Re: [PATCH v4 2/4] gdb/cli: add '.' as an argument for 'list' command

[Pedro Alves <pedro@palves.net>]

Hi,

Sorry for coming late to the party here, but while trying to catch up on the
list I spotted a few things that I think should be fixed.  See below.

BTW, I think the feature is cool!

On 2023-07-13 11:24, Bruno Larsen via Gdb-patches wrote:
> Currently, after the user has used the list command once, there is no
> self-contained way to ask GDB to print the location where the inferior is
> stopped.  The current best options require either using a separate
> command to scope out where the inferior is stopped, or using "list *$pc"
> requiring knowledge of GDB standard registers.  This commit adds a way
> to do that using '.' as a new argument for the 'list' command.  If the
> inferior isn't running, the command prints around the main function.
> 
> Because this necessitated having the inferior running and the test was
> (seemingly unnecessarily) using printf in a non-essential way and it
> would make the resulting log harder to read for no benefit, it was
> replaced by a differen t statement.
> ---
>  gdb/NEWS                        |  4 ++++
>  gdb/cli/cli-cmds.c              | 31 ++++++++++++++++++++++++--
>  gdb/doc/gdb.texinfo             |  5 +++++
>  gdb/testsuite/gdb.base/list.exp | 39 +++++++++++++++++++++++++++++++++
>  gdb/testsuite/gdb.base/list1.c  |  2 +-
>  5 files changed, 78 insertions(+), 3 deletions(-)
> 
> diff --git a/gdb/NEWS b/gdb/NEWS
> index b924834d3d7..eef440a5242 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -84,6 +84,10 @@
>    is 64k.  To print longer strings you should increase
>    'max-value-size'.
>  
> +* The 'list' command now accepts '.' as an argument, which tells GDB to
> +  print the location where the inferior is stopped.  If the inferior hasn't
> +  started yet, the command will print around the main function.

It would be more accurate to say that it's where the current thread is stopped.

Say you run to breakpoint hit in thread 1, and then switch to thread 2, and then do
"list .".  That will show you the current frame of thread 2, while "where the
inferior is stopped" could very well be interpreted as "thread 1".

> +
>  * New commands
>  
>  maintenance print record-instruction [ N ]
> diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
> index 00977bc2ee3..1c459afdc97 100644
> --- a/gdb/cli/cli-cmds.c
> +++ b/gdb/cli/cli-cmds.c
> @@ -1234,14 +1234,14 @@ list_command (const char *arg, int from_tty)
>    const char *p;
>  
>    /* Pull in the current default source line if necessary.  */
> -  if (arg == NULL || ((arg[0] == '+' || arg[0] == '-') && arg[1] == '\0'))
> +  if (arg == NULL || ((arg[0] == '+' || arg[0] == '-' || arg[0] == '.') && arg[1] == '\0'))
>      {
>        set_default_source_symtab_and_line ();
>        symtab_and_line cursal = get_current_source_symtab_and_line ();
>  
>        /* If this is the first "list" since we've set the current
>  	 source line, center the listing around that line.  */
> -      if (get_first_line_listed () == 0)
> +      if (get_first_line_listed () == 0 && (arg == nullptr || arg[0] != '.'))
>  	{
>  	  list_around_line (arg, cursal);
>  	}
> @@ -1263,6 +1263,32 @@ list_command (const char *arg, int from_tty)
>  	  print_source_lines (cursal.symtab, range, 0);
>  	}
>  
> +      /* "l ." lists the default location again.  */

Spelling out "list ." would be better for grepping, IMHO.

> +      else if (arg[0] == '.')
> +	{
> +	  try
> +	    {
> +	      /* Find the current line by getting the PC of the currently
> +		 selected frame, and finding the line associated to it.  */
> +	      frame_info_ptr frame = get_selected_frame (nullptr);

So this is actually using the selected frame, not the current frame.  So even
the "where the thread is stopped" description would be incorrect.  If you really
wanted to use frame #0 (where the thread stopped), then this should use get_current_frame.


> +	      CORE_ADDR curr_pc = get_frame_pc (frame);
> +	      cursal = find_pc_line (curr_pc, 0);
> +	    }
> +	  catch (const gdb_exception &e)
> +	    {
> +		  /* If there was an exception above, it means the inferior
> +		     is not running, so reset the current source location to
> +		     the default.  */

Aww.  Please don't use a try/catch for this...  You can just check target_has_execution.

Also, if an exception would be good for this (which it isn't), please don't catch
gdb_exception -- it catches QUIT/Ctrl-C as well, which is most often wrong.  You would
want gdb_exception_error normally.

> +		  clear_current_source_symtab_and_line ();
> +		  set_default_source_symtab_and_line ();
> +		  cursal = get_current_source_symtab_and_line ();
> +	    }
> +	  list_around_line (arg, cursal);
> +	  /* Advance argument so just pressing "enter" after using "list ."
> +	     will print the following lines instead of the same lines again. */
> +	  arg++;
> +	}
> +
>        return;
>      }
>  
> @@ -2770,6 +2796,7 @@ and send its output to SHELL_COMMAND."));
>      = add_com ("list", class_files, list_command, _("\
>  List specified function or line.\n\
>  With no argument, lists ten more lines after or around previous listing.\n\
> +\"list .\" lists ten lines arond where the inferior is stopped.\n\

arond -> around

>  \"list -\" lists the ten lines before a previous ten-line listing.\n\
>  One argument specifies a line, and ten lines are listed around that line.\n\
>  Two arguments with comma between specify starting and ending lines to list.\n\
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index b10c06ae91f..7619efe3de9 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -9148,6 +9148,11 @@ Stack}), this prints lines centered around that line.
>  
>  @item list -
>  Print lines just before the lines last printed.
> +
> +@item list .
> +Print the lines surrounding the location that is where the inferior
> +is stopped.  If the inferior is not running, print around the main
> +function instead.

This should be clarified as well.

>  @end table
>  
>  @cindex @code{list}, how many lines to display
> diff --git a/gdb/testsuite/gdb.base/list.exp b/gdb/testsuite/gdb.base/list.exp
> index 18ecd13ac0f..ed178a1dd95 100644
> --- a/gdb/testsuite/gdb.base/list.exp
> +++ b/gdb/testsuite/gdb.base/list.exp
> @@ -400,6 +400,42 @@ proc test_list_invalid_args {} {
>  	"second use of \"list +INVALID\""
>  }
>  
> +proc test_list_current_location {} {
> +    global binfile
> +    # If the first "list" command that GDB runs is "list ." GDB may be
> +    # unable to recognize that the inferior isn't running, so we should
> +    # reload the inferior to test that condition.

I don't understand this comment.  Why would it be unable to do so?

> +    clean_restart
> +    gdb_file_cmd ${binfile}
> +
> +    # Ensure that we are printing 10 lines
> +    if {![set_listsize 10]} {
> +	return
> +    }
> +
> +    # First guarantee that GDB prints around the main function correctly
> +    gdb_test "list ." \
> +	"1.*\r\n2\[ \t\]+\r\n3\[ \t\]+int main \[)(\]+.*5\[ \t\]+int x;.*" \
> +	"list . with inferior not running"
> +
> +    if {![runto_main]} {
> +	warning "couldn't start inferior"
> +	return
> +    }
> +
> +    # Walk forward some lines

Missing period.

> +    gdb_test "until 15" ".*15.*foo.*"
> +


> +    # Test that the correct location is printed and that
> +    # using just "list" will print the following lines.
> +    gdb_test "list ." ".*" "list current line after starting"
> +    gdb_test "list" ".*" "confirm we are printing the following lines"
> +
> +    # Test that list . will reset to current location
> +    gdb_test "list ." ".*" "list around current line again"
> +    gdb_test " " ".*" "testing repeated invocations with GDB's auto-repeat"

I don't understand -- these 4 tests match ".*", so how how they ensuring what
they claim they test?  Looks like WIP?

Pedro Alves

> +}
> +
>  clean_restart
>  gdb_file_cmd ${binfile}
>  
> @@ -519,4 +555,7 @@ test_list "list -" 10 2 "7-8" "5-6"
>  # the current line.
>  test_list "list -" 10 1 "7" "6"
>  
> +# Test printing the location where the inferior is stopped.
> +test_list_current_location
> +
>  remote_exec build "rm -f list0.h"
> diff --git a/gdb/testsuite/gdb.base/list1.c b/gdb/testsuite/gdb.base/list1.c
> index d694495c3fb..9297f958f46 100644
> --- a/gdb/testsuite/gdb.base/list1.c
> +++ b/gdb/testsuite/gdb.base/list1.c
> @@ -7,7 +7,7 @@ void bar (int x)
>     -
>     - */
>  {
> -    printf ("%d\n", x);
> +    x++;
>  
>      long_line ();
>  }