SLIST(3)

✨ SLIST(3) — the singly linked list glow-up ✨

okay bestie, so this is <sys/queue.h>'s singly linked list situation — literally the whole family of macros lives here: SLIST_EMPTY, SLIST_ENTRY, SLIST_FIRST, SLIST_FOREACH, SLIST_HEAD, SLIST_HEAD_INITIALIZER, SLIST_INIT, SLIST_INSERT_AFTER, SLIST_INSERT_HEAD, SLIST_NEXT, SLIST_REMOVE, SLIST_REMOVE_HEAD. it's the implementation of a singly linked list, and it's giving ✨library functions manual✨ energy, in case the header wasn't obvious 💅

📚 LIBRARY

standard C library — libc, -lc. the blueprint, the moment, the standard 💖

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🫶 SYNOPSIS

#include <sys/queue.h>

SLIST_ENTRY(TYPE);

SLIST_HEAD(HEADNAME, TYPE);
SLIST_HEAD SLIST_HEAD_INITIALIZER(SLIST_HEAD head);
void SLIST_INIT(SLIST_HEAD *head);

int SLIST_EMPTY(SLIST_HEAD *head);

void SLIST_INSERT_HEAD(SLIST_HEAD *head,
                        struct TYPE *elm, SLIST_ENTRY NAME);
void SLIST_INSERT_AFTER(struct TYPE *listelm,
                        struct TYPE *elm, SLIST_ENTRY NAME);

struct TYPE *SLIST_FIRST(SLIST_HEAD *head);
struct TYPE *SLIST_NEXT(struct TYPE *elm, SLIST_ENTRY NAME);

SLIST_FOREACH(struct TYPE *var, SLIST_HEAD *head, SLIST_ENTRY NAME);

void SLIST_REMOVE(SLIST_HEAD *head, struct TYPE *elm, TYPE,
                        SLIST_ENTRY NAME);
void SLIST_REMOVE_HEAD(SLIST_HEAD *head,
                        SLIST_ENTRY NAME);

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💜 DESCRIPTION

these macros define and operate on singly linked lists. no cap, that's the whole job 🦋

in the macro definitions, TYPE is the name of a user-defined structure, and it must have a field of type SLIST_ENTRY in it, named NAME. and HEADNAME is the name of a user-defined structure — one that must be declared using the macro SLIST_HEAD(). iykyk 🌸

🎀 Creation

a singly linked list gets headed by a structure defined by the SLIST_HEAD() macro. that structure holds a single pointer to the first element on the list. the elements are singly linked for minimum space and pointer manipulation overhead — iconic for leanness — but at the expense of O(n) removal for arbitrary elements. so yeah, cute but there's a tradeoff, lowkey 😭

new elements can be added to the list after an existing element, or at the head of the list. an SLIST_HEAD structure is declared like this:

SLIST_HEAD(HEADNAME, TYPE) head;

where struct HEADNAME is the structure to be defined, and struct TYPE is the type of the elements to be linked into the list. a pointer to the head of the list can later be declared as:

struct HEADNAME *headp;

(the names head and headp are user selectable — pick your own faves bestie 💅)

• SLIST_ENTRY() declares a structure that connects the elements in the list.
• SLIST_HEAD_INITIALIZER() evaluates to an initializer for the list head.
• SLIST_INIT() initializes the list referenced by head.
• SLIST_EMPTY() evaluates to true if there are no elements in the list.

🧚 Insertion

• SLIST_INSERT_HEAD() inserts the new element elm at the head of the list.
• SLIST_INSERT_AFTER() inserts the new element elm after the element listelm.

🌈 Traversal

• SLIST_FIRST() returns the first element in the list, or NULL if the list is empty.
• SLIST_NEXT() returns the next element in the list.
• SLIST_FOREACH() traverses the list referenced by head in the forward direction, assigning each element in turn to var.

🩷 Removal

• SLIST_REMOVE() removes the element elm from the list.
• SLIST_REMOVE_HEAD() removes the element elm from the head of the list.

for optimum efficiency, elements being removed from the head of the list should explicitly use this macro instead of the generic SLIST_REMOVE(). like, speak its name, queen 👑

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🌟 RETURN VALUE

• SLIST_EMPTY() returns nonzero if the list is empty, and zero if the list contains at least one entry.
• SLIST_FIRST(), and SLIST_NEXT() return a pointer to the first or next TYPE structure, respectively.
• SLIST_HEAD_INITIALIZER() returns an initializer that can be assigned to the list head.

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📜 STANDARDS

BSD. periodt 💅

🕰️ HISTORY

4.4BSD. the origin story fr fr ✨

💀 BUGS

SLIST_FOREACH() does not allow var to be removed or freed within the loop — that would interfere with the traversal, and we do not do that here bestie 😭

SLIST_FOREACH_SAFE(), which is present on the BSDs but is not present in glibc, fixes this limitation by allowing var to safely be removed from the list and freed from within the loop without interfering with the traversal. so if you're on glibc, know that this one is not in your roster 🙈

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👑 EXAMPLES

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct entry {
    int data;
    SLIST_ENTRY(entry) entries;             /* Singly linked list */
};

SLIST_HEAD(slisthead, entry);

int
main(void)
{
    struct entry *n1, *n2, *n3, *np;
    struct slisthead head;                  /* Singly linked list
                                               head */

    SLIST_INIT(&head);                      /* Initialize the queue */

    n1 = malloc(sizeof(struct entry));      /* Insert at the head */
    SLIST_INSERT_HEAD(&head, n1, entries);

    n2 = malloc(sizeof(struct entry));      /* Insert after */
    SLIST_INSERT_AFTER(n1, n2, entries);

    SLIST_REMOVE(&head, n2, entry, entries);/* Deletion */
    free(n2);

    n3 = SLIST_FIRST(&head);
    SLIST_REMOVE_HEAD(&head, entries);      /* Deletion from the head */
    free(n3);

    for (unsigned int i = 0; i < 5; i++) {
        n1 = malloc(sizeof(struct entry));
        SLIST_INSERT_HEAD(&head, n1, entries);
        n1->data = i;
    }

                                            /* Forward traversal */
    SLIST_FOREACH(np, &head, entries)
        printf("%i\n", np->data);

    while (!SLIST_EMPTY(&head)) {           /* List deletion */
        n1 = SLIST_FIRST(&head);
        SLIST_REMOVE_HEAD(&head, entries);
        free(n1);
    }
    SLIST_INIT(&head);

    exit(EXIT_SUCCESS);
}

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🦋 SEE ALSO

insque(3), queue(7)

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and that's the whole page bestie, you ate 💖 this one came from Linux man-pages 6.20, dated 2025-05-17 — so if you're quoting it in the group chat, that's the receipts. go forth and singly-link responsibly 🎀✨