diff --git a/os/various/syscalls.c b/os/various/syscalls.c
index fd2449b40..60b61c9a3 100644
--- a/os/various/syscalls.c
+++ b/os/various/syscalls.c
@@ -65,6 +65,11 @@
 #include "hal.h"
 #endif
 
+#ifndef __errno_r
+#include <sys/reent.h>
+#define __errno_r(reent) reent->_errno
+#endif
+
 /***************************************************************************/
 
 int _read_r(struct _reent *r, int file, char * ptr, int len)
diff --git a/readme.txt b/readme.txt
index 3f4f88620..73a5d47e7 100644
--- a/readme.txt
+++ b/readme.txt
@@ -91,8 +91,9 @@
 *** 2.7.0 ***
 - FIX: Fixed wrong RTC macro names in STM32L1xx HAL (bug #422)(backported to
   2.6.1 and 2.4.5).
-- FIX: Fixed FSMC reset on STM32F4xx (bug #420)(backported to 2.6.1
-  and 2.4.4).
+- FIX: Fixed CodeSourcery personal version fails to build with undefined
+  errno_r (bug #421)(backported to 2.6.1).
+- FIX: Fixed FSMC reset on STM32F4xx (bug #420)(backported to 2.6.1 and 2.4.4).
 - FIX: Fixed invalid directory links in the demo files (bug #419)(backported
   to 2.6.1).
 - FIX: Fixed missing casts in time-conversion macros (bug #418)(backported