Add a new $_inferior convenience variable

Like $_thread, but holds the current inferior number.

gdb/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention $_inferior.
	* inferior.c (inferior_id_make_value): New.
	(inferior_funcs): New.
	(_initialize_inferior): Create $_inferior variable.

gdb/testsuite/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/default.exp: Expect $_inferior as well.
	* gdb.multi/base.exp: Test $_inferior.

gdb/doc/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Inferiors and Programs): Document the $_inferior
	convenience variable.
	(Convenience Vars): Likewise.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2016-01-13  Pedro Alves  <palves@redhat.com>
+
+	* NEWS: Mention $_inferior.
+	* inferior.c (inferior_id_make_value): New.
+	(inferior_funcs): New.
+	(_initialize_inferior): Create $_inferior variable.
+
 2016-01-13  Pedro Alves  <palves@redhat.com>
 
 	PR breakpoints/19388

gdb/NEWS

@@ -3,6 +3,9 @@
 
 *** Changes since GDB 7.10
 
+* The new convenience variable $_inferior holds the number of the
+  current inferior.
+
 * Record btrace now supports non-stop mode.
 
 * Support for tracepoints on aarch64-linux was added in GDBserver.

gdb/doc/ChangeLog

@@ -1,3 +1,9 @@
+2016-01-13  Pedro Alves  <palves@redhat.com>
+
+	* gdb.texinfo (Inferiors and Programs): Document the $_inferior
+	convenience variable.
+	(Convenience Vars): Likewise.
+
 2016-01-12  Josh Stone  <jistone@redhat.com>
 	    Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 

gdb/doc/gdb.texinfo

@@ -2658,6 +2658,12 @@ Make inferior number @var{infno} the current inferior.  The argument
 in the first field of the @samp{info inferiors} display.
 @end table
 
+@vindex $_inferior@r{, convenience variable}
+The debugger convenience variable @samp{$_inferior} contains the
+number of the current inferior.  You may find this useful in writing
+breakpoint conditional expressions, command scripts, and so forth.
+@xref{Convenience Vars,, Convenience Variables}, for general
+information on convenience variables.
 
 You can get multiple executables into a debugging session via the
 @code{add-inferior} and @w{@code{clone-inferior}} commands.  On some
@@ -10382,6 +10388,10 @@ gdbserver that supports the @code{qGetTIBAddr} request.
 @xref{General Query Packets}.
 This variable contains the address of the thread information block.
 
+@item $_inferior
+The number of the current inferior.  @xref{Inferiors and
+Programs, ,Debugging Multiple Inferiors and Programs}.
+
 @end table
 
 @node Convenience Funs

gdb/inferior.c

@@ -1008,6 +1008,26 @@ show_print_inferior_events (struct ui_file *file, int from_tty,
   fprintf_filtered (file, _("Printing of inferior events is %s.\n"), value);
 }
 
+/* Return a new value for the selected inferior's id.  */
+
+static struct value *
+inferior_id_make_value (struct gdbarch *gdbarch, struct internalvar *var,
+			void *ignore)
+{
+  struct inferior *inf = current_inferior ();
+
+  return value_from_longest (builtin_type (gdbarch)->builtin_int, inf->num);
+}
+
+/* Implementation of `$_inferior' variable.  */
+
+static const struct internalvar_funcs inferior_funcs =
+{
+  inferior_id_make_value,
+  NULL,
+  NULL
+};
+
 
 
 void
@@ -1071,4 +1091,5 @@ Show printing of inferior events (e.g., inferior start and exit)."), NULL,
          show_print_inferior_events,
          &setprintlist, &showprintlist);
 
+  create_internalvar_type_lazy ("_inferior", &inferior_funcs, NULL);
 }

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-01-13  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/default.exp: Expect $_inferior as well.
+	* gdb.multi/base.exp: Test $_inferior.
+
 2016-01-13  Pedro Alves  <palves@redhat.com>
 
 	PR breakpoints/19388

gdb/testsuite/gdb.base/default.exp

@@ -587,6 +587,7 @@ set show_conv_list \
 	{$_sdata = void} \
 	{$_siginfo = void} \
 	{$_thread = 0} \
+	{$_inferior = 1} \
 	{$_exception = <error: No frame selected>} \
 	{$_probe_argc = <error: No frame selected>} \
 	{$_probe_arg0 = <error: No frame selected>} \

gdb/testsuite/gdb.multi/base.exp

@@ -44,12 +44,16 @@ if { [build_executable ${testfile}.exp ${exec3} "${srcfile3}" {debug}] == -1 } {
 
 clean_restart ${exec1}
 
+gdb_test {print $_inferior} " = 1"
+
 # Add an empty inferior, switch to it, and load a main executable into
 # it.
 gdb_test "add-inferior" "Added inferior 2.*" "add empty inferior 2"
 gdb_test "inferior 2" "Switching to inferior 2.*" "switch to inferior 2"
 gdb_test "file ${binfile2}" ".*" "load ${exec2} file in inferior 2"
 
+gdb_test {print $_inferior} " = 2" "print \$_inferior after switching"
+
 # Add a new inferior and load a main executable into it in one
 # command.
 gdb_test "add-inferior -exec ${binfile3}" \