diff --git a/sys/amd64/include/in_cksum.h b/sys/amd64/include/in_cksum.h
--- a/sys/amd64/include/in_cksum.h
+++ b/sys/amd64/include/in_cksum.h
@@ -37,8 +37,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#include <sys/cdefs.h>
-
 #define in_cksum(m, len)	in_cksum_skip(m, len, 0)
 
 #ifdef _KERNEL
diff --git a/sys/arm/include/in_cksum.h b/sys/arm/include/in_cksum.h
--- a/sys/arm/include/in_cksum.h
+++ b/sys/arm/include/in_cksum.h
@@ -37,8 +37,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#include <sys/cdefs.h>
-
 #ifdef _KERNEL
 
 #define	HAVE_MD_IN_CKSUM
diff --git a/sys/arm64/include/in_cksum.h b/sys/arm64/include/in_cksum.h
--- a/sys/arm64/include/in_cksum.h
+++ b/sys/arm64/include/in_cksum.h
@@ -35,8 +35,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#include <sys/cdefs.h>
-
 #ifdef _KERNEL
 #define	in_cksum(m, len)	in_cksum_skip(m, len, 0)
 u_short in_addword(u_short sum, u_short b);
diff --git a/sys/powerpc/include/in_cksum.h b/sys/powerpc/include/in_cksum.h
--- a/sys/powerpc/include/in_cksum.h
+++ b/sys/powerpc/include/in_cksum.h
@@ -37,8 +37,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#include <sys/cdefs.h>
-
 #define in_cksum(m, len)	in_cksum_skip(m, len, 0)
 
 #ifdef _KERNEL
diff --git a/sys/riscv/include/in_cksum.h b/sys/riscv/include/in_cksum.h
--- a/sys/riscv/include/in_cksum.h
+++ b/sys/riscv/include/in_cksum.h
@@ -35,8 +35,6 @@
 #ifndef _MACHINE_IN_CKSUM_H_
 #define	_MACHINE_IN_CKSUM_H_	1
 
-#include <sys/cdefs.h>
-
 #ifdef _KERNEL
 #define	in_cksum(m, len)	in_cksum_skip(m, len, 0)
 u_short in_addword(u_short sum, u_short b);