Re: [PATCH 1/1] futex.2: casting utime to uint32_t

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hello Heinrich,

On 04/13/2015 08:50 PM, Heinrich Schuchardt wrote:
> The kernel uses the following cast:
> 
>   if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
>       cmd == FUTEX_CMP_REQUEUE_PI || cmd == FUTEX_WAKE_OP)
>     val2 = (u32) (unsigned long) utime;
> 
> This ensures that always the least significant four bytes of the
> pointer are used, both on ILP32 and LP64 systems.
>
> On a big endian system a simple cast from 64 bit pointer to 32 bit
> integer would return the most significant four bytes.
> 
> We have to make the reader of the man-page aware of the usage of the
> least significant bytes.

Nice patch, and well explained. Applied.

Cheers,

Michael


> Signed-off-by: Heinrich Schuchardt <xypron.glpk-Mmb7MZpHnFY@public.gmane.org>
> ---
>  man2/futex.2 | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/futex.2 b/man2/futex.2
> index 9fd8a51..7732822 100644
> --- a/man2/futex.2
> +++ b/man2/futex.2
> @@ -136,11 +136,13 @@ argument is a pointer to a
>  .IR timespec
>  structure that specifies a timeout for the operation.
>  However,  notwithstanding the prototype shown above, for some operations,
> -this argument is instead a four-byte integer whose meaning
> +the least significant four bytes are used as an integer whose meaning
>  is determined by the operation.
>  For these operations, the kernel casts the
>  .I timeout
> -value to
> +value first to
> +.IR "unsigned long",
> +then to
>  .IR uint32_t ,
>  and in the remainder of this page, this argument is referred to as
>  .I val2
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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