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Re: [gpsd-users] gpsd and shared memory

From: Ed Simmons
Subject: Re: [gpsd-users] gpsd and shared memory
Date: Mon, 5 Oct 2015 09:53:07 +0100
User-agent: Mozilla/5.0 (X11; Linux i686; rv:38.0) Gecko/20100101 Thunderbird/38.2.0

Hi Gary, Roger,

On 05/10/15 09:28, Roger Oberholtzer wrote:
> ________________________________________
> From: Gary E. Miller address@hidden
> Sent: Friday, October 02, 2015 9:07 PM
> To: Roger Oberholtzer
> Cc: address@hidden
> Subject: Re: [gpsd-users] gpsd and shared memory
> Yo Roger!
> On Fri, 2 Oct 2015 06:54:20 +0000
> Roger Oberholtzer <address@hidden> wrote:
>> I think you may be the first person to use the shared memory interface. It 
>> is relatively new and probably only esr knows how it works.
> I like to go off in places with few others blocking the scenery...
>> It does not know it is 'new', but it does know it is valid or atomic. When 
>> the bookends match the the data was read in a consistent state.
> I guess I am trying to figure out why there is both a barrier and the 
> bookends. The barrier, I would imagine, ensures that the shm reader has 
> exclusive access so the contents do not change during the copy. If that is 
> the case, what purpose are the bookends? There is a comment in the code, but 
> it is unclear to me what is really trying to be accomplished. Are the 
> bookends some mechanism to determine if the contents have changed since a 
> previous copy of the data? Or something obtuse?
>> Doing shared memory is a twitchy thing, note the comments on barriers, 
>> read/write order, etc.  So just use the function, dont try to duplicate it.
> I have used shared memory in other situations to great effect. It is all in 
> what you claim to be putting in the segment that decides the degree of 
> twitchiness. 
>> Which iis a good reasson not to use this interfacec, but to use a socket 
>> instead.
> Yeah, but that changes the access logic I am hoping to implement. 
> Roger Oberholtzer
> RST Systems
> Office: +46 (0)10-615 6020
> Mobile: +46 (0)70-815 1696
> address@hidden
> ________________________________________
> Ramböll Sverige AB
> Krukmakargatan 21
> P.O. Box 17009
> SE-104 62 Stockholm, Sweden

Just my 2 cents, we used the shared memory interface with no problems on
a project which is sadly now defunct. The client was a c program reading
GPSD data (including some extra marine sensors that we added to GPSD)
that wrote data to a mysql database according to some settings it read
from a state table. This worked really well for our needs and I'd say
you have no worries making use of it!

Ed Simmons

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