Why am I reading garbage data or getting acces violations? - c++

Question

I'm using Windows Web Services to access a WCF service and it creates a an array of pointers of the results.
I had a similar issue where I was also reading garbage data and I though it might be caused by sendign this data to another function that was causing the pointers to get invalidated but now I'm storing it in a vector reference to try to avoid such issues but the problem persists. I'm not sure what do do anymore.
I also tried using ListaDeMaterialesBE** lstmat = new ListaDeMaterialesBE*; but that did not work at all.
Here is the code:
void svc_listMaterials(const size_t& idProject, std::vector<ListaDeMaterialesBE>& result) {
HRESULT hr = ERROR_SUCCESS;
WS_ERROR* error = NULL;
WS_HEAP* heap = NULL;
WS_SERVICE_PROXY* proxy = NULL;
WS_ENDPOINT_ADDRESS address = {};
const WS_STRING url = WS_STRING_VALUE(L"http://localhost/SIMSE_Service.ServicioEstadistico.svc");
address.url = url;
WS_HTTP_BINDING_TEMPLATE templateValue = {};
ULONG maxMessageSize = 2147483647;
WS_CHANNEL_PROPERTY channelProperty [ 1 ];
channelProperty [ 0 ].id = WS_CHANNEL_PROPERTY_MAX_BUFFERED_MESSAGE_SIZE;
channelProperty [ 0 ].value = &maxMessageSize;
channelProperty [ 0 ].valueSize = sizeof(maxMessageSize);
WS_CHANNEL_PROPERTIES channelProperties;
channelProperties.properties = channelProperty;
channelProperties.propertyCount = 1;
templateValue.channelProperties = channelProperties;
hr = WsCreateError(NULL, 0, &error);
if (FAILED(hr)) {
}
hr = WsCreateHeap(2048, 512, NULL, 0, &heap, error);
WS_HTTP_BINDING_TEMPLATE templ = {};
hr = BasicHttpBinding_IServicioEstadistico_CreateServiceProxy(&templateValue, NULL, 0, &proxy, error);
hr = WsOpenServiceProxy(proxy, &address, NULL, error);
// The issue starts here
ListaDeMaterialesBE** lstmat;
unsigned int rcount;
hr = BasicHttpBinding_IServicioEstadistico_ListarMaterialesPorProyecto(proxy,
idProject,
&rcount,
&lstmat,
heap,
NULL,
NULL,
NULL,
error);
// Here is where we seem to be reading garbage data
if (rcount > 1 && rcount < 10000) {
for (size_t i = 1; i <= rcount; ++i) {
ListaDeMaterialesBE t = **lstmat;
result.push_back(t);
lstmat++;
}
}
if (proxy) {
WsCloseServiceProxy(proxy, NULL, NULL);
WsFreeServiceProxy(proxy);
}
if (heap) {
WsFreeHeap(heap);
}
if (error) {
WsFreeError(error);
}
if (FAILED(hr)) {
std::cout << "errored \n";
}
}

Answer 1

I think the reason for the error is your wrong use of the returned secondary pointer.
I created a simple example to simulate the problem:
#define COUNT 10
void f(int** pp)
{
*pp = new int[COUNT];
for (int i = 0; i < COUNT; i++)
{
(*pp)[i] = i;
}
}
int main(int argc, const char* argv[])
{
int** pp = new int*;
f(pp);
for (int i = 0; i < COUNT; i++)
{
cout << **pp;
pp++;
}
return 0;
}
The function f obtains a secondary pointer, this secondary pointer saves the first address of an array, and then writes it.
Then use your method to traverse the array, it will cause an access exception:
The reason for the error is that when you use the secondary pointer to perform an auto-increment operation, the distance it moves each time is not the distance of an element you think.
So you should get the first address of the array it saves through the secondary pointer, and then traverse:
int** pp = new int*;
f(pp);
auto p = *pp;
for (int i = 0; i < COUNT; i++)
{
cout << *p;
p++;
}
Then we can read the correct data: