Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 4 Apr 2011 10:24:14 -0500

Hi Ben,

On Sunday 03 April 2011 11:34:39 Benjamin Franksen wrote:
> since CA does not support 64bit integer types, I wonder what to do about
> SNL programs with declarations like
>
> long x;
> assign x to "whatever";
>
> on a system where sizeof(long)==8 (like linux-x86_64). Note that in SNL
> code, the type of a variable determines the request type for CA requests.
> In this case DBR_LONG is certainly wrong, as it is defined to be 32bit.

Here's a naive question for FAQ purposes: Why can't you do what the database 
does and make the SNL 'long' type map to an epicsInt32 on all architectures?  
I personally understand the major disadvantage to doing that when interfacing 
to external APIs, but I think it's worth explaining exactly why you can't do 
that as part of this discussion.

I think making this an error is probably the right solution; the message 
should explain that the variable needs to be an int to assign it on any 64-bit 
systems, and the documentation should explain that the SNL code will need to 
marshal data values between two variables if the program really does need to 
use a 64-bit long variable.

> BTW, I doubt that the current stable version 2.0.12 works properly on such
> systems but I haven't tried it yet.

On little-endian (e.g. Intel/AMD) architecture systems the LSB is always at 
the pointed-to byte, thus long variables appear to work fine on most current 
64-bit systems as long as the values are all within the positive 32-bit range.  
This will fail as soon as either side tries to use a negative value, but many 
sites may already be using this version without realizing the problem because 
of that.  It would be good to see a 2.0.13 release with this fixed though, and 
the issue documented.

Good catch, thanks!

- Andrew
-- 
An error is only a mistake if you don't learn from it.
When you learn something from it, it becomes a lesson.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System