diff --git a/src/utl/mutex.h b/src/utl/mutex.h
index 7b51e3e..ef5be43 100644
--- a/src/utl/mutex.h
+++ b/src/utl/mutex.h
@@ -22,6 +22,9 @@
  *  - cmc_mtx_trylock
  */
 
+#ifndef CMC_MUTEX_H
+#define CMC_MUTEX_H
+
 #include <stdbool.h>
 
 #include "../cor/flags.h"
@@ -210,3 +213,5 @@ static inline bool cmc_mtx_trylock(struct cmc_mutex *mtx)
     return err == 0;
 #endif
 }
+
+#endif /* CMC_MUTEX_H */
diff --git a/src/utl/thread.h b/src/utl/thread.h
index 635860e..bf309fb 100644
--- a/src/utl/thread.h
+++ b/src/utl/thread.h
@@ -20,6 +20,9 @@
  *  - cmc_thrd_join
  */
 
+#ifndef CMC_THREAD_H
+#define CMC_THREAD_H
+
 #include <inttypes.h>
 #include <stdbool.h>
 
@@ -171,3 +174,5 @@ static inline bool cmc_thrd_join(struct cmc_thread *thr, int *result)
 
 #endif
 }
+
+#endif /* CMC_THREAD_H */