Question

Please see my pseudo-code below. The code comments should explain my problem. I’m new to both pthreads and linked lists in C so I’ve jumped into the deep end a bit. I just need to print the value of str out in the thread_work function. The sequential bit of code is fine but when each thread does its work, it can’t print out the value of str.

// linked list definition
struct linked_list {
   char *str;
   struct linked_list *next;
};

// linked list initiation
struct linked_list *root;
struct linked_list *next_info;
root = malloc( sizeof( struct linked_list ) );

// main code
some loop {
   next_node->str = str;
   printf( "%s\n", next_node ); // PRINTS FINE
   pthread_t thread;
   rc = pthread_create( &thread, NULL, thread_work, (void *) &next_node );
   next_node->next = malloc( sizeof( struct linked_list ) );
   next_node = next_node->next;
}

// code executed by each thread
void *thread_work( void *thread_arg ) {
   struct linked_list *ll;
   ll = ( struct linked_list * )thread_arg;
   printf( "%s\n", ll->str ); // PRINTS SOME MESS (��E#)
}

In my actual code there are a few more members of the linked_list struct.

Many thanks.

Answer 1

You have a pointer type mismatch: you are passing a pointer to a pointer to list node, but inside thread_work you treat it as a pointer to node. Either remove the ampersand before next_node in the call to pthread_create, or change your thread_work as follows:

void *thread_work( void *thread_arg ) {
    struct linked_list **llp, *ll;
    llp = ( struct linked_list ** )thread_arg;
    ll = *llp;
    printf( "%s\n", ll->str ); // PRINTS SOME GOOD STUFF
}