diff --git a/sys/posix4/_semaphore.h b/sys/posix4/_semaphore.h
index 2c330e881fe7..91106ff99e51 100644
--- a/sys/posix4/_semaphore.h
+++ b/sys/posix4/_semaphore.h
@@ -29,6 +29,7 @@
 #define __SEMAPHORE_H_
 
 typedef intptr_t semid_t;
+struct timespec;
 
 #ifndef _KERNEL
 
diff --git a/sys/posix4/semaphore.h b/sys/posix4/semaphore.h
index f451d5f6dc71..180625221f2e 100644
--- a/sys/posix4/semaphore.h
+++ b/sys/posix4/semaphore.h
@@ -47,6 +47,8 @@ typedef	struct sem *	sem_t;
 #ifndef _KERNEL
 #include <sys/cdefs.h>
 
+struct timespec;
+
 __BEGIN_DECLS
 int	 sem_close(sem_t *);
 int	 sem_destroy(sem_t *);
diff --git a/sys/sys/_semaphore.h b/sys/sys/_semaphore.h
index 2c330e881fe7..91106ff99e51 100644
--- a/sys/sys/_semaphore.h
+++ b/sys/sys/_semaphore.h
@@ -29,6 +29,7 @@
 #define __SEMAPHORE_H_
 
 typedef intptr_t semid_t;
+struct timespec;
 
 #ifndef _KERNEL
 
diff --git a/sys/sys/semaphore.h b/sys/sys/semaphore.h
index f451d5f6dc71..180625221f2e 100644
--- a/sys/sys/semaphore.h
+++ b/sys/sys/semaphore.h
@@ -47,6 +47,8 @@ typedef	struct sem *	sem_t;
 #ifndef _KERNEL
 #include <sys/cdefs.h>
 
+struct timespec;
+
 __BEGIN_DECLS
 int	 sem_close(sem_t *);
 int	 sem_destroy(sem_t *);