Allow resetting an empty inferior-tty

This patch allows the user to set the inferior-tty to "empty", in order
to come back to the default behaviour of using the same tty as gdb is
using.

This is already supported in MI (and tested in gdb.mi/mi-basics.exp).

I added a new test, set-inferior-tty.exp, where I test only the setting
and unsetting of the parameter.  It would be nice to actually test that
the inferior output properly goes to the separate tty, but that will be
for another day.

gdb/ChangeLog:

	* infcmd.c (set_inferior_io_terminal): Set inferior terminal to
	NULL if terminal_name is an empty string.
	(_initialize_infcmd): Make the argument of "set inferior-tty"
	optional, mention it in the help doc.

gdb/doc/ChangeLog:

	* gdb.texinfo (Input/Output): Mention possibility to unset
	inferior-tty.

gdb/testsuite/ChangeLog:

	* gdb.base/set-inferior-tty.exp: New file.
	* gdb.base/set-inferior-tty.c: New file.

1 files changed, 13 insertions, 7 deletions

diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index 58ba1cb..44a1fd1 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -151,7 +151,11 @@ void
 set_inferior_io_terminal (const char *terminal_name)
 {
   xfree (current_inferior ()->terminal);
-  current_inferior ()->terminal = terminal_name ? xstrdup (terminal_name) : 0;
+
+  if (terminal_name != NULL && *terminal_name != '\0')
+    current_inferior ()->terminal = xstrdup (terminal_name);
+  else
+    current_inferior ()->terminal = NULL;
 }
 
 const char *
@@ -3224,14 +3228,16 @@ _initialize_infcmd (void)
   const char *cmd_name;
 
   /* Add the filename of the terminal connected to inferior I/O.  */
-  add_setshow_filename_cmd ("inferior-tty", class_run,
-			    &inferior_io_terminal_scratch, _("\
+  add_setshow_optional_filename_cmd ("inferior-tty", class_run,
+				     &inferior_io_terminal_scratch, _("\
 Set terminal for future runs of program being debugged."), _("\
 Show terminal for future runs of program being debugged."), _("\
-Usage: set inferior-tty /dev/pts/1"),
-			    set_inferior_tty_command,
-			    show_inferior_tty_command,
-			    &setlist, &showlist);
+Usage: set inferior-tty [TTY]\n\n\
+If TTY is omitted, the default behavior of using the same terminal as GDB\n\
+is restored."),
+				     set_inferior_tty_command,
+				     show_inferior_tty_command,
+				     &setlist, &showlist);
   add_com_alias ("tty", "set inferior-tty", class_alias, 0);
 
   cmd_name = "args";