

    for(i=0; i<5; i++) {
        phil = &philosophers[i];
        if ( !phil->fail && pthread_join( phil->thread, NULL) ) {
            printf("error joining thread for %s", phil->name);
            exit(1);
        }
    }
}

int main()
{
    Ponder();
    return 0;
}