Re: [PATCH v2 10/19] introduce container_of macro

From: Derrick Stolee <stolee@gmail.com>
To: Eric Wong <e@80x24.org>, Junio C Hamano <gitster@pobox.com>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Phillip Wood <phillip.wood@dunelm.org.uk>,
	git@vger.kernel.org
Subject: Re: [PATCH v2 10/19] introduce container_of macro
Date: Wed, 25 Sep 2019 09:12:12 -0400	[thread overview]
Message-ID: <059c05bb-685a-7933-55ff-f18ed271ac3a@gmail.com> (raw)
In-Reply-To: <20190924010324.22619-11-e@80x24.org>

On 9/23/2019 9:03 PM, Eric Wong wrote:
> This macro is popular within the Linux kernel for supporting
> intrusive data structures such as linked lists, red-black trees,
> and chained hash tables while allowing the compiler to do
> type checking.
> 
> I intend to use this to remove the limitation of "hashmap_entry"
> being location-dependent and to allow more compile-time type
> checking.

nit: I don't know why the first-person singular language caused
me to stumble during this message. Perhaps the following rewrite
would convey the same information:

  Later patches will use container_of() to remove the limitation
  of "hashmap_entry" being location-dependent. This will complete
  the transition to compile-time type checking for the hashmap API.

> This macro already exists in our source as "list_entry" in
> list.h and making "list_entry" an alias to "container_of"
> as the Linux kernel has done is a possibility.

If it is the same code, then I would prefer you do this conversion
now so we can see that equivalence in the patch AND we know that
existing code will test it.

Thanks,
-Stolee

> 
> Signed-off-by: Eric Wong <e@80x24.org>
> ---
>  git-compat-util.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 83be89de0a..4cc2c8283a 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -1312,4 +1312,14 @@ void unleak_memory(const void *ptr, size_t len);
>   */
>  #include "banned.h"
>  
> +/*
> + * container_of - Get the address of an object containing a field.
> + *
> + * @ptr: pointer to the field.
> + * @type: type of the object.
> + * @member: name of the field within the object.
> + */
> +#define container_of(ptr, type, member) \
> +	((type *) ((char *)(ptr) - offsetof(type, member)))
> +
>  #endif
> 