Re: [xml] Need help understanding customer error handlers

On Mon, Aug 13, 2012 at 09:43:25PM -0400, John Schember wrote:
I'm having trouble getting custom error functions working properly. I
want to be able to get the error information from parsing errors so I
can store it for use later. Specifically with Relax-NG

I would like to know this also. In fact, I already inquired about it on 
this list.


I've attached a sample application which demonstrates how I'm setting
up error handlers. I'm using 2.8.0 on OS X 10.8.0. I'm compiling and
running with:

$ gcc -I /usr/local/Cellar/libxml2/2.8.0/include/libxml2/ -lxml2 test.c
$ /a.out 1 b.xml c.xml schema.xml
$ /a.out 2 b.xml c.xml schema.xml
$ /a.out 3 b.xml c.xml schema.xml
$ /a.out 4 b.xml c.xml schema.xml
$ /a.out 5 b.xml c.xml schema.xml

The numbers on the command line represent different functions I'm
setting for the error handler and different types of errors generated
by the parser.

1: error function is fprintf and a validation error occurs
2: error function is err_func (a varg function that passes input to
fprintf) and a validation error occurs.
3. error function is fprintf and a structural error occurs.
4. error function is err_func and a structural error occurs.

I'm seeing strange output when comparing the various tests.

A) Test 1 gives, "b.xml:5: element note: Relax-NG validity error : Did
not expect element note there" as the output but test 2 gives, "Did
not expect element note there". Test 2 just passes the vargs on to
fprintf which test 1 is calling directly. Yet as far as I can tell the
varg list that err_func receives "%s" as the message and the last part
of the error message as the output. So, why is fprintf getting extra
error text?

B) Test 3 and Test 4 gives the same output and my marker for err_func
is never displayed. Test 4 thus isn't every calling err_func but must
be using the internal error handler. I've tried multiple ways to set
structural error handlers (comments in test.c) but none seem to work.
There are 5 different error handler setting functions in test.c I'm
not sure which if any of the 5 are correct.

xmlRelaxNGSetParserErrors and xmlRelaxNGSetParserStructuredErrors
don't appear to do anything.

Setting xmlRelaxNGSetValidStructuredErrors causes no error messages to
be relayed to the error handling functions. fprintf (as far as I can
tell) and err_func (marker is displayed) do not have any message text
and display not error output.

xmlSetGenericErrorFunc and xmlSetStructuredErrorFunc both end up with
strange output when they are set. When 5 is used for the test number
(5 runs all 4 tests in a row) fprintf test 3 calls err_func multiple
times. It's the full error message just each part is sent to err_func
separately. That in itself is strange but what is more strange is
error fprintf is the error handler for that test. However, this could
be due to some kind of misuse due to the test function calling this on
each test. But I don't see how redefining a general error function
would cause error handling to use a function other than what was

How do I properly define an error handler for both validation and
malformed xml errors? How do I get the full error message from the
error function? Basically, I'm confused and I'm hoping someone can
point me in the right direction as to what I'm missing.

John Schember

xml mailing list, project page
xml gnome org

John J. Boyer; President, Chief Software Developer
Abilitiessoft, Inc.
Madison, Wisconsin USA
Developing software for people with disabilities

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