[xml] xmlNewNode allocates memory that ist already in use

[Eric Zurcher <Eric Zurcher csiro au>]

Thomas Kutzer wrote:

> With the second solution a new problem occured which I want to describe
> here:
>
> Following two lines of code:
>
>       temp2 = (xmlChar *)IntToStr(i_value).c_str();
>       nodePtr3 = xmlNewTextChild(NodePtr3, NULL, (xmlChar *)"value",
> temp2);
>
> After the call of xmlNewTextChild there is just "0" in the part of memory
> to which temp2 adresses.

This is NOT a problem with libxml2, the Borland compiler, its runtime 
library or byte alignment. It IS a problem with the way Thomas is using 
a C++ "temporary" object.

In this example, the Borland runtime's IntToStr function returns an 
object of type String. This object's c_str() member function returns a 
pointer to its internal string buffer, and this pointer is then stored 
in "temp2". But once this statement has executed, the String object goes 
out of scope, and so its destructor is called. This leaves temp2 holding 
an invalid pointer, since it points to an address which has been 
deallocated.

The same problem would occur with the following equivalent code, but 
here the scoping problem is a little more explicit.

 {
   String s = IntToStr(i_value);
   temp2 = (xmlChar *)s.c_str();
 } // At this point, s is destroyed; temp2 now points to freed memory!
 nodePtr3 = xmlNewTextChild(NodePtr3, NULL, (xmlChar *)"value", temp2);

> xmlMalloc allocates a part of memory to which temp2 also adresses.
> When I compile the programm with these lines of code the fault doesn't
> occur:
>
>       String s = IntToStr(i_value);
>       temp2 = (xmlChar *)s.c_str();
>       nodePtr3 = xmlNewTextChild(NodePtr3, NULL, (xmlChar *)"value",
> temp2);
>
> Has anyone an idea how I can eliminate this fault?

Yes, the code you show here will eliminate this fault, since the String 
object remains in scope. It should also work to use:

 nodePtr3 = xmlNewTextChild(NodePtr3, NULL, (xmlChar *)"value", 
(xmlChar *)IntToStr(i_value).c_str());

> Thanks
> Thomas Kutzer

--
Eric Zurcher
CSIRO Livestock Industries
Canberra, Australia
Eric Zurcher csiro au