diff --git a/include/errno.h b/include/errno.h
index c762e8b1c..8f329e3cc 100644
--- a/include/errno.h
+++ b/include/errno.h
@@ -101,11 +101,21 @@ int __fastcall__ __osmaperrno (unsigned char oserror);
 unsigned char __fastcall__ __seterrno (unsigned char code);
 /* Set errno to a specific error code and return zero. Used by the library */
 
+#if __CC65_STD__ >= __CC65_STD_CC65__
+/* define the name with just one underscore for backwards compatibility */
+#define _seterrno __seterrno
+#endif
+
 int __fastcall__ __directerrno (unsigned char code);
 /* Set errno to a specific error code, clear __oserror and return -1. Used
 ** by the library.
 */
 
+#if __CC65_STD__ >= __CC65_STD_CC65__
+/* define the name with just one underscore for backwards compatibility */
+#define _directerrno __directerrno
+#endif
+
 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
@@ -113,7 +123,10 @@ int __fastcall__ __mappederrno (unsigned char code);
 ** Used by the library.
 */
 
-
+#if __CC65_STD__ >= __CC65_STD_CC65__
+/* define the name with just one underscore for backwards compatibility */
+#define _mappederrno __mappederrno
+#endif
 
 /* End of errno.h */
 #endif
diff --git a/libsrc/cbm/dir.s b/libsrc/cbm/dir.s
index 808fcf982..734485aaf 100644
--- a/libsrc/cbm/dir.s
+++ b/libsrc/cbm/dir.s
@@ -92,8 +92,8 @@ L3:     sta     tmp1            ; Save returned count
 ; Didn't read enough bytes. This is an error for us, but errno is not set
 
         lda     #<EIO
-        sta     __errno
-        stx     __errno+1       ; X is zero
+        sta     ___errno
+        stx     ___errno+1      ; X is zero
         bne     L1              ; Branch always
 
 
diff --git a/libsrc/cbm/readdir.c b/libsrc/cbm/readdir.c
index 1512edc4e..25627a9cb 100644
--- a/libsrc/cbm/readdir.c
+++ b/libsrc/cbm/readdir.c
@@ -126,7 +126,7 @@ struct dirent* __fastcall__ readdir (register DIR* dir)
     }
 
     /* Something went wrong when parsing the directory entry */
-    _errno = EIO;
+    __errno = EIO;
 exitpoint:
     return 0;
 }
diff --git a/libsrc/common/_seterrno.s b/libsrc/common/_seterrno.s
index 30394a9cb..4344a3302 100644
--- a/libsrc/common/_seterrno.s
+++ b/libsrc/common/_seterrno.s
@@ -1,7 +1,7 @@
 ;
 ; Ullrich von Bassewitz, 2004-05-13
 ;
-; ___seterrno: Will set __errno to the value in A and return zero in A. Other
+; ___seterrno: Will set ___errno to the value in A and return zero in A. Other
 ;             registers aren't changed. The function is C callable, but
 ;             currently only called from asm code.
 ;
@@ -12,9 +12,9 @@
 
 .proc   ___seterrno
 
-        sta     __errno
+        sta     ___errno
         lda     #0
-        sta     __errno+1
+        sta     ___errno+1
         rts
 
 .endproc
diff --git a/libsrc/common/errno.s b/libsrc/common/errno.s
index f448c3c14..43bbe3fb0 100644
--- a/libsrc/common/errno.s
+++ b/libsrc/common/errno.s
@@ -2,7 +2,7 @@
 ; 2003-08-12, Ullrich von Bassewitz
 ; 2015-09-24, Greg King
 ;
-; extern int _errno;
+; extern int __errno;
 ; /* Library errors go here. */
 ;
 
@@ -10,5 +10,5 @@
 
 .bss
 
-__errno:
+___errno:
         .word   0
diff --git a/libsrc/common/perror.c b/libsrc/common/perror.c
index ddc015c1e..9b0dac7de 100644
--- a/libsrc/common/perror.c
+++ b/libsrc/common/perror.c
@@ -42,7 +42,7 @@
 void __fastcall__ perror (const char* msg)
 {
     /* Fetch the message that corresponds to errno */
-    const char* errormsg = strerror (_errno);
+    const char* errormsg = strerror (__errno);
 
     /* Different output depending on msg */
     if (msg) {
diff --git a/libsrc/common/strerror.s b/libsrc/common/strerror.s
index 0f41331ea..8cc7c7441 100644
--- a/libsrc/common/strerror.s
+++ b/libsrc/common/strerror.s
@@ -19,9 +19,9 @@ _strerror:
 ; The given error code is invalid
 
 @L1:    lda     #<EINVAL
-        sta     __errno
+        sta     ___errno
         lda     #>EINVAL        ; = 0
-        sta     __errno+1
+        sta     ___errno+1
 ;       lda     #$00            ; A contains zero: "Unknown error"
 
 ; Load the pointer to the error message and return