Re: [xml] libxml and threads

[Daniel Veillard <veillard redhat com>]

On Wed, Feb 23, 2005 at 09:54:27AM +0100, Massimo Cafaro wrote:
> Everything works fine, but I have found that many memory leaks are 
> associated with libxml2 APIs. The leaks are small, but they are so many 
> that this is a serious problem. Now, I strongly believe that it is 
> entirely my fault, but I need to understand exactly how to use libxml2 
> in threaded applications. 

[...]
> Leak: 0x0110f690  size=48       string 'MUTX' 
>         Call stack: [thread 280c000]: | 0x0 | _pthread_body | 
> thread_starter | provider_thr | grelc_get_sql | 
> xmlSAXParseMemoryWithData | xmlCreateMemoryParserCtxt | 
> xmlNewParserCtxt | xmlInitParserCtxt | xmlDefaultSAXHandlerInit | 
> __xmlDefaultSAXHandler | xmlGetGlobalState | xmlNewGlobalState | 
> xmlInitializeGlobalState | xmlInitGlobals | xmlNewMutex | malloc | 
> malloc_zone_malloc  

  The threads code is the only one which can legally do direct malloc()
without using xmlMalloc(), so it's among the memory use I won't detect
leaks in regression tests.

> It looks like a mutex object is never deallocated. 
>
> The code looks like 
>
> xmlDocPtr doc; 
> ... 
>
> doc = xmlParseMemory (docname, strlen(docname) - 1); 
> ... 
> xmlCleanupParser(); 
>
>
> Is this the way to write this kind of code? I was not able to find any 
> example of threaded code. I have found some APIs for dealing with 
> threads tough. 

  You must make sure you made the call to the library initialization
xmlInitParser() before any call to the library in the main thread,
and make sure you never call xmlParserCleanup() unless nothing will
ever use the library or library produced data.

> mutex object and call related lock/unlock functions). The libxml2 I am 
> using is the latest one officially released on January. I am working on 
> Mac oS X 10.3.8 

  I don't use Mac oS X, I know Apple people now use libxml2 and they
should be able to track down that problem better than me. It is a
thread local storage problem and is usually platform specific. The
memory you're pointed out should be freed by xmlCleanupGlobals() 
which is called by xmlCleanupParser() and to be released only once
in the whole application lifetime. I can't see how this specific call
can lead to " many memory leaks".
  I.e. I looked at your problems but don't see how this could lead to
what you are describing.

Daniel

-- 
Daniel Veillard      | Red Hat Desktop team http://redhat.com/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
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