diff --git a/include/stddef.h b/include/stddef.h
index 2f525d7..ccd3857 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -8,4 +8,6 @@ typedef long ptrdiff_t;
 typedef unsigned int wchar_t;
 typedef long max_align_t;
 
+#define offsetof(type, member) ((size_t)&(((type *)0)->member))
+
 #endif
diff --git a/test/offsetof.c b/test/offsetof.c
new file mode 100644
index 0000000..7f16ec9
--- /dev/null
+++ b/test/offsetof.c
@@ -0,0 +1,19 @@
+#include "test.h"
+#include <stddef.h>
+
+typedef struct {
+  int a;
+  char b;
+  int c;
+  double d;
+} T;
+
+int main() {
+  ASSERT(0, offsetof(T, a));
+  ASSERT(4, offsetof(T, b));
+  ASSERT(8, offsetof(T, c));
+  ASSERT(16, offsetof(T, d));
+
+  printf("OK\n");
+  return 0;
+}