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Re: [PATCH v3] C++ify gdb/common/environ.c
- From: Pedro Alves <palves at redhat dot com>
- To: Sergio Durigan Junior <sergiodj at redhat dot com>
- Cc: GDB Patches <gdb-patches at sourceware dot org>, Simon Marchi <simon dot marchi at polymtl dot ca>
- Date: Thu, 4 May 2017 16:30:37 +0100
- Subject: Re: [PATCH v3] C++ify gdb/common/environ.c
- Authentication-results: sourceware.org; auth=none
- References: <20170413040455.23996-1-sergiodj@redhat.com> <20170418030319.12637-1-sergiodj@redhat.com> <e8360438-0edc-2020-5622-34d4344981e3@redhat.com> <87o9vdjoz4.fsf@redhat.com>
On 05/01/2017 03:22 AM, Sergio Durigan Junior wrote:
>>> + this->m_environ_vector.insert (this->m_environ_vector.end () - 1,
>>> + xstrdup (std::string (var
>>> + + '='
>>> + + value).c_str ()));
>>
>> I don't really understand the "m_environ_vector.end () - 1" here.
>
> This is needed because the last element of m_environ_vector needs to be
> always NULL. Therefore, this code is basically inserting the new
> variable in the second-to-last position of the vector. I made a comment
> on top of it to clarify this part.
Ah, OK.
>
> Also, there are other places where I need to iterate through the
> elements of the vector, and I'm also using the "- 1" in these places.
> I'll put comments where applicable.
OK, I'll take another look when you post it.
>>> -extern struct gdb_environ *make_environ (void);
>>> +class gdb_environ
>>> +{
>>> +public:
>>> + /* Regular constructor and destructor. */
>>> + gdb_environ ();
>>> + ~gdb_environ ();
>>>
>>> -extern void free_environ (struct gdb_environ *);
>>> + /* Reinitialize the environment stored. This is used when the user
>>> + wants to delete all environment variables added by him/her. */
>>> + void reinit ();
>>
>> This should mention that the initial state is copied from the host's
>> environ. I think the default ctor could use a similar comment.
>> I was going to suggest to call this clear() instead, to go
>> with the standard containers' terminology, until I realized what
>> "init" really does. Or maybe even find a more explicit name,
>> reset_from_host_environ or some such.
>
> Sorry, I guess I wasn't aware of the importance of specifying that the
> variables come from the host. Somehow I thought this was implied.
>
> I guess reset_from_host_environ is a good name for the method; my other
> option would be "reinit_using_host_environ", but that's longer.
>
>> Perhaps an even clearer approach would be to make the default ctor
>> not do a deep copy of the host's "environ", but instead add a
>> static factor method that returned such a new gdb_environ, like:
>>
>> static gdb_environ
>> gdb_environ::from_host_environ ()
>> {
>> // build/return a gdb_environ that wraps the host's environ global.
>> }
>>
>> Not sure. Only experimenting would tell.
>
> Sorry, I'm not sure I understand your suggestion. Please correct me if
> I'm wrong.
>
> You're basically saying that the default ctor shouldn't actually do
> anything; instead, the class should have this new from_host_environ
> method which would be the "official" ctor. The users would then call
> from_host_environ directly when they wanted an instance of the class,
> and the method would be responsible for initializing the object. Is
> that correct?
>
> If yes, I think I fail to see the advantage of this method over having a
> normal ctor (aside from explicitly naming the new ctor after the fact
> that we're using the host environ to build the object).
The advantage was all in that "aside" -- to make the code
document itself. It was totally non-obvious to me at first
that:
gdb_environ env;
makes a copy of the host's environment. I just assumed it created
an empty environment. Probably because in my mind I don't think
it'll make sense for a remote process to inherit the host's
environment variables.
>
> ... after some reading ...
>
> Oh, I think I see what you're suggesting. Instead of building one
> gdb_environ every time someone requests it, we build just one and pass
> it along. OK, now it makes sense.
That wasn't actually what I was saying. :-)
>
>> Speaking of copying the host environ:
>>
>>> _initialize_mi_cmd_env (void)
>>> {
>>> - struct gdb_environ *environment;
>>> + gdb_environ environment;
>>> const char *env;
>>>
>>> /* We want original execution path to reset to, if desired later.
>>> @@ -278,13 +278,10 @@ _initialize_mi_cmd_env (void)
>>> current_inferior ()->environment. Also, there's no obvious
>>> place where this code can be moved such that it surely run
>>> before any code possibly mangles original PATH. */
>>> - environment = make_environ ();
>>> - init_environ (environment);
>>> - env = get_in_environ (environment, path_var_name);
>>> + env = environment.get (path_var_name);
>>>
>>> /* Can be null if path is not set. */
>>> if (!env)
>>> env = "";
>>> orig_path = xstrdup (env);
>>> - free_environ (environment);
>>> }
>>
>> This usage of gdb_environ looks like pointless
>> wrapping / dupping / freeing to me -- I don't see why we
>> need to dup the whole host environment just to get at some
>> env variable. Using good old getenv(3) directly should do,
>> and end up trimming off a bit of work from gdb's startup.
>
> Absolutely. I didn't pay close attention to this bit; I was just
> mechanically converting the code.
>
>> I could see perhaps wanting to avoid / optimize the linear
>> walks that getenv must do, if we have many getenv calls in
>> gdb, but then that suggests keeping a gdb_environ global that
>> is initialized early on and is reused. But that would miss
>> any setenv call that changes the environment since that
>> gdb_environ is created, so I'd prefer not do that unless
>> we find a real need.
>
> I'll make the code use getenv instead.
Thanks. That bit could/should go in as its own
preparatory/obvious patch first, no need to carry it in
the same patch.
Thanks,
Pedro Alves