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=========================================BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK=========================================
.. note:: - ``BPF_MAP_TYPE_QUEUE`` and ``BPF_MAP_TYPE_STACK`` were introduced in kernel version 4.20
``BPF_MAP_TYPE_QUEUE`` provides FIFO storage and ``BPF_MAP_TYPE_STACK``provides LIFO storage for BPF programs. These maps support peek, pop andpush operations that are exposed to BPF programs through the respectivehelpers. These operations are exposed to userspace applications usingthe existing ``bpf`` syscall in the following way:
- ``BPF_MAP_LOOKUP_ELEM`` -> peek- ``BPF_MAP_LOOKUP_AND_DELETE_ELEM`` -> pop- ``BPF_MAP_UPDATE_ELEM`` -> push
``BPF_MAP_TYPE_QUEUE`` and ``BPF_MAP_TYPE_STACK`` do not support``BPF_F_NO_PREALLOC``.
Usage=====
Kernel BPF----------
bpf_map_push_elem()~~~~~~~~~~~~~~~~~~~
.. code-block:: c
long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags)
An element ``value`` can be added to a queue or stack using the``bpf_map_push_elem`` helper. The ``flags`` parameter must be set to``BPF_ANY`` or ``BPF_EXIST``. If ``flags`` is set to ``BPF_EXIST`` then,when the queue or stack is full, the oldest element will be removed tomake room for ``value`` to be added. Returns ``0`` on success, ornegative error in case of failure.
bpf_map_peek_elem()~~~~~~~~~~~~~~~~~~~
.. code-block:: c
long bpf_map_peek_elem(struct bpf_map *map, void *value)
This helper fetches an element ``value`` from a queue or stack withoutremoving it. Returns ``0`` on success, or negative error in case offailure.
bpf_map_pop_elem()~~~~~~~~~~~~~~~~~~
.. code-block:: c
long bpf_map_pop_elem(struct bpf_map *map, void *value)
This helper removes an element into ``value`` from a queue orstack. Returns ``0`` on success, or negative error in case of failure.
Userspace---------
bpf_map_update_elem()~~~~~~~~~~~~~~~~~~~~~
.. code-block:: c
int bpf_map_update_elem (int fd, const void *key, const void *value, __u64 flags)
A userspace program can push ``value`` onto a queue or stack using libbpf's``bpf_map_update_elem`` function. The ``key`` parameter must be set to``NULL`` and ``flags`` must be set to ``BPF_ANY`` or ``BPF_EXIST``, with thesame semantics as the ``bpf_map_push_elem`` kernel helper. Returns ``0`` onsuccess, or negative error in case of failure.
bpf_map_lookup_elem()~~~~~~~~~~~~~~~~~~~~~
.. code-block:: c
int bpf_map_lookup_elem (int fd, const void *key, void *value)
A userspace program can peek at the ``value`` at the head of a queue or stackusing the libbpf ``bpf_map_lookup_elem`` function. The ``key`` parameter must beset to ``NULL``. Returns ``0`` on success, or negative error in case offailure.
bpf_map_lookup_and_delete_elem()~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. code-block:: c
int bpf_map_lookup_and_delete_elem (int fd, const void *key, void *value)
A userspace program can pop a ``value`` from the head of a queue or stack usingthe libbpf ``bpf_map_lookup_and_delete_elem`` function. The ``key`` parametermust be set to ``NULL``. Returns ``0`` on success, or negative error in case offailure.
Examples========
Kernel BPF----------
This snippet shows how to declare a queue in a BPF program:
.. code-block:: c
struct { __uint(type, BPF_MAP_TYPE_QUEUE); __type(value, __u32); __uint(max_entries, 10); } queue SEC(".maps");
Userspace---------
This snippet shows how to use libbpf's low-level API to create a queue fromuserspace:
.. code-block:: c
int create_queue() { return bpf_map_create(BPF_MAP_TYPE_QUEUE, "sample_queue", /* name */ 0, /* key size, must be zero */ sizeof(__u32), /* value size */ 10, /* max entries */ NULL); /* create options */ }
References==========
https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/