Re: [xml] thread saftey and xmlXPathEvalExpression

On Wed, Aug 28, 2013 at 07:26:36PM +0200, Oliver Friedrich wrote:
I have the following multi thread related problem.

xmlParserCtxtPtr globalContext = xmlSchemaValidCtxtGetParserCtxt(ctxt);

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 Veillard      | Open Source and Standards, Red Hat
veillard redhat com  | libxml Gnome XML XSLT toolkit | virtualization library

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