Re: [Boost-users] [Interprocess] error_code_t = 9 (already_exists_error) while opening a fixed_managed_shared_memory

Subject: Re: [Boost-users] [Interprocess] error_code_t = 9 (already_exists_error) while opening a fixed_managed_shared_memory
From: Gaetan Gaumer (gaetan.gaumer_at_[hidden])
Date: 2011-05-19 12:26:04

2011/5/19 Marsh Ray <marsh_at_[hidden]>:
> On 05/19/2011 10:52 AM, Gaetan Gaumer wrote:
>>
>> So I have two questions :
>> 1) Do you have an idea why the mmap function maps the segment at a
>> different address from the provided one, knowing that this address
>> is the one returned from code below on another process (so should be
>> valid) ?
>
> Perhaps that address space was already being used by something. This is
> likely to be the case with large regions on a 32-bit OS.

It's indeed used by something as it is the address of the segment return by a
new fixed_managed_shared_memory(boost::interprocess::create_only,...)

>
> How large is is the segment you're trying to map?

Currently 250MB but we want to use bigger segments soon.

>
> Is your process running as 64- or 32-bits?

64 bits

>
>> 2) In the mapped_region constructor, when a non null address value is
>> given as a parameter, could we use the MAP_FIXED flag  to force
>> the mmap function to map the segment at the provided address ?
>
> What happens if something else is already at that address?

According to mmap spec
(http://pubs.opengroup.org/onlinepubs/009695399/functions/mmap.html)
"mmap() may return MAP_FAILED and set errno to [EINVAL]"

But in our case the address is used by the segment we want to open...