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

[Steven Rostedt <rostedt@goodmis.org>]

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