From: Rong Tao <rongtao@xxxxxxxx> since commit 5854a09b4957("net/ipv4: Use __DECLARE_FLEX_ARRAY() helper") linux/in.h use __DECLARE_FLEX_ARRAY() macro, and sync to tools/ in commit 036b8f5b8970("tools headers uapi: Update linux/in.h copy"), this macro define in linux/stddef.h, which introduced in commit 3080ea5553cc("stddef: Introduce DECLARE_FLEX_ARRAY() helper"), thus, stddef.h should be included in in.h, which resolves the compilation error below: How to reproduce this compilation error: $ make -C tools/testing/selftests/bpf/ In file included from progs/bpf_flow.c:8: linux/in.h:199:3: error: type name requires a specifier or qualifier __DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex); ^ linux/in.h:199:32: error: type specifier missing, defaults to 'int' [-Werror,-Wimplicit-int] __DECLARE_FLEX_ARRAY(__be32, imsf_slist_flex); ^ 2 errors generated. Same error occurs with cgroup_skb_sk_lookup_kern.c, connect_force_port4.c, connect_force_port6.c, etc. that contain the header linux/in.h. Signed-off-by: Rong Tao <rongtao@xxxxxxxx> --- include/uapi/linux/in.h | 1 + tools/include/uapi/linux/in.h | 1 + 2 files changed, 2 insertions(+) diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h index f243ce665f74..07a4cb149305 100644 --- a/include/uapi/linux/in.h +++ b/include/uapi/linux/in.h @@ -20,6 +20,7 @@ #define _UAPI_LINUX_IN_H #include <linux/types.h> +#include <linux/stddef.h> #include <linux/libc-compat.h> #include <linux/socket.h> diff --git a/tools/include/uapi/linux/in.h b/tools/include/uapi/linux/in.h index f243ce665f74..07a4cb149305 100644 --- a/tools/include/uapi/linux/in.h +++ b/tools/include/uapi/linux/in.h @@ -20,6 +20,7 @@ #define _UAPI_LINUX_IN_H #include <linux/types.h> +#include <linux/stddef.h> #include <linux/libc-compat.h> #include <linux/socket.h> -- 2.31.1