diff --git a/asminc/errno.inc b/asminc/errno.inc
index 1efe88cda..2e876c7ac 100644
--- a/asminc/errno.inc
+++ b/asminc/errno.inc
@@ -4,10 +4,10 @@
 
 ; Variables and functions
 
-        .global         __errno, __oserror
-        .global         __osmaperrno
-        .global         __seterrno
-        .global         __directerrno, __mappederrno
+        .global         ___errno, ___oserror
+        .global         ___osmaperrno
+        .global         ___seterrno
+        .global         ___directerrno, ___mappederrno
 
 ; Error codes, must match the values in the C headers
 .enum
diff --git a/include/cbm.h b/include/cbm.h
index cceb76b1b..0679b2d65 100644
--- a/include/cbm.h
+++ b/include/cbm.h
@@ -225,7 +225,7 @@ void cbm_k_untlk (void);
 
 
 
-/* The cbm_* I/O functions below set _oserror (see errno.h),
+/* The cbm_* I/O functions below set __oserror (see errno.h),
 ** in case of an error.
 **
 ** error-code   BASIC error
@@ -251,7 +251,7 @@ unsigned int __fastcall__ cbm_load (const char* name, unsigned char device, void
 ** address of the file if "data" is the null pointer (like load"name",8,1
 ** in BASIC).
 ** Returns number of bytes that were loaded if loading was successful;
-** otherwise 0, "_oserror" contains an error-code, then (see table above).
+** otherwise 0, "__oserror" contains an error-code, then (see table above).
 */
 
 unsigned char __fastcall__ cbm_save (const char* name, unsigned char device,
@@ -274,7 +274,7 @@ void __fastcall__ cbm_close (unsigned char lfn);
 int __fastcall__ cbm_read (unsigned char lfn, void* buffer, unsigned int size);
 /* Reads up to "size" bytes from a file into "buffer".
 ** Returns the number of actually-read bytes, 0 if there are no bytes left.
-** -1 in case of an error; then, _oserror contains an error-code (see table
+** -1 in case of an error; then, __oserror contains an error-code (see table
 ** above).  (Remember:  0 means end-of-file; -1 means error.)
 */
 
@@ -282,7 +282,7 @@ int __fastcall__ cbm_write (unsigned char lfn, const void* buffer,
                             unsigned int size);
 /* Writes up to "size" bytes from "buffer" to a file.
 ** Returns the number of actually-written bytes, or -1 in case of an error;
-** _oserror contains an error-code, then (see above table).
+** __oserror contains an error-code, then (see above table).
 */
 
 unsigned char cbm_opendir (unsigned char lfn, unsigned char device, ...);
diff --git a/include/dio.h b/include/dio.h
index 4201728cc..2521a90ff 100644
--- a/include/dio.h
+++ b/include/dio.h
@@ -35,9 +35,9 @@
 
 
 
-/* Please note: All functions in this file will set _oserror *and* return its
-** value. The only exception is dio_open, which will return NULL, but _oserror
-** will be set. All function will also set _oserror in case of successful
+/* Please note: All functions in this file will set __oserror *and* return its
+** value. The only exception is dio_open, which will return NULL, but __oserror
+** will be set. All function will also set __oserror in case of successful
 ** execution, effectively clearing it.
 */
 
diff --git a/include/errno.h b/include/errno.h
index 92d304938..c762e8b1c 100644
--- a/include/errno.h
+++ b/include/errno.h
@@ -45,12 +45,17 @@
 
 
 /* Operating system specific error code */
-extern unsigned char _oserror;
+extern unsigned char __oserror;
 
-extern int _errno;
+#if __CC65_STD__ >= __CC65_STD_CC65__
+/* define the name with just one underscore for backwards compatibility */
+#define _oserror __oserror
+#endif
+
+extern int __errno;
 /* System errors go here */
 
-#define errno   _errno
+#define errno   __errno
 /* errno must be a macro */
 
 
@@ -83,21 +88,26 @@ extern int _errno;
 
 
 
-int __fastcall__ _osmaperrno (unsigned char oserror);
-/* Map an operating system specific error code (for example from _oserror)
+int __fastcall__ __osmaperrno (unsigned char oserror);
+/* Map an operating system specific error code (for example from __oserror)
 ** into one of the E... codes above. It is user callable.
 */
 
-unsigned char __fastcall__ _seterrno (unsigned char code);
+#if __CC65_STD__ >= __CC65_STD_CC65__
+/* define the name with just one underscore for backwards compatibility */
+#define _osmaperrno __osmaperrno
+#endif
+
+unsigned char __fastcall__ __seterrno (unsigned char code);
 /* Set errno to a specific error code and return zero. Used by the library */
 
-int __fastcall__ _directerrno (unsigned char code);
-/* Set errno to a specific error code, clear _oserror and return -1. Used
+int __fastcall__ __directerrno (unsigned char code);
+/* Set errno to a specific error code, clear __oserror and return -1. Used
 ** by the library.
 */
 
-int __fastcall__ _mappederrno (unsigned char code);
-/* Set _oserror to the given platform specific error code. If it is a real
+int __fastcall__ __mappederrno (unsigned char code);
+/* Set __oserror to the given platform specific error code. If it is a real
 ** error code (not zero) set errno to the corresponding system error code
 ** and return -1. Otherwise return zero.
 ** Used by the library.