From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 06 Jun 2008 23:09:42 GMT
Subject: Re: [PATCH v2] builtin-fast-export: Add importing and exporting of revision marks
Message-ID: <7v8wxirwi1.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <1212663163-43064-1-git-send-email-pdebie@ai.rug.nl>

Pieter de Bie <pdebie@ai.rug.nl> writes:

> +static void export_marks(char *file)
> +{
> +	unsigned int i;
> +	uint32_t mark;
> +	struct object_decoration *deco = idnums.hash;
> +	FILE *f;
> +
> +	f = fopen(file, "w");
> +	if (!f)
> +		error("Unable to open marks file %s for writing", file);
> +
> +	for (i = 0; i < idnums.size; ++i) {
> +		deco++;
> ...
> +			mark = (uint32_t *)deco->decoration - (uint32_t *)NULL;
> +			fprintf(f, ":%u %s\n", mark,
> +				sha1_to_hex(deco->base->sha1));
> ...
> +}
> +
> +static void import_marks(char * input_file)
> ...
> +		add_decoration(&idnums, object, ((uint32_t *)NULL) + mark);

I am confused.

The type of object_decoration.decorattion is a (void*).  Why isn't it
sufficient to do it in a naïve and straightforward way?

	mark = (uint32_t)(deco->decoration);
        add_decoration(&idnums, object, (void*) mark);

Is this twisted pointer arithmetic done in order to avoid cast between int
and pointer of different size in the code?  Even if that is the case,
doesn't "(uint32_t *)deco->decoration - (uint32_t *)NULL" mean the value
range for deco->decoration is one-fourth of U32?  What are you gaining
from using "uint32_t *" instead of some other pointer types, say "char *"?