diff --git a/src/util.h b/src/util.h
index 8da47ae..20c1984 100644
--- a/src/util.h
+++ b/src/util.h
@@ -1,6 +1,8 @@
 #ifndef EPOLL_UTIL_H_
 #define EPOLL_UTIL_H_
 
+#include <stddef.h>
+
 #ifndef _SSIZE_T_DEFINED
 #define SSIZE_T_DEFINED
 typedef intptr_t ssize_t;
@@ -8,4 +10,7 @@ typedef intptr_t ssize_t;
 
 #define array_count(a) (sizeof(a) / (sizeof((a)[0])))
 
+#define container_of(ptr, type, member) \
+  ((type*) ((char*) (ptr) -offsetof(type, member)))
+
 #endif /* EPOLL_UTIL_H_ */