linux-kernel - Re: [alsa-devel] [PATCH v6 5/8] ALSA: Avoid using timespec for struct snd_rawmidi

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Date:   Tue, 12 Nov 2019 21:04:10 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     ALSA Development Mailing List <alsa-devel@...a-project.org>,
        Takashi Iwai <tiwai@...e.com>,
        Baolin Wang <baolin.wang@...aro.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Baolin Wang <baolin.wang7@...il.com>
Subject: Re: [alsa-devel] [PATCH v6 5/8] ALSA: Avoid using timespec for struct snd_rawmidi_status

On Tue, Nov 12, 2019 at 5:38 PM Takashi Iwai <tiwai@...e.de> wrote:
> On Tue, 12 Nov 2019 16:16:39 +0100, Arnd Bergmann wrote:
> > +#ifndef __KERNEL__
> >  struct snd_rawmidi_status {
> >       int stream;
> > +     unsigned char pad1[sizeof(time_t) - sizeof(int)];
> >       struct timespec tstamp;         /* Timestamp */
> >       size_t avail;                   /* available bytes */
> >       size_t xruns;                   /* count of overruns since last status (in bytes) */
> >       unsigned char reserved[16];     /* reserved for future use */
> >  };
>
> Can we use union instead of padding?  Something like:
>
> struct snd_rawmidi_status {
>         union {
>                 int stream;
>                 time_t stream_alignment;
>         };
>         struct timespec tstamp;         /* Timestamp */
>         ....

I think this would work most of the time, though I don't feel this is more
readable than the other version.

More importantly, it requires compiling user applications with GNU extensions
(--std=gnu89 or higher) or C11, but not C99, so this could be a problem
for some applications.

If you feel there is a problem with the padding syntax, how about enclosing
it in a typedef like:

typedef struct { char pad[sizeof(time_t) - sizeof(int)]; } __time_pad;

This typedef could be used in several structures from the other patches
as well.

      Arnd