Re: [xml] using xmlDocSetRootElement to move nodes between documents

[Igor Zlatkovic <igor zlatkovic com>]

On 27/07/04 19:11, Martijn Faassen wrote:
> Hi there,
>
> I've spent quite a bit of trying to track down a segfault in my code, 
> and I think I may have found the cause. The following test code:
>
> #include "libxml/tree.h"
>
> int main() {
>   xmlDoc* doc1;
>   xmlDoc* doc2;
>   xmlNode* node;
>
>   doc1 = xmlParseDoc("<doc>Foo</doc>");
>   doc2 = xmlNewDoc("1.0");
>   node = xmlDocGetRootElement(doc1);
>   xmlDocSetRootElement(doc2, node);
>   xmlFreeDoc(doc1);
>   xmlFreeDoc(doc2);
> }
>
> doesn't seem to be safe; xmlFreeDoc(doc2) apparently tries to free 
> something already freed in xmlFreeDoc(doc1). This is what valgrind says:
>
> ==14012== Memcheck, a memory error detector for x86-linux.
> ==14012== Copyright (C) 2002-2004, and GNU GPL'd, by Julian Seward.
> ==14012== Using valgrind-2.1.1, a program supervision framework for 
> x86-linux.
> ==14012== Copyright (C) 2000-2004, and GNU GPL'd, by Julian Seward.
> ==14012== For more details, rerun with: -v
> ==14012==
> ==14012== Invalid read of size 4
> ==14012==    at 0x3C06BD64: xmlFreeNodeList (in /usr/lib/libxml2.so.2.6.11)
> ==14012==    by 0x3C06BD9F: xmlFreeNodeList (in /usr/lib/libxml2.so.2.6.11)
> ==14012==    by 0x3C06972C: xmlFreeDoc (in /usr/lib/libxml2.so.2.6.11)
> ==14012==    by 0x8048607: main (in /home/faassen/working/lxml/foo)
> ==14012==  Address 0x3C163918 is 80 bytes inside a block of size 88 free'd
> ==14012==    at 0x3C01F918: free (vg_replace_malloc.c:127)
> ==14012==    by 0x3C0696FD: xmlFreeDoc (in /usr/lib/libxml2.so.2.6.11)
> ==14012==    by 0x80485FC: main (in /home/faassen/working/lxml/foo)
> ==14012==
> ==14012== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 17 from 1)
> ==14012== malloc/free: in use at exit: 670 bytes in 20 blocks.
> ==14012== malloc/free: 43 allocs, 23 frees, 13112 bytes allocated.
> ==14012== For a detailed leak analysis,  rerun with: --leak-check=yes
> ==14012== For counts of detected errors, rerun with: -v
>
> is moving nodes between documents in general not supported this way?

No, it isn't. You must call xmlUnlinkNode to get the node out of the 
first document. Then you can insert it into the new document. And take a 
good care about namespaces, should either document use them.

Ciao,
Igor