libbpf: Add helper macro to clear opts structs

Add a small and generic LIBBPF_OPTS_RESET() helper macros which clears an
opts structure and reinitializes its .sz member to place the structure
size. Additionally, the user can pass option-specific data to reinitialize
via varargs.

I found this very useful when developing selftests, but it is also generic
enough as a macro next to the existing LIBBPF_OPTS() which hides the .sz
initialization, too.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf_common.h

@@ -70,4 +70,20 @@
  }; \
  })
 
+/* Helper macro to clear and optionally reinitialize libbpf options struct
+ *
+ * Small helper macro to reset all fields and to reinitialize the common
+ * structure size member. Values provided by users in struct initializer-
+ * syntax as varargs can be provided as well to reinitialize options struct
+ * specific members.
+ */
+#define LIBBPF_OPTS_RESET(NAME, ...) \
+ do { \
+ memset(&NAME, 0, sizeof(NAME)); \
+ NAME = (typeof(NAME)) { \
+ .sz = sizeof(NAME), \
+ __VA_ARGS__ \
+ }; \
+ } while (0)
+
 #endif /* __LIBBPF_LIBBPF_COMMON_H */