Re: [PATCH v2 3/4] strbuf_setlen: don't write to strbuf_slopbuf

From: Brandon Casey <drafnel@gmail.com>
Date: Aug 23, 2017, 21:20 UTC
Message-ID: <CA+sFfMe56itAMDXOJybf0yHj+BqU1Ai1aU7inoTG3FJtdtZxyw@mail.gmail.com>
In-Reply-To: <xmqqh8wyxag1.fsf@gitster.mtv.corp.google.com>

On Wed, Aug 23, 2017 at 2:04 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 8 quoted lines

> Brandon Casey <drafnel@gmail.com> writes:
>
>> So is there any reason why didn't do something like the following in
>> the first place?
>
> My guess is that we didn't bother; if we cared, we would have used a
> single instance of const char in a read-only segment, instead of
> such a macro.

I think you mean something like this:

   const char * const strbuf_slopbuf = "";

..with or without "const" at the beginning. We can't use an actual variable like that since we also want to be able to do initialization like:

   struct strbuf b = STRBUF_INIT;

i.e.

   struct strbuf b = { 0, 0, strbuf_slopbuf };

So the compiler needs to be able to determine that everything within the curly braces is constant and apparently gcc cannot.

On a related note... I was just looking at object.c which also uses a slopbuf. We could similarly protect it from inadvertent modification by doing something like this:

diff --git a/object.c b/object.c
index 321d7e9..4c7a041 100644
--- a/object.c
+++ b/object.c
@@ -303,7 +303,7 @@ int object_list_contains(struct object_list *list, struct ob
ject *obj)
  * A zero-length string to which object_array_entry::name can be
  * initialized without requiring a malloc/free.
  */
-static char object_array_slopbuf[1];
+static const char * const object_array_slopbuf = "";

 void add_object_array_with_path(struct object *obj, const char *name,
                                struct object_array *array,
@@ -326,7 +326,7 @@ void add_object_array_with_path(struct object
*obj, const char *name,
                entry->name = NULL;
        else if (!*name)
                /* Use our own empty string instead of allocating one: */
-               entry->name = object_array_slopbuf;
+               entry->name = (char*) object_array_slopbuf;
        else
                entry->name = xstrdup(name);
        entry->mode = mode;