diff --git a/debug.c b/debug.c
index f811ec6779..e4ad163ad1 100644
--- a/debug.c
+++ b/debug.c
@@ -11,6 +11,7 @@
 
 #include "ruby/ruby.h"
 #include "ruby/encoding.h"
+#include "ruby/io.h"
 #include "ruby/util.h"
 #include "vm_debug.h"
 #include "eval_intern.h"
diff --git a/gc.c b/gc.c
index b00dda8ffb..d0144f5266 100644
--- a/gc.c
+++ b/gc.c
@@ -14,13 +14,13 @@
 #define rb_data_object_alloc rb_data_object_alloc
 #define rb_data_typed_object_alloc rb_data_typed_object_alloc
 
-#include "internal.h"
+#include "ruby/io.h"
 #include "ruby/st.h"
 #include "ruby/re.h"
-#include "ruby/io.h"
 #include "ruby/thread.h"
 #include "ruby/util.h"
 #include "ruby/debug.h"
+#include "internal.h"
 #include "eval_intern.h"
 #include "vm_core.h"
 #include "gc.h"
diff --git a/internal.h b/internal.h
index 119726b384..1a965991d3 100644
--- a/internal.h
+++ b/internal.h
@@ -14,7 +14,6 @@
 
 #include "ruby.h"
 #include "ruby/encoding.h"
-#include "ruby/io.h"
 
 #if defined(__cplusplus)
 extern "C" {
@@ -1315,7 +1314,9 @@ void ruby_set_inplace_mode(const char *);
 ssize_t rb_io_bufread(VALUE io, void *buf, size_t size);
 void rb_stdio_set_default_encoding(void);
 VALUE rb_io_flush_raw(VALUE, int);
+#ifdef RUBY_IO_H
 size_t rb_io_memsize(const rb_io_t *);
+#endif
 int rb_stderr_tty_p(void);
 
 /* load.c */
diff --git a/string.c b/string.c
index 75db0d22e8..390a9c3a58 100644
--- a/string.c
+++ b/string.c
@@ -24,6 +24,7 @@
 #define BEG(no) (regs->beg[(no)])
 #define END(no) (regs->end[(no)])
 
+#include <errno.h>
 #include <math.h>
 #include <ctype.h>
 
diff --git a/win32/win32.c b/win32/win32.c
index 5cb842ff76..025c4a3726 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -23,6 +23,7 @@
 
 #include "ruby/ruby.h"
 #include "ruby/encoding.h"
+#include "ruby/io.h"
 #include "ruby/util.h"
 #include <fcntl.h>
 #include <process.h>