Single-linked List

Zephyr provides a sys_slist_t type for storing simple singly-linked list data (i.e. data where each list element stores a pointer to the next element, but not the previous one). This supports constant-time access to the first (head) and last (tail) elements of the list, insertion before the head and after the tail of the list and constant time removal of the head. Removal of subsequent nodes requires access to the “previous” pointer and thus can only be performed in linear time by searching the list.

The sys_slist_t struct may be instantiated by the user in any accessible memory. It should be initialized with either sys_slist_init() or by static assignment from SYS_SLIST_STATIC_INIT before use. Its interior fields are opaque and should not be accessed by user code.

The end nodes of a list may be retrieved with sys_slist_peek_head() and sys_slist_peek_tail(), which will return NULL if the list is empty, otherwise a pointer to a sys_snode_t struct.

The sys_snode_t struct represents the data to be inserted. In general, it is expected to be allocated/controlled by the user, usually embedded within a struct which is to be added to the list. The container struct pointer may be retrieved from a list node using SYS_SLIST_CONTAINER, passing it the struct name of the containing struct and the field name of the node. Internally, the sys_snode_t struct contains only a next pointer, which may be accessed with sys_slist_peek_next().

Lists may be modified by adding a single node at the head or tail with sys_slist_prepend() and sys_slist_append(). They may also have a node added to an interior point with sys_slist_insert(), which inserts a new node after an existing one. Similarly sys_slist_remove() will remove a node given a pointer to its predecessor. These operations are all constant time.

Convenience routines exist for more complicated modifications to a list. sys_slist_merge_slist() will append an entire list to an existing one. sys_slist_append_list() will append a bounded subset of an existing list in constant time. And sys_slist_find_and_remove() will search a list (in linear time) for a given node and remove it if present.

Finally the slist implementation provides a set of “for each” macros that allows for iterating over a list in a natural way without needing to manually traverse the next pointers. SYS_SLIST_FOR_EACH_NODE will enumerate every node in a list given a local variable to store the node pointer. SYS_SLIST_FOR_EACH_NODE_SAFE behaves similarly, but has a more complicated implementation that requires an extra scratch variable for storage and allows the user to delete the iterated node during the iteration. Each of those macros also exists in a “container” variant (SYS_SLIST_FOR_EACH_CONTAINER and SYS_SLIST_FOR_EACH_CONTAINER_SAFE) which assigns a local variable of a type that matches the user’s container struct and not the node struct, performing the required offsets internally. And SYS_SLIST_ITERATE_FROM_NODE exists to allow for enumerating a node and all its successors only, without inspecting the earlier part of the list.

Single-linked List Internals

The slist code is designed to be minimal and conventional. Internally, a sys_slist_t struct is nothing more than a pair of “head” and “tail” pointer fields. And a sys_snode_t stores only a single “next” pointer.

slist example — Fig. 11 An slist containing three elements.

empty slist example — Fig. 12 An empty slist

The specific implementation of the list code, however, is done with an internal “Z_GENLIST” template API which allows for extracting those fields from arbitrary structures and emits an arbitrarily named set of functions. This allows for implementing more complicated single-linked list variants using the same basic primitives. The genlist implementor is responsible for a custom implementation of the primitive operations only: an “init” step for each struct, and a “get” and “set” primitives for each of head, tail and next pointers on their relevant structs. These inline functions are passed as parameters to the genlist macro expansion.

Only one such variant, sflist, exists in Zephyr at the moment.

Flagged List

The sys_sflist_t is implemented using the described genlist template API. With the exception of symbol naming (“sflist” instead of “slist”) and the additional API described next, it operates in all ways identically to the slist API.

It adds the ability to associate exactly two bits of user defined “flags” with each list node. These can be accessed and modified with sys_sfnode_flags_get() and sys_sfnode_flags_get(). Internally, the flags are stored unioned with the bottom bits of the next pointer and incur no SRAM storage overhead when compared with the simpler slist code.

Single-linked List API Reference

group single-linked-list_apis

Defines

SYS_SLIST_FOR_EACH_NODE(__sl, __sn)

Provide the primitive to iterate on a list Note: the loop is unsafe and thus __sn should not be removed.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SLIST_FOR_EACH_NODE(l, n) {
    <user code>
}

This and other SYS_SLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_slist_t to iterate on
__sn – A sys_snode_t pointer to peek each node of the list

SYS_SLIST_ITERATE_FROM_NODE(__sl, __sn)

Provide the primitive to iterate on a list, from a node in the list Note: the loop is unsafe and thus __sn should not be removed.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SLIST_ITERATE_FROM_NODE(l, n) {
    <user code>
}

Like SYS_SLIST_FOR_EACH_NODE(), but __dn already contains a node in the list where to start searching for the next entry from. If NULL, it starts from the head.

This and other SYS_SLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_slist_t to iterate on
__sn – A sys_snode_t pointer to peek each node of the list it contains the starting node, or NULL to start from the head

SYS_SLIST_FOR_EACH_NODE_SAFE(__sl, __sn, __sns)

Provide the primitive to safely iterate on a list Note: __sn can be removed, it will not break the loop.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SLIST_FOR_EACH_NODE_SAFE(l, n, s) {
    <user code>
}

This and other SYS_SLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_slist_t to iterate on
__sn – A sys_snode_t pointer to peek each node of the list
__sns – A sys_snode_t pointer for the loop to run safely

SYS_SLIST_CONTAINER(__ln, __cn, __n)

SYS_SLIST_PEEK_HEAD_CONTAINER(__sl, __cn, __n)

SYS_SLIST_PEEK_TAIL_CONTAINER(__sl, __cn, __n)

SYS_SLIST_PEEK_NEXT_CONTAINER(__cn, __n)

SYS_SLIST_FOR_EACH_CONTAINER(__sl, __cn, __n)

Provide the primitive to iterate on a list under a container Note: the loop is unsafe and thus __cn should not be detached.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SLIST_FOR_EACH_CONTAINER(l, c, n) {
    <user code>
}

Parameters

__sl – A pointer on a sys_slist_t to iterate on
__cn – A pointer to peek each entry of the list
__n – The field name of sys_node_t within the container struct

SYS_SLIST_FOR_EACH_CONTAINER_SAFE(__sl, __cn, __cns, __n)

Provide the primitive to safely iterate on a list under a container Note: __cn can be detached, it will not break the loop.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SLIST_FOR_EACH_NODE_SAFE(l, c, cn, n) {
    <user code>
}

Parameters

__sl – A pointer on a sys_slist_t to iterate on
__cn – A pointer to peek each entry of the list
__cns – A pointer for the loop to run safely
__n – The field name of sys_node_t within the container struct

SYS_SLIST_STATIC_INIT(ptr_to_list)

Functions

static inline void sys_slist_init(sys_slist_t *list)

Initialize a list.

Parameters

list – A pointer on the list to initialize

static inline sys_snode_t *sys_slist_peek_head(sys_slist_t *list)

Peek the first node from the list.

Parameters

list – A point on the list to peek the first node from

Returns

A pointer on the first node of the list (or NULL if none)

static inline sys_snode_t *sys_slist_peek_tail(sys_slist_t *list)

Peek the last node from the list.

Parameters

list – A point on the list to peek the last node from

Returns

A pointer on the last node of the list (or NULL if none)

static inline bool sys_slist_is_empty(sys_slist_t *list)

Test if the given list is empty.

Parameters

list – A pointer on the list to test

Returns

a boolean, true if it’s empty, false otherwise

static inline sys_snode_t *sys_slist_peek_next_no_check(sys_snode_t *node)

Peek the next node from current node, node is not NULL.

Faster then sys_slist_peek_next() if node is known not to be NULL.

Parameters

node – A pointer on the node where to peek the next node

Returns

a pointer on the next node (or NULL if none)

static inline sys_snode_t *sys_slist_peek_next(sys_snode_t *node)

Peek the next node from current node.

Parameters

node – A pointer on the node where to peek the next node

Returns

a pointer on the next node (or NULL if none)

static inline void sys_slist_prepend(sys_slist_t *list, sys_snode_t *node)

Prepend a node to the given list.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to prepend

static inline void sys_slist_append(sys_slist_t *list, sys_snode_t *node)

Append a node to the given list.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to append

static inline void sys_slist_append_list(sys_slist_t *list, void *head, void *tail)

Append a list to the given list.

Append a singly-linked, NULL-terminated list consisting of nodes containing the pointer to the next node as the first element of a node, to list. This and other sys_slist_*() functions are not thread safe.

FIXME: Why are the element parameters void *?

Parameters

list – A pointer on the list to affect
head – A pointer to the first element of the list to append
tail – A pointer to the last element of the list to append

static inline void sys_slist_merge_slist(sys_slist_t *list, sys_slist_t *list_to_append)

merge two slists, appending the second one to the first

When the operation is completed, the appending list is empty. This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
list_to_append – A pointer to the list to append.

static inline void sys_slist_insert(sys_slist_t *list, sys_snode_t *prev, sys_snode_t *node)

Insert a node to the given list.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
prev – A pointer on the previous node
node – A pointer on the node to insert

static inline sys_snode_t *sys_slist_get_not_empty(sys_slist_t *list)

Fetch and remove the first node of the given list.

List must be known to be non-empty. This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect

Returns

A pointer to the first node of the list

static inline sys_snode_t *sys_slist_get(sys_slist_t *list)

Fetch and remove the first node of the given list.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect

Returns

A pointer to the first node of the list (or NULL if empty)

static inline void sys_slist_remove(sys_slist_t *list, sys_snode_t *prev_node, sys_snode_t *node)

Remove a node.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
prev_node – A pointer on the previous node (can be NULL, which means the node is the list’s head)
node – A pointer on the node to remove

static inline bool sys_slist_find_and_remove(sys_slist_t *list, sys_snode_t *node)

Find and remove a node from a list.

This and other sys_slist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to remove from the list

Returns

true if node was removed

Flagged List API Reference

group flagged-single-linked-list_apis

Defines

SYS_SFLIST_FOR_EACH_NODE(__sl, __sn)

Provide the primitive to iterate on a list Note: the loop is unsafe and thus __sn should not be removed.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SFLIST_FOR_EACH_NODE(l, n) {
    <user code>
}

This and other SYS_SFLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_sflist_t to iterate on
__sn – A sys_sfnode_t pointer to peek each node of the list

SYS_SFLIST_ITERATE_FROM_NODE(__sl, __sn)

Provide the primitive to iterate on a list, from a node in the list Note: the loop is unsafe and thus __sn should not be removed.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SFLIST_ITERATE_FROM_NODE(l, n) {
    <user code>
}

Like SYS_SFLIST_FOR_EACH_NODE(), but __dn already contains a node in the list where to start searching for the next entry from. If NULL, it starts from the head.

This and other SYS_SFLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_sflist_t to iterate on
__sn – A sys_sfnode_t pointer to peek each node of the list it contains the starting node, or NULL to start from the head

SYS_SFLIST_FOR_EACH_NODE_SAFE(__sl, __sn, __sns)

Provide the primitive to safely iterate on a list Note: __sn can be removed, it will not break the loop.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SFLIST_FOR_EACH_NODE_SAFE(l, n, s) {
    <user code>
}

This and other SYS_SFLIST_*() macros are not thread safe.

Parameters

__sl – A pointer on a sys_sflist_t to iterate on
__sn – A sys_sfnode_t pointer to peek each node of the list
__sns – A sys_sfnode_t pointer for the loop to run safely

SYS_SFLIST_CONTAINER(__ln, __cn, __n)

SYS_SFLIST_PEEK_HEAD_CONTAINER(__sl, __cn, __n)

SYS_SFLIST_PEEK_TAIL_CONTAINER(__sl, __cn, __n)

SYS_SFLIST_PEEK_NEXT_CONTAINER(__cn, __n)

SYS_SFLIST_FOR_EACH_CONTAINER(__sl, __cn, __n)

Provide the primitive to iterate on a list under a container Note: the loop is unsafe and thus __cn should not be detached.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SFLIST_FOR_EACH_CONTAINER(l, c, n) {
    <user code>
}

Parameters

__sl – A pointer on a sys_sflist_t to iterate on
__cn – A pointer to peek each entry of the list
__n – The field name of sys_sfnode_t within the container struct

SYS_SFLIST_FOR_EACH_CONTAINER_SAFE(__sl, __cn, __cns, __n)

Provide the primitive to safely iterate on a list under a container Note: __cn can be detached, it will not break the loop.

User MUST add the loop statement curly braces enclosing its own code:

SYS_SFLIST_FOR_EACH_NODE_SAFE(l, c, cn, n) {
    <user code>
}

Parameters

__sl – A pointer on a sys_sflist_t to iterate on
__cn – A pointer to peek each entry of the list
__cns – A pointer for the loop to run safely
__n – The field name of sys_sfnode_t within the container struct

SYS_SFLIST_STATIC_INIT(ptr_to_list)

SYS_SFLIST_FLAGS_MASK

Functions

static inline void sys_sflist_init(sys_sflist_t *list)

Initialize a list.

Parameters

list – A pointer on the list to initialize

static inline uint8_t sys_sfnode_flags_get(sys_sfnode_t *node)

Fetch flags value for a particular sfnode.

Parameters

node – A pointer to the node to fetch flags from

Returns

The value of flags, which will be between 0 and 3

static inline sys_sfnode_t *sys_sflist_peek_head(sys_sflist_t *list)

Peek the first node from the list.

Parameters

list – A point on the list to peek the first node from

Returns

A pointer on the first node of the list (or NULL if none)

static inline sys_sfnode_t *sys_sflist_peek_tail(sys_sflist_t *list)

Peek the last node from the list.

Parameters

list – A point on the list to peek the last node from

Returns

A pointer on the last node of the list (or NULL if none)

static inline void sys_sfnode_init(sys_sfnode_t *node, uint8_t flags)

Initialize an sflist node.

Set an initial flags value for this slist node, which can be a value between 0 and 3. These flags will persist even if the node is moved around within a list, removed, or transplanted to a different slist.

This is ever so slightly faster than sys_sfnode_flags_set() and should only be used on a node that hasn’t been added to any list.

Parameters

node – A pointer to the node to set the flags on
flags – A value between 0 and 3 to set the flags value

static inline void sys_sfnode_flags_set(sys_sfnode_t *node, uint8_t flags)

Set flags value for an sflist node.

Set a flags value for this slist node, which can be a value between 0 and 3. These flags will persist even if the node is moved around within a list, removed, or transplanted to a different slist.

Parameters

node – A pointer to the node to set the flags on
flags – A value between 0 and 3 to set the flags value

static inline bool sys_sflist_is_empty(sys_sflist_t *list)

Test if the given list is empty.

Parameters

list – A pointer on the list to test

Returns

a boolean, true if it’s empty, false otherwise

static inline sys_sfnode_t *sys_sflist_peek_next_no_check(sys_sfnode_t *node)

Peek the next node from current node, node is not NULL.

Faster then sys_sflist_peek_next() if node is known not to be NULL.

Parameters

node – A pointer on the node where to peek the next node

Returns

a pointer on the next node (or NULL if none)

static inline sys_sfnode_t *sys_sflist_peek_next(sys_sfnode_t *node)

Peek the next node from current node.

Parameters

node – A pointer on the node where to peek the next node

Returns

a pointer on the next node (or NULL if none)

static inline void sys_sflist_prepend(sys_sflist_t *list, sys_sfnode_t *node)

Prepend a node to the given list.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to prepend

static inline void sys_sflist_append(sys_sflist_t *list, sys_sfnode_t *node)

Append a node to the given list.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to append

static inline void sys_sflist_append_list(sys_sflist_t *list, void *head, void *tail)

Append a list to the given list.

Append a singly-linked, NULL-terminated list consisting of nodes containing the pointer to the next node as the first element of a node, to list. This and other sys_sflist_*() functions are not thread safe.

FIXME: Why are the element parameters void *?

Parameters

list – A pointer on the list to affect
head – A pointer to the first element of the list to append
tail – A pointer to the last element of the list to append

static inline void sys_sflist_merge_sflist(sys_sflist_t *list, sys_sflist_t *list_to_append)

merge two sflists, appending the second one to the first

When the operation is completed, the appending list is empty. This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
list_to_append – A pointer to the list to append.

static inline void sys_sflist_insert(sys_sflist_t *list, sys_sfnode_t *prev, sys_sfnode_t *node)

Insert a node to the given list.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
prev – A pointer on the previous node
node – A pointer on the node to insert

static inline sys_sfnode_t *sys_sflist_get_not_empty(sys_sflist_t *list)

Fetch and remove the first node of the given list.

List must be known to be non-empty. This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect

Returns

A pointer to the first node of the list

static inline sys_sfnode_t *sys_sflist_get(sys_sflist_t *list)

Fetch and remove the first node of the given list.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect

Returns

A pointer to the first node of the list (or NULL if empty)

static inline void sys_sflist_remove(sys_sflist_t *list, sys_sfnode_t *prev_node, sys_sfnode_t *node)

Remove a node.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
prev_node – A pointer on the previous node (can be NULL, which means the node is the list’s head)
node – A pointer on the node to remove

static inline bool sys_sflist_find_and_remove(sys_sflist_t *list, sys_sfnode_t *node)

Find and remove a node from a list.

This and other sys_sflist_*() functions are not thread safe.

Parameters

list – A pointer on the list to affect
node – A pointer on the node to remove from the list

Returns

true if node was removed