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

[Florian Weimer <fweimer at redhat dot com>]

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.

Thanks,
Florian