Allow setting errno instead of just clearing it

basis/libc/libc.factor

@@ -9,8 +9,14 @@ IN: libc
 : errno ( -- int )
     int "factor" "err_no" { } alien-invoke ;
 
+: set-errno ( int -- )
+    void "factor" "set_err_no" { int } alien-invoke ;
+
 : clear-errno ( -- )
-    void "factor" "clear_err_no" { } alien-invoke ;
+    0 set-errno ;
+
+: preserve-errno ( quot -- )
+    errno [ call ] dip set-errno ; inline
 
 <PRIVATE
 

vm/io.cpp

@@ -245,8 +245,8 @@ VM_C_API int err_no()
 	return errno;
 }
 
-VM_C_API void clear_err_no()
+VM_C_API void set_err_no(int err)
 {
-	errno = 0;
+	errno = err;
 }
 }

vm/io.hpp

@@ -8,6 +8,6 @@ int safe_fclose(FILE *stream);
 /* Platform specific primitives */
 
 VM_C_API int err_no();
-VM_C_API void clear_err_no();
+VM_C_API void set_err_no(int err);
 
 }