+# ifndef FILE_SYSTEM_PREFIX_LEN
+# if FILE_SYSTEM_ACCEPTS_DRIVE_LETTER_PREFIX
+ /* This internal macro assumes ASCII, but all hosts that support drive
+ letters use ASCII. */
+# define _IS_DRIVE_LETTER(c) (((unsigned int) (c) | ('a' - 'A')) - 'a' \
+ <= 'z' - 'a')
+# define FILE_SYSTEM_PREFIX_LEN(Filename) \
+ (_IS_DRIVE_LETTER ((Filename)[0]) && (Filename)[1] == ':' ? 2 : 0)
+# else
+# define FILE_SYSTEM_PREFIX_LEN(Filename) 0
+# endif
+# endif
+
+# ifndef FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+# define FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE 0
+# endif
+
+# ifndef DOUBLE_SLASH_IS_DISTINCT_ROOT
+# define DOUBLE_SLASH_IS_DISTINCT_ROOT 0
+# endif
+
+# if FILE_SYSTEM_DRIVE_PREFIX_CAN_BE_RELATIVE
+# define IS_ABSOLUTE_FILE_NAME(F) ISSLASH ((F)[FILE_SYSTEM_PREFIX_LEN (F)])
+# else
+# define IS_ABSOLUTE_FILE_NAME(F) \
+ (ISSLASH ((F)[0]) || 0 < FILE_SYSTEM_PREFIX_LEN (F))