diff --git c/common.mk i/common.mk
index 89c042e..43ff7fb 100644
--- c/common.mk
+++ i/common.mk
@@ -94,6 +94,7 @@ COMMONOBJS    = array.$(OBJEXT) \
 		vm_trace.$(OBJEXT) \
 		thread.$(OBJEXT) \
 		cont.$(OBJEXT) \
+		sizes.$(OBJEXT) \
 		$(BUILTIN_ENCOBJS) \
 		$(BUILTIN_TRANSOBJS) \
 		$(MISSING)
@@ -817,6 +818,8 @@ golf_prelude.$(OBJEXT): {$(VPATH)}golf_prelude.c $(RUBY_H_INCLUDES) \
 goruby.$(OBJEXT): {$(VPATH)}goruby.c {$(VPATH)}main.c $(RUBY_H_INCLUDES) \
   {$(VPATH)}vm_debug.h {$(VPATH)}node.h $(hdrdir)/ruby.h
 
+sizes.$(OBJEXT): {$(VPATH)}sizes.c $(RUBY_H_INCLUDES)
+
 ascii.$(OBJEXT): {$(VPATH)}ascii.c {$(VPATH)}regenc.h {$(VPATH)}config.h \
   {$(VPATH)}oniguruma.h {$(VPATH)}missing.h $(RUBY_H_INCLUDES)
 us_ascii.$(OBJEXT): {$(VPATH)}us_ascii.c {$(VPATH)}regenc.h \
diff --git c/inits.c i/inits.c
index fe0aade..f6611a0 100644
--- c/inits.c
+++ i/inits.c
@@ -61,5 +61,6 @@ rb_call_inits(void)
     CALL(Complex);
     CALL(version);
     CALL(vm_trace);
+    CALL(sizes);
 }
 #undef CALL
diff --git c/sizes.c i/sizes.c
new file mode 100644
index 0000000..398ee95
--- /dev/null
+++ i/sizes.c
@@ -0,0 +1,52 @@
+#include "ruby/ruby.h"
+
+void
+Init_sizes(void)
+{
+    VALUE s = rb_hash_new();
+    rb_define_const(rb_define_module("RbConfig"), "SIZEOF", s);
+
+#define DEFINE(type, size) rb_hash_aset(s, rb_str_new_cstr(#type), INT2FIX(SIZEOF_##size));
+
+#ifdef SIZEOF_INT
+    DEFINE(int, INT);
+#endif
+#ifdef SIZEOF_SHORT
+    DEFINE(short, SHORT);
+#endif
+#ifdef SIZEOF_LONG
+    DEFINE(long, LONG);
+#endif
+#ifdef SIZEOF_LONG_LONG
+    DEFINE(long long, LONG_LONG);
+#endif
+#ifdef SIZEOF___INT64
+    DEFINE(__int64, __INT64);
+#endif
+#ifdef SIZEOF___INT128
+    DEFINE(__int128, __INT128);
+#endif
+#ifdef SIZEOF_OFF_T
+    DEFINE(off_t, OFF_T);
+#endif
+#ifdef SIZEOF_VOIDP
+    DEFINE(void*, VOIDP);
+#endif
+#ifdef SIZEOF_FLOAT
+    DEFINE(float, FLOAT);
+#endif
+#ifdef SIZEOF_DOUBLE
+    DEFINE(double, DOUBLE);
+#endif
+#ifdef SIZEOF_TIME_T
+    DEFINE(time_t, TIME_T);
+#endif
+#ifdef SIZEOF_SIZE_T
+    DEFINE(size_t, SIZE_T);
+#endif
+#ifdef SIZEOF_PTRDIFF_T
+    DEFINE(ptrdiff_t, PTRDIFF_T);
+#endif
+
+#undef DEFINE
+}