How do I solve memory leaks that are reported to reside in STL string?

For a school project, we have to send big files across the network., we must use Poco::XML for our data.

After our files are send over the network, it appears that the memory does not free.

Here is an example for a file of ~9 Mb on the receiving part:

valgrind --leak-check=full --show-reachable=yes -v ourExecutable parms returns:

 12,880,736 bytes in 37 blocks are definitely lost in loss record 101 of 101
    at 0x4C2747E: operator new(unsigned long) (vg_replace_malloc.c:261)
    by 0x5A3AC88: std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) (in /usr/lib64/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6.0.13)
    by 0x5A3BC4A: std::string::_Rep::_M_clone(std::allocator<char> const&, unsigned long) (in /usr/lib64/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6.0.13)
    by 0x5A3C1BB: std::string::reserve(unsigned long) (in /usr/lib64/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6.0.13)
    by 0x5A3C68E: std::string::append(std::string const&) (in /usr/lib64/gcc/x86_64-pc-linux-gnu/4.4.4/libstdc++.so.6.0.13)
    by 0x5202359: Poco::XML::Element::innerText() const (in /home/tomwij/IGS/trunk/Project/external/lib/libPocoXML.so.8)
    by 0x4145BF: NodeProtocol::getChildNodeStrValue(Poco::XML::Element*, std::string) (NodeProtocol.cpp:82)
    by 0x41544F: NodeProtocol::deserialize(std::string const&) (NodeProtocol.cpp:200)
    by 0x40B088: Node::handleClientPacket(PriorityElement*) (Node.cpp:760)
    by 0x40A04C: Node::handlePackets() (Node.cpp:574)
    by 0x4078EA: Node::run() (Node.cpp:162)
    by 0x40772D: Node::activate() (Node.cpp:138)

 LEAK SUMMARY:
    definitely lost: 12,888,036 bytes in 190 blocks
    indirectly lost: 644,979 bytes in 1,355 blocks
      possibly lost: 10,089 bytes in 27 blocks
    still reachable: 306,020 bytes in 43 blocks
         suppressed: 0 bytes in 0 blocks

The function which is right before Poco is

const string NodeProtocol::getChildNodeStrValue(Element * elem, string child)
{
    Element*  tempNode = elem->getChildElement(child);
    XMLString result(tempNode->innerText());
    string ret = string(fromXMLString(result));
    result.clear();
    return ret;
}

which calls

XMLString Element::innerText() const
{
    XMLString result;
    Node* pChild = firstChild();
    while (pChild)
    {
        result.append(pChild->innerText());
        pChild = pChild->nextSibling();
    }
    return result;
}

(Note that XMLString is std::string)

Why is the append of STL string leaking memory?

If I just assign instead of using the copy constructors it gives the same problem.

EDIT:

I'm using the latest stable GNU GCC 4.4.4 on Gentoo x64 (linux-2.6.34-gentoo-r12).

More functions from the call stack (stripped irrelevant big chunks of code / if structures):

Command * NodeProtocol::deserialize(const string & msg)
{
    DOMParser xmlParser;

    // Get the root node.
    AutoPtr<Document> doc = xmlParser.parseString(msg);
    AutoPtr<Element> rootElement = doc->documentElement();

    string root = fromXMLString(rootElement->nodeName());
    string name = getChildNodeStrValue(rootElement, "name");
    string data = getChildNodeStrValue(rootElement, "data");
    return new PutCommand(name, data);
}

and

void Node::handleClientPacket(PriorityElement * prio)
{
        Command * command = NodeProtocol::deserialize(prio->fPacket);

        // CUT: Access some properties of command, let the command execute.

        delete command;
}

and

void Node::handlePackets()
{
    PriorityElement * prio = fQueue->top();
    fQueue->pop();

    if (prio->fSource == kCLIENT)
        handleClientPacket(prio);
    else if (prio->fSource == kNODE)
        handleNodePacket(prio);

    delete prio;
}

where fQueue is:

priority_queue< PriorityElement*,  vector<PriorityElement*>, ComparisonFunction >

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