Re: [Xenomai-help] Some problems with shared memory

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Wolfgang Denk wrote:
> Dear Gilles,
> 
> in message <49FDFCC7.6000009@domain.hid> you wrote:
>>> 1.) If I don´t page align the size of shared memory (multiple of 4096)
>> I get the following error message :
>>> "createshm mmap: No such device or address"
> ...
>> Are you sure this issue is specific to xenomai, do not you have the same
>> effect with plain Linux shared memories ?
> 
> To avoid misunderstandings, we should first point  out  that  we  are
> here  in  fact *not* discussing classic shared memory in the sense of
> the SysV SHM code, but the creation of shared memory regions by using
> the mmap() system call.

Yes, obviously.

> 
> 
> For completeness: shmget(2) *does* allow to allocate sizes which are
> not a multiple of pages sizes; but it then rounds up internally:
> 
> SHMGET(2) ... DESCRIPTION:
> 
>         ... A new shared memory segment, with size equal to the value
>         of size rounded up to a multiple of PAGE_SIZE, is created ...
> 
> As far as mmap()  is  concerned,  there  is  no  restriction  on  the
> "length"  parameter  (though  it  is  clear  that  internally this is
> rounded up to a multiple of pages, too, but only "length"  bytes  are
> visible to the user).
> 
> The restrictions for page allignments affect the addr and offset
> paraemters only.
> 
> 
> To me it seems clear that mmap() in plain Linux and Xenomai behave
> differently.

It will be clear to me when someone will have run the tests.

-- 
					    Gilles.