Re: [RFC PATCH 2/2] Allow and evaluate expressions in command arguments

[Andrew Burgess <aburgess@redhat.com>]

Keith Seitz via Gdb-patches <gdb-patches@sourceware.org> writes:

> This patch adds support for arbitrary expressions to be passed to
> GDB commands, evaluated, and the results substituted into command
> arguments.
>
> This allows users to pass arbitrary expressions into commands which
> currently do not support any way of doing this. This is especially
> useful with, for example, convenience variables and functions.
>
> Example:
> $ export MYDIR="tmp"
> $ export MYFILE="simple"
> $ gdb -nx -q
> (gdb) file $($_env("HOME"))/$($_env("MYDIR"))/$($_env("MYFILE"))
> Reading symbols from /home/keiths/tmp/simple...
> (gdb)
>
> In order to mark the bounds of expressions to be evaluated, I've
> introduced the (arbitrary) marker "$()". Everything contained inside
> this marker will be passed to the expression parser, evaluated, printed
> as a string, and then substituted for the original expression.

I really like the goal of this series, and think something like this
would be awesome to add.

I do worry about the extra $(..) wrapper though, that feels a little
clunky, if the common case turns out to be just doing simple things like
environment variables, I wonder if we could make things like:

  (gdb) file $_env("HOME")/$_env("BLAH")

just work?

We could still retain $(...) for full expression evaluation, because I
guess, in theory this would allow things like:

  (gdb) some_gdb_command $(some_inferior_function())

thought I suspect things like this will be far less common ... maybe?

Thanks,
Andrew

> ---
>  gdb/top.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
>
> diff --git a/gdb/top.c b/gdb/top.c
> index 205eb360ba3..3f394b5b63c 100644
> --- a/gdb/top.c
> +++ b/gdb/top.c
> @@ -57,6 +57,8 @@
>  #include "gdbsupport/pathstuff.h"
>  #include "cli/cli-style.h"
>  #include "pager.h"
> +#include "valprint.h"
> +#include "cp-support.h" 	/* for find_toplevel_char  */
>  
>  /* readline include files.  */
>  #include "readline/readline.h"
> @@ -567,6 +569,60 @@ set_repeat_arguments (const char *args)
>    repeat_arguments = args;
>  }
>  
> +/* Evaluate and expand any expressions in the command line list of arguments
> +   given by ORIG_ARGS.  All occurrences of "$(expr)" will be replaced with a
> +   string representation of the evaluated EXPR.  */
> +
> +#define EVAL_START_STRING "$("
> +#define EVAL_END_TOKEN ')'
> +
> +static std::string
> +expand_command_arg (const char *orig_arg)
> +{
> +  std::string new_arg = orig_arg;
> +  std::string::size_type n = new_arg.find (EVAL_START_STRING);
> +
> +  /* These needs adjustment to output "simple" strings.  For example,
> +     if a char value is used, we end up with "97 'a'" instead of simply "a".  */
> +  struct value_print_options opts;
> +  get_user_print_options (&opts);
> +
> +  string_file stb;
> +
> +  while (n != std::string::npos)
> +    {
> +      const char *args = new_arg.c_str ();
> +      const char *c = find_toplevel_char (args + n + 2, EVAL_END_TOKEN);
> +
> +      if (c != nullptr)
> +	{
> +	  std::string expr = new_arg.substr (n + 2, c - args - n - 2);
> +	  struct value *val = parse_and_eval (expr.c_str ());
> +
> +	  value_print (val, &stb, &opts);
> +
> +	  /* If value_print returns a quote-enclosed string, remove the
> +	     quote characters.  */
> +	  std::string r = stb.release ();
> +
> +	  if (*r.begin () == '\"' && *r.rbegin () == '\"')
> +	    {
> +	      r.erase (r.begin ());
> +	      r.pop_back ();
> +	    }
> +
> +	  new_arg.replace (n, c - args - n + 1, r);
> +	  n = new_arg.find (EVAL_START_STRING);
> +	}
> +    }
> +
> +#if 0
> +  if (new_arg != orig_arg)
> +    printf ("NEW_ARG = %s\n", new_arg.c_str ());
> +#endif
> +  return new_arg;
> +}
> +
>  /* Execute the line P as a command, in the current user context.
>     Pass FROM_TTY as second argument to the defining function.  */
>  
> @@ -657,6 +713,15 @@ execute_command (const char *p, int from_tty)
>        if (c->deprecated_warn_user)
>  	deprecated_cmd_warning (line, cmdlist);
>  
> +      /* Do any expression substitutions on ARG.  */
> +      std::string expanded_arg;
> +
> +      if (arg != nullptr)
> +	{
> +	  expanded_arg = expand_command_arg (arg);
> +	  arg = expanded_arg.c_str ();
> +	}
> +
>        /* c->user_commands would be NULL in the case of a python command.  */
>        if (c->theclass == class_user && c->user_commands)
>  	execute_user_command (c, arg);
> -- 
> 2.38.1