Question

I have been looking at how I could use Single linked lists using the macros defined in sys/queue.h and I have a few doubts. Currently I am trying to create a simple list using the same. Please note that the program below is incomplete, I have put in only the part I feel is relevant.

int main() {
    SLIST_HEAD(slisthead, entry) head = SLIST_HEAD_INITIALIZER(head);
    struct slisthead *headp;

    struct entry {
        SLIST_ENTRY(entry) entries;
    }*n1, *n2, *n3, *np;


    /* Upon expanding the macro we would get,
     * struct entry {
     *      struct {
     *          struct entry *sle_first;
     *      } entries;
     *  }*n1, *n2, *np;
     */

I fail to understand why “sle_first” is stored in another struct altogether. Couldnt SLIST_ENTRY expand to something like the following instead?

#define SLIST_ENTRY(type) struct type* sle_first;

Answer 1

sys/queue.h also contain double linked lists and queues, that require more than one pointer in entry.

So I think SLIST_ENTRY expanded this way to be similar to another type entries, like queues.