On 10/4/22 23:19, Donald Hunter wrote:
> +Examples
> +========
> +
> +Please see the ``tools/testing/selftests/bpf`` directory for functional
> +examples. The sample code below demonstrates API usage.
> +
Since you have many code snippets, better say "The code samples below".
> +Kernel
> +------
> +
> +This snippet shows how to declare an array in a BPF program.
> +
> +.. code-block:: c
> +
> + struct {
> + __uint(type, BPF_MAP_TYPE_ARRAY);
> + __type(key, u32);
> + __type(value, long);
> + __uint(max_entries, 256);
> + } my_map SEC(".maps");
> +
> +
> +This example shows how to access an array element.
> +
> +.. code-block:: c
> +
> + int bpf_prog(struct __sk_buff *skb)
> + {
> + int index = load_byte(skb,
> + ETH_HLEN + offsetof(struct iphdr, protocol));
> + long *value;
> +
> + if (skb->pkt_type != PACKET_OUTGOING)
> + return 0;
> +
> + value = bpf_map_lookup_elem(&my_map, &index);
> + if (value)
> + __sync_fetch_and_add(value, skb->len);
> +
> + return 0;
> + }
> +
> +Userspace
> +---------
> +
> +BPF_MAP_TYPE_ARRAY
> +~~~~~~~~~~~~~~~~~~
> +
> +This example shows array creation, initialisation and lookup from userspace.
> +
"Initialize the array, set elements, and perform lookup".
> +.. code-block:: c
> +
> + #include <assert.h>
> + #include <bpf/libbpf.h>
> + #include <bpf/bpf.h>
> +
> + int main(int argc, char **argv)
> + {
> + int fd;
> + int ret = 0;
> + long value;
> + __u32 index = 42;
> + __u32 i;
> +
> + fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "example_array",
> + sizeof(__u32), sizeof(long),
> + 256, 0);
> + if (fd < 0)
> + return fd;
> +
> + /* fill the map with values from 0-255 */
> + for (i = 0; i < 256 ; i++) {
> + value = i;
> + ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY);
> + if (ret < 0)
> + return ret;
> + }
> +
> + ret = bpf_map_lookup_elem(fd, &index, &value);
> + if (ret < 0)
> + return ret;
> +
> + assert(value == 42);
> +
> + return ret;
> + }
> +
> +BPF_MAP_TYPE_PERCPU_ARRAY
> +~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +This example shows per CPU array usage.
> +
> +.. code-block:: c
> +
> + #include <assert.h>
> + #include <bpf/libbpf.h>
> + #include <bpf/bpf.h>
> +
> + int main(int argc, char **argv)
> + {
> + int ncpus = libbpf_num_possible_cpus();
> + if (ncpus < 0)
> + return ncpus;
> +
> + int fd;
> + int ret = 0;
> + __u32 i, j;
> + __u32 index = 42;
> + long v[ncpus], value[ncpus];
> +
> + fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "example_percpu",
> + sizeof(__u32), sizeof(long), 256, 0);
> + if (fd < 0)
> + return -1;
> +
> + /* fill the map with values from 0-255 for each cpu */
> + for (i = 0; i < 256 ; i++) {
> + for (j = 0; j < ncpus; j++)
> + v[j] = i;
> + ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY);
> + if (ret < 0)
> + return ret;
> + }
> +
> + ret = bpf_map_lookup_elem(fd, &index, &value);
> + if (ret < 0)
> + return ret;
> +
> + for (j = 0; j < ncpus; j++)
> + assert(value[j] == 42);
> +
> + return ret;
> + }
> +
What is the purpose of above snippet? Give more detailed explanation.
Thanks.
--
An old man doll... just what I always wanted! - Clara
