lib/utmp.c: Replace UT_LINESIZE by a NITEMS() calculation

A difference between 'struct utmp' and 'struct utmpx' is that
the former uses UT_LINESIZE for the size of its array members,
while the latter doesn't have a standard variable to get its
size.  Therefore, we need to get the number of elements in
the array with NITEMS().

Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/utmp.c

@@ -31,19 +31,23 @@
 #ident "$Id$"
 
 
+#define UTX_LINESIZE  NITEMS(memberof(struct utmpx, ut_line))
+
+
 /*
  * is_my_tty -- determine if "tty" is the same TTY stdin is using
  */
-static bool is_my_tty (const char tty[UT_LINESIZE])
+static bool
+is_my_tty(const char tty[UTX_LINESIZE])
 {
-	char         full_tty[STRLEN("/dev/") + UT_LINESIZE + 1];
+	char         full_tty[STRLEN("/dev/") + UTX_LINESIZE + 1];
 	/* tmptty shall be bigger than full_tty */
 	static char  tmptty[sizeof(full_tty) + 1];
 
 	full_tty[0] = '\0';
 	if (tty[0] != '/')
 		strcpy (full_tty, "/dev/");
-	strncat (full_tty, tty, UT_LINESIZE);
+	strncat(full_tty, tty, UTX_LINESIZE);
 
 	if ('\0' == tmptty[0]) {
 		const char *tname = ttyname (STDIN_FILENO);

src/logoutd.c

@@ -212,7 +212,7 @@ main(int argc, char **argv)
 				tty_name[0] = '\0';
 			}
 
-			strncat (tty_name, ut->ut_line, UT_LINESIZE);
+			strncat(tty_name, ut->ut_line, NITEMS(ut->ut_line));
 #ifndef O_NOCTTY
 #define O_NOCTTY 0
 #endif