Re: [PATCH] libtracefs: An API to set the filtering of functions

[Sameeruddin Shaik <sameeruddin.shaik8@gmail.com>]

Okay steve

Actually I chose this API randomly from bugzilla, i don't have any
plans as of now.
I am developing it, as per your requirement.

Out of curiosity, I started implementing this API. Actually I am a
novice in development,
i know somewhat like how to write code logically and make it work, but
i haven't followed any standard
till now. I am learning some new things from your reviews.
Thanks,
sameer.

On Sat, 6 Mar, 2021, 9:08 am Steven Rostedt, <rostedt@goodmis.org> wrote:
>
> On Sat, 6 Mar 2021 07:25:18 +0530
> Sameeruddin Shaik <sameeruddin.shaik8@gmail.com> wrote:
>
> > hi steve,
> >
> > i have one doubt.
> > >Note, @filters should be of type: const char * const * filters,  as not
> > >only is filters pointing to constant strings, the array itself will not be
> > >modified.
> >
> > what If the user wants to capture the filters at run time like below ?
> > let's say
> >
> >   filters = malloc(sizeof(char *));
> >         if (!filters)
> >                 return 1;
> >         printf("please enter the input filters count\n");
> >         scanf("%d", &fil_count);
> >         while(i < fil_count) {
> >                 scanf("%s", buf);
> >                 slen = strlen(buf);
> >                 if (!slen)
> >                         return 1;
> >                 filters[i] = calloc(1, slen);
> >                 strncpy(filters[i++], buf, slen);
> >         }
> > at that time, this declaration will be problematic right?, because we
> > are trying to modify
>
> No it wont. You can assign const pointers to dynamic pointers, but not
> the other way around. It's a way to show that the function you are
> calling wont do anything with the array you pass to it.
>
> > the read-only memory. Are we expecting the user to supply filters at
> > compile time like below?
> > const char * const *filters = {"kvm_pmu_reset", "kvm_pmu_init",
> > "dir_item_err", NULL};
>
> No, as explained above.
>
> >
> > Tzvetomir & steve,
> > >Since a triple pointer is difficult to manage in the code, you could have:
> > >
> > >       const char **e = NULL;
> > >
> > >
> > >               if (errs) {
> > >                        e = realloc(sizeof(*e), j + 1);
> > >                        e[j++] = filters[i];
> > >               }
> > >
> > >Then at the end:
> > >
> > >       if (errs)
> > >                *errs = e;
> > i have a concern here
> > when a double pointer is doing our work here without any overhead, why
> > we want to make it a triple pointer?
>
> What overhead? A string is a pointer, an array of strings is a double
> pointer, and passing in the address to an array of strings so you can
> modify that array is a triple pointer, and that's exactly what you need
> for errs.
>
> This is basic C coding, nothing special here.
>
> I'm curious to why you picked this particular API to implement. Is
> there something you are planning on using this for?
>
> -- Steve