Re: [PATCH] string-list.h: Add a value to string_list initializer lists

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 2, 2014, 20:00 UTC
Message-ID: <xmqqlhtf3wvc.fsf@gitster.dls.corp.google.com>
In-Reply-To: <1401699504-29826-1-git-send-email-tanayabh@gmail.com>

Tanay Abhra <tanayabh@gmail.com> writes:

> Add a NULL value in the end of STRING_LIST_INIT_NODUP and
> STRING_LIST_DUP to initialize `compare_strings_fn`.

Hmph. That is "what change is proposed", which we can read from the patch text itself below. We would want to see "why is this change a good thing" to justify it.

As you mentioned later "In C, ...", there is nothing wrong in what we have (and we can not quite read what exactly you tried to do with uninitialized memory with these macros), so we need an excuse other than correctness to justify this change. Perhaps like this?

	STRING_LIST_INIT_{NODUP,DUP} initializers list values only
	for earlier structure members, relying on the usual
	convention in C that the omitted members are initailized to
	0, i.e. the former is expanded to the latter:

	    struct string_list l = STRING_LIST_INIT_DUP;
            struct string_list l = { NULL, 0, 0, 1 };

	and the last member that is not mentioned (i.e. 'cmp') is
	initialized to NULL.

	While there is nothing wrong in this construct, spelling out
	all the values where the macros are defined will serve also
	as a documentation, so let's do so.

Show 27 quoted lines

> Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
> ---
> When I used a malloced string_list to play around with string-list API and
> used the default init_list, it caused a seg fault. After an hour of debugging
> I saw that comapre_strings_fn should be initialized to NULL. In C, even an
> incomplete initialzer initializes every value to NULl or 0, so in normal
> usage in the codebase this problem never occurs. Still it is better to be
> thorough.
>
>  string-list.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/string-list.h b/string-list.h
> index de6769c..87ee419 100644
> --- a/string-list.h
> +++ b/string-list.h
> @@ -15,8 +15,8 @@ struct string_list {
>  	compare_strings_fn cmp; /* NULL uses strcmp() */
>  };
>  
> -#define STRING_LIST_INIT_NODUP { NULL, 0, 0, 0 }
> -#define STRING_LIST_INIT_DUP   { NULL, 0, 0, 1 }
> +#define STRING_LIST_INIT_NODUP { NULL, 0, 0, 0, NULL }
> +#define STRING_LIST_INIT_DUP   { NULL, 0, 0, 1, NULL }
>  
>  void print_string_list(const struct string_list *p, const char *text);
>  void string_list_clear(struct string_list *list, int free_util);