diff --git a/src/common.c b/src/common.c
new file mode 100644
index 0000000..eb86c18
--- /dev/null
+++ b/src/common.c
@@ -0,0 +1,151 @@
+/* common.c */
+
+
+
+#ifdef COMMON_MODE
+# include <ctype.h>
+
+/* mode
+ * PRE	*mask exists
+ * POST	mode is converted a mask
+ * 	1	an error happened */
+static int _mode_alpha(char const * mode, mode_t * mask);
+static int _mode_digit(char const * mode, mode_t * mask);
+static int _mode(char const * mode, mode_t * mask)
+	/* FIXME need a way to tell whether to =, & or | the resulting mask */
+{
+	if(*mode == '\0')
+		return 1;
+	if(_mode_digit(mode, mask) == 0)
+		return 0;
+	return _mode_alpha(mode, mask);
+}
+
+static int _mode_digit(char const * mode, mode_t * mask)
+{
+	char const * p;
+
+	for(p = mode; isdigit(*p) && *p < '8'; p++);
+	if(*p == '\0')
+	{
+		*mask = strtol(mode, NULL, 8);
+		return 0;
+	}
+	return 1;
+}
+
+static int _symbolic_mode(char const ** mode);
+static int _mode_alpha(char const * mode, mode_t * mask)
+{
+	/* FIXME doesn't give any result */
+	return _symbolic_mode(&mode);
+}
+
+static void _clause(char const ** mode);
+static int _symbolic_mode(char const ** mode)
+	/* clause { ',' clause } */
+{
+	_clause(mode);
+	while(**mode == ',')
+	{
+		(*mode)++;
+		_clause(mode);
+	}
+	return **mode == '\0' ? 0 : 1;
+}
+
+static void _wholist(char const ** mode);
+static void _actionlist(char const ** mode);
+static void _clause(char const ** mode)
+	/* [wholist] actionlist */
+{
+	char c = **mode;
+
+	if(c == 'u' || c == 'g' || c == 'o' || c == 'a')
+		_wholist(mode);
+	_actionlist(mode);
+}
+
+static void _who(char const ** mode);
+static void _wholist(char const ** mode)
+	/* who { who } */
+{
+	char c;
+
+	_who(mode);
+	for(c = ** mode; c == 'u' || c == 'g' || c == 'o' || c == 'a';
+			c = **mode)
+		_who(mode);
+}
+
+static void _who(char const ** mode)
+	/* 'u' | 'g' | 'o' | 'a' */
+{
+	/* FIXME */
+	(*mode)++;
+}
+
+static void _action(char const ** mode);
+static void _actionlist(char const ** mode)
+	/* action { action } */
+{
+	char c;
+
+	for(c = **mode; c == '+' || c == '-' || c == '='; c = **mode)
+		_action(mode);
+}
+
+static void _op(char const ** mode);
+static void _permlist(char const ** mode);
+static void _permcopy(char const ** mode);
+static void _action(char const ** mode)
+	/* op [(permlist | permcopy)] */
+{
+	char c;
+
+	_op(mode);
+	for(c = **mode; c != '\0'; c = **mode)
+	{
+		if(c == 'u' || c == 'g' || c == 'o')
+			_permcopy(mode);
+		else if(c == 'r' || c == 'w' || c == 'x' || c == 'X' || c == 's'
+				|| c == 't')
+			_permlist(mode);
+		else
+			break;
+	}
+}
+
+static void _permcopy(char const ** mode)
+	/* 'u' | 'g' | 'o' */
+{
+	/* FIXME */
+	(*mode)++;
+}
+
+static void _op(char const ** mode)
+	/* '+' | '-' | '=' */
+{
+	/* FIXME */
+	(*mode)++;
+}
+
+static void _perm(char const ** mode);
+static void _permlist(char const ** mode)
+	/* perm { perm } */
+{
+	char c;
+
+	_perm(mode);
+	for(c = **mode; c == 'r' || c == 'w' || c == 'x' || c == 'X'
+			|| c == 's' || c == 't'; c = **mode)
+		_perm(mode);
+}
+
+static void _perm(char const ** mode)
+	/* 'r' | 'w' | 'x' | 'X' | 's' | 't' */
+{
+	/* FIXME */
+	(*mode)++;
+}
+#endif /* COMMON_MODE */