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Re: [igraph] Multithreading and GraphML

From: Tamas Nepusz
Subject: Re: [igraph] Multithreading and GraphML
Date: Tue, 18 Aug 2009 23:49:11 +0100

Hi Chris,

Thanks for reporting and fixing that. My only concern is that you cannot be sure that igraph_read_graph_graphml is called in the "main" thread, so xmlInitParser() may be called from a different thread. I don't know if that matters to libxml2 or not, but the webpage states that xmlInitParser() must be called from the main thread. Do you have any comments on that? If it really matters, then we should maybe add an extra igraph_init() function to the library and make sure that Python calls it when it loads the igraph module.


On 2009.08.18., at 22:19, Chris Wj wrote:

Hi all, this is just a FYI for anyone working with igraph and doing multithreading. I fixed a bug where igraph crashes in multithreaded apps using libxml2 by adding the following line to 'foreign- graphml.c', line 997 in 'igraph_read_graph_graphml' after the variable declarations.


From http://xmlsoft.org/threads.html:
call xmlInitParser() in the "main" thread before using any of the libxml2 API

While 'igraph_read_graph_graphml' may be called multiple times, I checked the 'xmlInitParser' function to find this source:
xmlInitParser(void) {
    if (xmlParserInitialized != 0)
    if (xmlParserInitialized == 0) {

Where it a) locks on __xmlGlobalInitMutexLock(); and b) subsequently sets xmlParserInitialized to 1, so calling it multiple times doesn't really hurt (I am fairly sure of this and will report back if otherwise).

This bug manifested itself using the Python library and Python threads, which are pthreads after all.

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