Safely reading a variable shared between threads

QNX Newsgroups (Archives) qdn.public.qnxrtp.os
#1

I want to protect access to a couple of shared variables (circular list
pointers).

they are declared as

volatile unsigned usRead, usWrite;

I have used the atomic_add / atomic _sub functions to write to the
variables.

Two questions:

1 - Is there any need to protect reads of these variables? Can a read of a
short be interrupted and somehow corrupted? (both threads that access the
buffer run at the same priority.

2 - Can / should I safely us a atomic_add_value to get the current value as
follows?

Value = atomic_add_value( &usRead, 0);

#2

Michael Nunan <m.nunan@ieee.ca> wrote:

I want to protect access to a couple of shared variables (circular list
pointers).

they are declared as

volatile unsigned usRead, usWrite;

I have used the atomic_add / atomic _sub functions to write to the
variables.

Two questions:

1 - Is there any need to protect reads of these variables? Can a read of a

Not on an x86 at least – dunno about PPC/MIPS etc…

short be interrupted and somehow corrupted? (both threads that access the
buffer run at the same priority.

2 - Can / should I safely us a atomic_add_value to get the current value as
follows?

Value = atomic_add_value( &usRead, 0);

Again, not if you’re on an x86…

Someone else will hafta comment on the non-x86 platforms…

Cheers,
-RK

–
Robert Krten, PARSE Software Devices +1 613 599 8316.
Realtime Systems Architecture, Books, Video-based and Instructor-led
Training and Consulting at www.parse.com.
Email my initials at parse dot com.

#3

It is an x86 platform - Thanks.

Mike Nunan

“Robert Krten” <nospam88@parse.com> wrote in message
news:ag01cs$jcn$1@inn.qnx.com…

Michael Nunan <> m.nunan@ieee.ca> > wrote:
I want to protect access to a couple of shared variables (circular list
pointers).

they are declared as

volatile unsigned usRead, usWrite;

I have used the atomic_add / atomic _sub functions to write to the
variables.

Two questions:

1 - Is there any need to protect reads of these variables? Can a read of
a

Not on an x86 at least – dunno about PPC/MIPS etc…

short be interrupted and somehow corrupted? (both threads that access
the
buffer run at the same priority.

2 - Can / should I safely us a atomic_add_value to get the current value
as
follows?

Value = atomic_add_value( &usRead, 0);

Again, not if you’re on an x86…

Someone else will hafta comment on the non-x86 platforms…

Cheers,
-RK

–
Robert Krten, PARSE Software Devices +1 613 599 8316.
Realtime Systems Architecture, Books, Video-based and Instructor-led
Training and Consulting at > www.parse.com> .
Email my initials at parse dot com.