Dining Philosophers in C memory leaks

Question

i am trying to implement Dining Philosophers in C using Resource hierarchy solution. when i am using valgrind everything goes fine. Unfortunately when i done this using console im getting random seqfaults. One time my program will be succesful,one time it will broke on the beginning. I would be grateful if anybody could point where i did mistake and why it's 100% succesfull.

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

#define NUM_PHILOSPHERS 5

sem_t forks[NUM_PHILOSPHERS];
void *philosopher(void *param){
    printf("Thread created!");
    int *id = (int *)param;
    int L = 0;
    sem_t* Myforks[2];
    int par = *id;
    if(par == 4){
        Myforks[0]  = &forks[4];
        Myforks[1]  = &forks[0];
    }else{
        Myforks[0]  = &forks[par];
        Myforks[1]  = &forks[par+1];
    }
    while(L!=5){    
        printf("Eat spaghetti!",*id);
        sem_wait(Myforks[0]);
        sem_wait(Myforks[1]);
        //.....
        printf("EAT spaghetti!",*id);
        sem_post(Myforks[1]);
        sem_post(Myforks[0]);
        L=L+1;
    }
    pthread_exit(NULL);
}

int main(){
    int i;
    pthread_t threads[NUM_PHILOSPHERS];

    for(i = 0; i < NUM_PHILOSPHERS; i++)
        sem_init(&forks[i], 0, 1);
    for(i = 0; i < NUM_PHILOSPHERS; i++)
        pthread_create(&threads[i], NULL, philosopher, (void *)&i);
    return 0;
}

Answer 1

int i;

...

for(i = 0; i < NUM_PHILOSPHERS; i++) 
  pthread_create(&threads[i], NULL, philosopher, (void *)&i);
                                                         ^^

Passing a pointer to a local variable isn't going to work. You're passing the same address to all of the threads, so there's an inherent race condition. You point them a pointer to i and them almost immediately you increment i . What value will they read when they access *param ? Who knows!

You'll want to create an array with NUM_PHILOSPHERS (sic) slots in it and pass a different address to each thread. You'll also want to make sure that array isn't destroyed when main() exits—ie, make the array global or static, not local.