Re: [gpsd-users] gpsd and shared memory

[Roger Oberholtzer]

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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

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