Re: [RFC PATCH v3 09/37] bpf tools: Open eBPF object file and do basic validation

["Wangnan (F)" <wangnan0@huawei.com>]

On 2015/5/25 21:30, Arnaldo Carvalho de Melo wrote:
> Em Fri, May 22, 2015 at 06:00:58PM -0700, Alexei Starovoitov escreveu:
>> On 5/22/15 10:23 AM, Jiri Olsa wrote:
>>>> +struct bpf_object *bpf_open_object(const char *path)
>>> another suggestion for the namespace.. Arnaldo forces us ;-)
>>> to use the object name first plus '__(method name)' for
>>> interface functions so that would be:
>>>    bpf_object__open
>>>    bpf_object__close
>>> not sure we want to keep that standard in here though.. Arnaldo?
>   
>> have been thinking back and forth on this one.
>> Finally convinced myself that we shouldn't be forcing it here.
>> object__method style would force the library to look like fake
>> object oriented whereas it's not. It's a normal C. Let's keep it
> Why "fake"? Just because C doesn't have explicit support for OO doesn't
> mean we can't use the concept of OO with structs and functions :-)
>
>> simple. Objects are not needed here. May be 'bpf_object' is an
>> unfortunate name, but it doesn't make the library to be 'ooo'.
> Well, I don't think that what leads one to think about using some
> convention was because "object" was in its name, but because OO _is_
> being used in this case, albeit a restricted set, the one possible while
> using C.
>
> For instance, in this patch:
>
> struct bpf_object {
> 	/*
>          * Information when doing elf related work. Only valid if fd
>          * is valid.
>          */
> 	struct {
>                 int fd;
>                 Elf *elf;
>                 GElf_Ehdr ehdr;
> 	} elf;
>         char path[];  /* Changed from being a pointer to here, to avoid one alloc */
> };
>
> static struct bpf_object *__bpf_obj_alloc(const char *path)
> {
> 	struct bpf_object *obj;
>
> 	obj = calloc(1, sizeof(struct bpf_object));
> 	if (!obj) {
>                 pr_warning("alloc memory failed for %s\n", path);
>                 return NULL;
> 	}
>
> 	obj->path = strdup(path);
> 	if (!obj->path) {
>                 pr_warning("failed to strdup '%s'\n", path);
>                 free(obj);
>                 return NULL;
> 	}
> 	return obj;
> }
>
> The above is for me naturally a constructor, in the restricted OO
> possible with C used in tools/perf (or anywhere else :)), and thus we
> have a convention for this, short one, struct being instantiated + __ +
> new.
>
> struct bpf_object *bpf_object__new(const char *path)
> {
> 	struct bpf_object *obj = zalloc(sizeof(*obj) + strlen(path) + 1);
>
> 	if (obj) {
> 		strcpy(obj->path, path);
> 		obj->elf.fd = -1;
> 	}
>
> 	return obj;
> }
>
> ---
>
> If it doesn't allocates, i.e. it is embedded in another struct, then, we
> have struct being initiated + __ + init, and so on for __delete +
> __exit, etc.
>
> Just a convention, that we (or at least I) try to follow as judiciously
> as possible in tools/perf/.
>
> Like others in the kernel, like using, hey, "__" in front of functions
> to state that they do slightly less than the a function with the same
> name, normally locking is done on foo() that calls __foo() to do the
> unlocked part.
>
> It avoids ambiguity as what is the struct being acted upon by the
> function, since we use _ to separate words in the struct name
> (bpf_object, perf_evlist, etc) and in the function name (findnew_thread,
> process_events, etc), helps with grepping the source code base, etc.
>
>> libtraceevent doesn't use this style either...
> Well, there are many styles to pick, the fact that perf uses __ to
> separate class name from class method doesn't mean that you should as
> well, as you may find it inconvenient or useless to you, you may prefer
> CamelCase notation, for instance ;-)
>
> In the same fashion the fact that libtraceevent doesn't doesn't mean you
> shouldn't use what the perf tooling uses.
>
> - Arnaldo
I'll try OO style naming and see the results in my next version. 
However, I'm
not very sure whether such naming make sense, since we have only 2 
classes in
libbpf: 'bpf_object, bpf_program', 'bpf_map' has potential to become a 
class but
currently not. In addition, the internal functions are hidden to user, 
so the only
meaning to user of such API is an additional '_' in each function.

Anyway, let's see the result then decide whether it is good enough.

Thank you.