Re: [PATCH/RFC] Implement the ability to set the current working directory in GDBserver

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Thursday, August 31 2017, Pedro Alves wrote:

> On 08/30/2017 06:38 AM, Sergio Durigan Junior wrote:
>> I didn't want to implement a gdbserver-specific command (e.g., "set
>> remote directory"), which means that my approach has some drawbacks.
>> For example, if you want gdbserver to cd to "/abc", but "/abc" doesn't
>> exist in the host, then you still won't be able to do this, because
>> GDB obviously won't allow you to "cd" into a non-existing dir.  So you
>> will have to have the same directory structure in both host and target
>> if you want to do that.
>
> I'm not sure this is the right approach.  I'd like to have a
> better understanding of what are the use cases "cd" is used for.
> Beyond affecting the inferior's cwd when it is started, what
> else is/can "cd" used for?  Or IOW, what else does GDB's
> current working directory affect?

Good, I was also not 100% this was the right approach either.

I gave this all a thought yesterday and, before I answer your questions,
I'd like to know if I'm understanding the goal correctly.  We want to be
able to instruct gdbserver to change the current working directory
before starting the inferior, correct?  I had the impression that this
was the only goal to achieve, but I'm afraid I'm not seeing the entire
picture here.

As for your questions.  I looked at the code to find places where the
"current_directory" variable was being used.  This is the variable that
ultimately gets changed when "cd" is used.

Aside from impacting the inferior's cwd, current_directory is also used
on the ".gdb_history" machinery.

    tmpenv = getenv ("GDBHISTFILE");
    if (tmpenv)
      history_filename = xstrdup (tmpenv);
    else if (!history_filename)
      {
        /* We include the current directory so that if the user changes
           directories the file written will be the same as the one
           that was read.  */
  #ifdef __MSDOS__
        /* No leading dots in file names are allowed on MSDOS.  */
        history_filename = concat (current_directory, "/_gdb_history",
                                   (char *)NULL);
  #else
        history_filename = concat (current_directory, "/.gdb_history",
                                   (char *)NULL);
  #endif
      }
    read_history (history_filename);

As John Baldwin also mentioned, 'cd' has an effect when loading GDB
scripts.  And probably has an effect when loading other stuff.

Since gdbserver doesn't really support loading scripts and also doesn't
use .gdb_history, I don't think they are relevant in this case.

Having said that, I'd like to discuss more about the ultimate goal, so
that I know I'm pursuing the right things here.

Thanks,

-- 
Sergio
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