Experimental Physics and
Industrial Control System

Eric Norum <[email protected]> · Mon, 3 Nov 2008 15:25:45 -0600

On Nov 3, 2008, at 2:40 PM, Andrew Johnson wrote:

......

The middle sentence "by the same thread" is important in

understanding why the

existing code is like it is. Developers don't particularly like it

if their

errlogPrintf() output doesn't come out immediately; they expect the

sequence

of calls
	printf("A\n");
	errlogPrintf("B\n");
	printf("C\n");

to generate "A" "B" and "C" in order, which is why the "by the same

thread"

part was added. Now if epicsThreadIsOkToBlock() returns false we

must avoid

blocking the thread (any kind of printf() to a file descriptor is

potentially

blocking) so we have no choice but to punt the printf() to the

errlogThread,

but where blocking isn't a problem it's nicer to do the printf()

directly in

the calling thread.

..............

The question I have for any core-talk readers still paying attention

is:

   How important is it that errlogPrintf() and friends directly call
   fprintf(stderr) when called in a thread that is allowed to block?

   My inclination would be to do what Lewis proposes and do all the
   console output from the errlogThread, but that would often cause
   the example above to output "A", "C" and then later "B".

The decision comes down to how heavily you weight the expectations

that you mention in the first paragraph above. As you pointed out,

the developer's guide does state (or at least imply pretty strongly)

that

 	printf("A\n");
	errlogPrintf("B\n");
	printf("C\n");

will generate "A" "B" and "C" in order. Now this promise is to be

rescinded. About par for the course at this time of year in this

country, I suppose.........

I think that it is fairly easy to argue that there should not be any

expectation that errlogPrintf output appear in any fixed relation to

printf/fprintf output.

--
Eric Norum <[email protected]>
Advanced Photon Source
Argonne National Laboratory
(630) 252-4793

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System