diff --git a/lib/Mathlib/asm/_d_sqrt.s b/lib/Mathlib/asm/_d_sqrt.s
index 6b8042c434..e7e454f0ee 100644
--- a/lib/Mathlib/asm/_d_sqrt.s
+++ b/lib/Mathlib/asm/_d_sqrt.s
@@ -1,6 +1,6 @@
 	.include "macros/function.inc"
 
-	.extern Unk_021D0D40
+	.extern errno
 
 	.text
 
@@ -122,5 +122,5 @@ _020E070C:
 	; .align 2, 0
 _020E0728: .word 0x7FF00000
 _020E072C: .word 0x7FF80000
-_020E0730: .word Unk_021D0D40
+_020E0730: .word errno
 	arm_func_end _dsqrt
\ No newline at end of file
diff --git a/lib/NitroDWC/asm/auth/dwc_auth.s b/lib/NitroDWC/asm/auth/dwc_auth.s
index 19461a1ba6..5648e1ba8f 100644
--- a/lib/NitroDWC/asm/auth/dwc_auth.s
+++ b/lib/NitroDWC/asm/auth/dwc_auth.s
@@ -2,7 +2,7 @@
 	.include "include/dwc_auth.inc"
 
 	.extern OSi_ThreadInfo
-	.extern Unk_021D0D40
+	.extern errno
 	.extern Unk_ov4_0221A43C
 
 	.text
@@ -879,7 +879,7 @@ _021D3C3C:
 	; .align 2, 0
 _021D3C44: .word Unk_ov4_0221A414
 _021D3C48: .word Unk_ov4_02215F40
-_021D3C4C: .word Unk_021D0D40
+_021D3C4C: .word errno
 _021D3C50: .word Unk_ov4_02215F48
 _021D3C54: .word 0x00004E85
 _021D3C58: .word Unk_ov4_02215F54
diff --git a/lib/NitroDWC/asm/auth/dwc_nastime.s b/lib/NitroDWC/asm/auth/dwc_nastime.s
index 570a6f57b3..6e3762c6fa 100644
--- a/lib/NitroDWC/asm/auth/dwc_nastime.s
+++ b/lib/NitroDWC/asm/auth/dwc_nastime.s
@@ -1,7 +1,7 @@
 	.include "macros/function.inc"
 	.include "include/dwc_nastime.inc"
 
-	.extern Unk_021D0D40
+	.extern errno
 
 	.text
 
@@ -144,7 +144,7 @@ _021D6C88:
 	ldmia sp!, {r3, r4, r5, r6, r7, r8, pc}
 	; .align 2, 0
 _021D6C94: .word Unk_ov4_02216570
-_021D6C98: .word Unk_021D0D40
+_021D6C98: .word errno
 _021D6C9C: .word Unk_ov4_02216540
 _021D6CA0: .word Unk_ov4_0221A4B4
 	arm_func_end DWC_Auth_SetNasTimeDiff
diff --git a/lib/NitroDWC/asm/auth/dwc_netcheck.s b/lib/NitroDWC/asm/auth/dwc_netcheck.s
index 858ce5258b..13e5fdf380 100644
--- a/lib/NitroDWC/asm/auth/dwc_netcheck.s
+++ b/lib/NitroDWC/asm/auth/dwc_netcheck.s
@@ -2,7 +2,7 @@
 	.include "include/dwc_netcheck.inc"
 
 	.extern OSi_ThreadInfo
-	.extern Unk_021D0D40
+	.extern errno
 
 	.extern Unk_ov4_02215EA0
 	.text
@@ -1259,7 +1259,7 @@ _021D69C4: .word Unk_ov4_0221635C
 _021D69C8: .word Unk_ov4_0221A450
 _021D69CC: .word OSi_ThreadInfo
 _021D69D0: .word Unk_ov4_02216470
-_021D69D4: .word Unk_021D0D40
+_021D69D4: .word errno
 _021D69D8: .word 0x0000012E
 _021D69DC: .word Unk_ov4_02215EA0
 _021D69E0: .word 0x00004E20
diff --git a/lib/NitroDWC/asm/auth/dwc_svl.s b/lib/NitroDWC/asm/auth/dwc_svl.s
index b56c90b9c3..d137cd5d23 100644
--- a/lib/NitroDWC/asm/auth/dwc_svl.s
+++ b/lib/NitroDWC/asm/auth/dwc_svl.s
@@ -2,7 +2,7 @@
 	.include "include/dwc_svl.inc"
 
 	.extern OSi_ThreadInfo
-	.extern Unk_021D0D40
+	.extern errno
 
 	.extern Unk_ov4_02215EA0
 	.text
@@ -444,7 +444,7 @@ _021D72B4: .word Unk_ov4_0221A4C8
 _021D72B8: .word 0xFFFF9DF3
 _021D72BC: .word Unk_ov4_022165E4
 _021D72C0: .word Unk_ov4_022165EC
-_021D72C4: .word Unk_021D0D40
+_021D72C4: .word errno
 _021D72C8: .word 0xFFFF9E58
 _021D72CC: .word Unk_ov4_022165F8
 _021D72D0: .word 0xFFFFA240
diff --git a/lib/msl/asm/MSL_C/e_pow.s b/lib/msl/asm/MSL_C/e_pow.s
index 0429007ede..741f97ba7a 100644
--- a/lib/msl/asm/MSL_C/e_pow.s
+++ b/lib/msl/asm/MSL_C/e_pow.s
@@ -1,7 +1,7 @@
 	.include "macros/function.inc"
 	.include "include/e_pow.inc"
 
-	.extern Unk_021D0D40
+	.extern errno
 
 	.text
 
@@ -398,7 +398,7 @@ _020DD488: .word 0x43400000
 _020DD48C: .word 0xFFFFFC01
 _020DD490: .word 0x3FE00000
 _020DD494: .word 0x021015B0
-_020DD498: .word Unk_021D0D40
+_020DD498: .word errno
 _020DD49C: .word 0x41E00000
 _020DD4A0: .word 0x3FEFFFFF
 _020DD4A4: .word 0x3FD00000
diff --git a/lib/msl/asm/MSL_C/errno.s b/lib/msl/asm/MSL_C/errno.s
deleted file mode 100644
index ce32e9f2ac..0000000000
--- a/lib/msl/asm/MSL_C/errno.s
+++ /dev/null
@@ -1,11 +0,0 @@
-	.include "macros/function.inc"
-
-	
-
-	.bss
-
-
-	.global Unk_021D0D40
-Unk_021D0D40: ; 0x021D0D40
-	.space 0x4
-
diff --git a/lib/msl/asm/MSL_C/file_pos.s b/lib/msl/asm/MSL_C/file_pos.s
index 7d12b7da16..1f34c6b197 100644
--- a/lib/msl/asm/MSL_C/file_pos.s
+++ b/lib/msl/asm/MSL_C/file_pos.s
@@ -5,7 +5,7 @@
 	.extern Unk_021D0920
 	.extern Unk_021D0944
 	.extern Unk_021D0968
-	.extern Unk_021D0D40
+	.extern errno
 	.extern __files
 
 
@@ -47,7 +47,7 @@ _020D4AA4:
 	subhs r0, r0, r1
 	bx lr
 	; .align 2, 0
-_020D4ADC: .word Unk_021D0D40
+_020D4ADC: .word errno
 	arm_func_end _ftell
 
 	arm_func_start ftell
@@ -264,7 +264,7 @@ _020D4DD0:
 	add sp, sp, #0x10
 	bx lr
 	; .align 2, 0
-_020D4DE0: .word Unk_021D0D40
+_020D4DE0: .word errno
 	arm_func_end _fseek
 
 	arm_func_start fseek
diff --git a/lib/msl/asm/MSL_C/strtold.s b/lib/msl/asm/MSL_C/strtold.s
index 0fe2b09c04..b3c8bdf5fd 100644
--- a/lib/msl/asm/MSL_C/strtold.s
+++ b/lib/msl/asm/MSL_C/strtold.s
@@ -1,7 +1,7 @@
 	.include "macros/function.inc"
 	.include "include/strtold.inc"
 
-	.extern Unk_021D0D40
+	.extern errno
 	.extern Unk_021015AC
 	.extern Unk_021015B4
 
@@ -1403,7 +1403,7 @@ _020DA42C:
 	; .align 2, 0
 _020DA43C: .word __StringRead
 _020DA440: .word 0x7FEFFFFF
-_020DA444: .word Unk_021D0D40
+_020DA444: .word errno
 	arm_func_end strtod
 
 	arm_func_start atof
diff --git a/lib/msl/asm/MSL_C/strtoul.s b/lib/msl/asm/MSL_C/strtoul.s
index 2bfd1c79da..30c57f7522 100644
--- a/lib/msl/asm/MSL_C/strtoul.s
+++ b/lib/msl/asm/MSL_C/strtoul.s
@@ -1,7 +1,7 @@
 	.include "macros/function.inc"
 	.include "include/strtoul.inc"
 
-	.extern Unk_021D0D40
+	.extern errno
 
 	.text
 
@@ -541,7 +541,7 @@ _020DAD28:
 	ldmia sp!, {r3, r4, r5, pc}
 	; .align 2, 0
 _020DAD3C: .word __StringRead
-_020DAD40: .word Unk_021D0D40
+_020DAD40: .word errno
 	arm_func_end strtoul
 
 	arm_func_start strtol
@@ -599,7 +599,7 @@ _020DADF4:
 	ldmia sp!, {r3, r4, r5, pc}
 	; .align 2, 0
 _020DAE04: .word __StringRead
-_020DAE08: .word Unk_021D0D40
+_020DAE08: .word errno
 	arm_func_end strtol
 
 	arm_func_start atoi
diff --git a/lib/msl/src/MSL_C/errno.c b/lib/msl/src/MSL_C/errno.c
new file mode 100644
index 0000000000..10ea7ea558
--- /dev/null
+++ b/lib/msl/src/MSL_C/errno.c
@@ -0,0 +1,14 @@
+#pragma ANSI_strict off
+
+#include <ansi_params.h>
+#include <errno.h>
+
+#if !_MSL_THREADSAFE || !_MSL_LOCALDATA_AVAILABLE
+    #if !defined(__MACH__)
+int errno;
+    #endif
+
+    #if ((__dest_os == __mac_os) || (__dest_os == __mac_os_x))
+short __MacOSErrNo;
+    #endif
+#endif
diff --git a/main.lsf b/main.lsf
index fcaa0e2f3c..121c03065a 100644
--- a/main.lsf
+++ b/main.lsf
@@ -576,7 +576,7 @@ Static main
 	Object lib/msl/asm/MSL_C/math_arm.o
 	Object lib/msl/src/MSL_C/alloc.o
 	Object lib/msl/src/MSL_C/ansi_files.o
-	Object lib/msl/asm/MSL_C/errno.o
+	Object lib/msl/src/MSL_C/errno.o
 	Object lib/msl/src/MSL_C/arith.o
 	Object lib/msl/src/MSL_C/assert.o
 	Object lib/msl/src/MSL_C/buffer_io.o