SegFault after the call of pthread_join - c

Question

I am new to threads and I have problem using pthread_join.
This is my code :
int create_threads(t_lemin *lem)
{
int i;
pthread_t **threads;
int j;
void *ret;
t_tree *tmp;
j = -1;
printf("starting algo\n");
threads = xmalloc(sizeof(pthread_t));
i = count_leaf(lem->start);
tmp = lem->start;
printf("%d\n", tmp->visited[0]);
while (++j != i)
{
tmp->leaf[j]->thread_nbr = j;
pthread_create(threads[j], NULL, find_way_out, tmp->leaf[j]);
usleep(100);
}
i = -1;
while (++j != i)
(void)pthread_join (*threads[j], &ret);
//printf("%d\n", *((int*)ret));
return (0);
}
One of my thread finished his work and after the second one finish, I have a Segmentation Fault.
My functions find_way_out returns pthread_exit((void*)j); or pthread_exit(0);
where j is a integer pointer.
Do you have an idea where it can comes from ?
Thanks !

Answer 1

The problem is that you should pass a pointer to an actual pthread_t variable as first argument to pthread_create, as a way of emulating pass by reference, pthread_create will then initialize the object.
Since you pass an uninitialized pointer, you will have undefined behavior when pthread_create dereferences the pointer. And you will have it too when you try to dereference the pointer for the pthread_join call.
And it's even worse, since you only allocate space for one single pthread_t object, so you will go way out of bounds of the "array".
And then for the pthread_join loop, you will have j being some small positive number, while i is a negative number, meaning your loop will iterate a lot since it has to iterate until j overflows and becomes negative and then continue looping until j equals -1.
Instead I suggest these changes:
Use a variable-length array for the pthread_t object
Save the "size" of the array in a variable that you don't change
And finally, use for loops instead
Something like
size_t count = count_leaf(lem->start);
pthread_t threads[count];
for (size_t i = 0; i < count; ++i)
pthread_create(&threads[i], ...);
...
for (size_t i = 0; i < count; ++i)
pthread_join(threads[i], NULL);