Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Richard Biener <rguenther@suse.de>]

On Tue, 11 Apr 2017, Florian Weimer wrote:

> On 04/06/2017 09:12 PM, Richard Biener wrote:
> > > Right.  The kernel also has many APIs which return multiple
> > > variable-length data blocks, such as getdents64, and many more
> > > interfaces in combination with read/recv system calls.  Variable length
> > > 
> > > means that you cannot declare the appropriate type after the first data
> > > 
> > > item, so you technically have to use malloc.
> > > 
> > > POSIX interfaces which exhibit a similar pattern are getpwnam_r and
> > > friends, but for them, you can probably use malloc without ill effect
> > > (although there are still performance concerns).
> 
> > Can you give a concrete example which shows the issue and how
> > typeless_storage helps?
> 
> An example is in libffi/src/closures.c:
> 
>       char buf[MAXPATHLEN * 3];
> 
>       if (getmntent_r (last_mntent, &mnt, buf, sizeof (buf)) == NULL)
>         return -1;
> 
> The intent is that buf is untyped storage, from which the getmntent_r function
> can allocate objects as needed (instead of using malloc).
> 
> Based on your earlier comments, GCC already supports that without any further
> source code annotations.

Yes.

Richard.