linux-kernel - Re: [PATCH 2/2] riscv/include/uapi: Define a custom _

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Date:   Thu, 4 Jul 2019 10:34:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alistair Francis <alistair23@...il.com>
Cc:     Alistair Francis <alistair.francis@....com>,
        linux-riscv-bounces@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] riscv/include/uapi: Define a custom __SIGINFO struct
 for RV32

On Thu, Jul 4, 2019 at 12:18 AM Alistair Francis <alistair23@...il.com> wrote:
> On Wed, Jul 3, 2019 at 12:47 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Wed, Jul 3, 2019 at 8:45 PM Alistair Francis <alistair23@...il.com> wrote:
> > > What I don't understand though is how that impacted this struct, it
> > > doesn't use clock_t at all, everything in the struct is an int or
> > > void*.
> >
> > si_utime/si_stime in siginfo are clock_t.
>
> But they are further down the struct. I just assumed that GCC would
> align those as required, I guess it aligns the start of the struct to
> match some 64-bit members which seems strange.

These are the regular struct alignment rules. Essentially you would
get something like

struct s {
    int a;
    int b;
    int c;
    union {
         int d;
         long long e;
   };
   int f;
};

Since 'e' has 8 byte alignment, the same is true for the union,
and putting the union in a struct also requires the same alignment
for the struct itself, so now you get padding after 'c' and 'f'.

       Arnd