bpf: add a new API libbpf_num_possible_cpus()

Adding a new API libbpf_num_possible_cpus() that helps user with
per-CPU map operations.

Signed-off-by: Hechao Li <hechaol@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

src/libbpf.h

@@ -454,6 +454,22 @@ bpf_program__bpil_addr_to_offs(struct bpf_prog_info_linear *info_linear);
 LIBBPF_API void
 bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear);
 
+/*
+ * A helper function to get the number of possible CPUs before looking up
+ * per-CPU maps. Negative errno is returned on failure.
+ *
+ * Example usage:
+ *
+ *     int ncpus = libbpf_num_possible_cpus();
+ *     if (ncpus < 0) {
+ *          // error handling
+ *     }
+ *     long values[ncpus];
+ *     bpf_map_lookup_elem(per_cpu_map_fd, key, values);
+ *
+ */
+LIBBPF_API int libbpf_num_possible_cpus(void);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif