Re: [PATCH] init-if-undefined command

[Jim Blandy <jimb@red-bean.com>]

Okay, that looks better.  We agreed we need a NEWS entry, too; could
you include that in your patch?  Once Eli approves the NEWS entry,
this can go in.

On 11/30/05, Andrew STUBBS <andrew.stubbs@st.com> wrote:
> Here is another version with the various help strings updated.
>
> I have rewritten the main help because I noticed it did not start with
> something suitable for 'help data'. I also added a little more
> information about its exact behaviour.
>
> I hope this is better now.
>
> Andrew Stubbs
>
>
> 2005-11-30  Andrew Stubbs  <andrew.stubbs@st.com>
>
>         * value.c (init_if_undefined_command): New function.
>         (_initialize_values): Add command init-if-undefined.
>
> doc/
>         * gdb.texinfo (Convenience variables): Add init-if-undefined command.
>
> Index: src/gdb/value.c
> ===================================================================
> --- src.orig/gdb/value.c        2005-11-29 16:35:46.000000000 +0000
> +++ src/gdb/value.c     2005-11-30 12:46:34.000000000 +0000
> @@ -727,6 +727,47 @@ show_values (char *num_exp, int from_tty
>
>  static struct internalvar *internalvars;
>
> +/* If the variable does not already exist create it and give it the value given.
> +   If no value is given then the default is zero.  */
> +static void
> +init_if_undefined_command (char* args, int from_tty)
> +{
> +  struct internalvar* intvar;
> +
> +  /* Parse the expression - this is taken from set_command().  */
> +  struct expression *expr = parse_expression (args);
> +  register struct cleanup *old_chain =
> +    make_cleanup (free_current_contents, &expr);
> +
> +  /* Validate the expression.
> +     Was the expression an assignment?
> +     Or even an expression at all?  */
> +  if (expr->nelts == 0 || expr->elts[0].opcode != BINOP_ASSIGN)
> +    {
> +      printf_unfiltered ("Init-if-undefined requires an assignment expression.\n");
> +      do_cleanups (old_chain);
> +      return;
> +    }
> +
> +  /* Extract the variable from the parsed expression.
> +     In the case of an assign the lvalue will be in elts[1] and elts[2].  */
> +  if (expr->elts[1].opcode != OP_INTERNALVAR)
> +    {
> +      printf_unfiltered ("The first parameter to init-if-undefined should be a GDB variable\n");
> +      do_cleanups (old_chain);
> +      return;
> +    }
> +  intvar = expr->elts[2].internalvar;
> +
> +  /* Only evaluate the expression if the lvalue is void.
> +     This may still fail if the expresssion is invalid.  */
> +  if (TYPE_CODE (value_type (intvar->value)) == TYPE_CODE_VOID)
> +    evaluate_expression (expr);
> +
> +  do_cleanups (old_chain);
> +}
> +
> +
>  /* Look up an internal variable with name NAME.  NAME should not
>     normally include a dollar sign.
>
> @@ -1639,4 +1680,11 @@ A few convenience variables are given va
>    add_cmd ("values", no_class, show_values,
>            _("Elements of value history around item number IDX (or last ten)."),
>            &showlist);
> +
> +  add_com ("init-if-undefined", class_vars, init_if_undefined_command, _("\
> +Initialize a convenience variable if necessary.\n\
> +init-if-undefined VARIABLE = EXPRESSION\n\
> +Set an internal VARIABLE to the result of the EXPRESSION if it does not\n\
> +exist or does not contain a value.  The EXPRESSION is not evaluated if the\n\
> +VARIABLE is already initialized."));
>  }
> Index: src/gdb/doc/gdb.texinfo
> ===================================================================
> --- src.orig/gdb/doc/gdb.texinfo        2005-11-29 16:36:23.000000000 +0000
> +++ src/gdb/doc/gdb.texinfo     2005-11-30 12:24:23.000000000 +0000
> @@ -6131,6 +6131,18 @@ variable, when used as an expression, ha
>  @item show convenience
>  Print a list of convenience variables used so far, and their values.
>  Abbreviated @code{show conv}.
> +
> +@kindex init-if-undefined
> +@cindex convenience variables, initializing
> +@item init-if-undefined $@var{variable} = @var{expression}
> +Set a convenience variable if it has not already been set.  This is useful
> +for user-defined commands that keep some state.  It is similar, in concept,
> +to using local static variables with initializers in C (except that
> +convenience variables are global).  It can also be used to allow users to
> +override default values used in a command script.
> +
> +If the variable is already defined then the expression is not evaluated so
> +any side-effects do not occur.
>  @end table
>
>  One of the ways to use a convenience variable is as a counter to be
>
>
>