Re: [xml] thread saftey and xmlXPathEvalExpression

[Daniel Veillard <veillard redhat com>]

On Wed, Aug 28, 2013 at 07:26:36PM +0200, Oliver Friedrich wrote:
> Hello.
> I have the following multi thread related problem.
>
> main(){
> xmlInitParser();
> //validationstuff
> xmlParserCtxtPtr globalContext = xmlSchemaValidCtxtGetParserCtxt(ctxt);
> }
>
> threadFunction(){
> xPathObjectPtr foos = xmlXPathEvalExpression ("foo", globalContext);
> //read all the foos
> }
> (sorry I dont have the actual code here)

  Your pseudo code makes no sense.
In the first routine globalContext is of type xmlParserCtxtPtr, i.e. a
context for parsing a document
In the tread routines globalContext is of type xmlXPathContextPtr.

> The program makes a lot of new threads and some of these segfault if
> there are to many of them. So e.g. when I create 50 threads, approx.
> 10 of them segfault - the rest don't.
>
> I first thought, I could fix this by using a globalDocPtr instead of
> context and create a new context in each thread - but this will lead
> to the segfaults.
>
> I could reduce the problem to the xmlXPathEvalExpression call (since
> if I return before, everything wents fine).
> Did I use the API wrong or what is the problem (at least as you can
> say based on the piece of code)?

  You can have concurrent XPath evaluations, using each separate
XPath evaluation context, but all of them pointing to the same
*read only* document.
  Parse the document in main, allocate XPath contexts and run
XPath queries in parallel on that shared document.

Daniel

-- 
Daniel Veillard      | Open Source and Standards, Red Hat
veillard redhat com  | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | virtualization library  http://libvirt.org/