[liblouis-liblouisxml] Re: Unsure how to do a change to logging
- From: "Michael Whapples" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "mwhapples@xxxxxxx" for DMARC)
- To: liblouis-liblouisxml@xxxxxxxxxxxxx
- Date: Thu, 22 May 2014 08:51:20 +0100
Hello,
The difficulty is for the initial state, if an application never calls
lbu_registerLogCallback (lbu_registerLogCallback being what I am calling
the liblouisutdml function for registering a callback).
The initial default callback in liblouis is set in global code like this:
static logCallback = defaultLogCallback;
The only way an external library or application can alter the callback
in liblouis is to call lou_registerLogCallback. However I believe a
function call in C can only be made from within another function. This
leads to the question of how liblouisutdml should change the logging
callback in liblouis to the one in liblouisutdml, before any
liblouisutdml function call is made?
So until lou_registerLogCallback is called, liblouis will be using its
own default logging callback, and liblouisutdml will be using its own
default logging callback (IE. there will be two different logging
callbacks in use in different places,). If these two default callbacks
try to write to one stream then we get the issue of resource access.
If liblouisutdml had a function which must be called before any other
function in liblouisutdml, then placing a call to
lou_registerLogCallback could be done there. However liblouisutdml has
no such entry/initialisation function.
Another option is if liblouisutdml had an initialisation function which
gets called by any public API function if liblouisutdml is not
initialised, but again liblouisutdml has no such initialisation function.
In Java I would use a static initialiser, but C has no such concept.
What I seem to be left with is that every public liblouisutdml function
should have a call to a function to check and set the default callback.
If taking this route I probably would actually structure it as a
initialisation function and look at refactoring liblouisutdml to migrate
other potential initialisation code into such a function.
Does this make it clear where the difficulty is?
Michael Whapples
On 21/05/2014 19:13, Keith Creasy wrote:
Michael.
I may need to look at your implementation but I don't understand why you'd ever end up
with more than one component trying to write to the same output stream. Is
"handler" in this case what you are calling the callback function or is it an
output stream?
LibLouis would use its default handler unless LibLouisUTDML or some other
module registered a different one. If LibLouisUTDML registers a callback then
all of LibLouis' messages would go through the LibLouisUTDML callback, in this
case LibLouisUTDML default.
BrailleBlaster adds another layer, registering a callback in LibLouisUTDML and
so we are now using BrailleBlaster's default.
When LibLouis generates a log message it calls the callback function in
LlibLouisUTDML. The function in LouisUTDML then calls the BrailleBlaster
callback. BrailleBlaster writes the message to an output stream.
-----Original Message-----
From: liblouis-liblouisxml-bounce@xxxxxxxxxxxxx
[mailto:liblouis-liblouisxml-bounce@xxxxxxxxxxxxx] On Behalf Of Michael Whapples
Sent: Wednesday, May 21, 2014 1:15 PM
To: liblouis-liblouisxml@xxxxxxxxxxxxx
Subject: [liblouis-liblouisxml] Unsure how to do a change to logging
Hello,
In changing the logging so that lou_log and lou_logPrint are not public, I have
hit a difficulty.
The problem is that in liblouisutdml, how I set a default log handler?
This might seem initially simple, just do it in the same way as in liblouis.
This however is not enough, it sets a default handler in liblouisutdml but not
in liblouis.
You may ask so what is the problem with that? It means that should someone set
a file name for the default log handler (eg.
lbu_logFilename), this gets set on liblouisutdml but ont on liblouis.
Having two handlers accessing one file (one handler in liblouis and one in
liblouisutdml) might cause file access problems.
What I need is a C version of a static initialiser (think of static
initialisers in Java) where I could call lou_registerLogCallback(NULL) from
liblouisutdml.
Failing that, I can only see two possible options:
1. Find every public function call in liblouisutdml and do a check/register
there. Risk being one will get missed and may become a maintanence issue for
the longer term.
2. The shared library option I suggested for logging code, then there is only a
single storing of the registered callback.
Michael Whapples
For a description of the software, to download it and links to project pages go
to http://www.abilitiessoft.com
For a description of the software, to download it and links to
project pages go to http://www.abilitiessoft.com
For a description of the software, to download it and links to
project pages go to http://www.abilitiessoft.com
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