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

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

[Heinrich Schuchardt]

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.

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
-- 
2.1.4

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

[Michael Kerrisk (man-pages)]

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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