How to use DirectShow and webcam to preview an image - c++

Question

I'm trying to use C++ and DirectShow to display my webcam... but I'm having some troubles.
The following code gives me a segmentation fault on:
m_pDF->EnumPins(&pinEnum);
my cpp code:
#include <tchar.h>
#include <strsafe.h>
#include <dshow.h>
#include <atlbase.h>
#include <d3d9.h>
#include <vmr9.h>
#pragma comment(lib,"Strmiids.lib")
#define SAFE_RELEASE(x) { if (x) x->Release(); x = NULL; }
HRESULT EnumerateDevices(REFGUID category, IEnumMoniker **ppEnum)
{
// Create the System Device Enumerator.
ICreateDevEnum *pDevEnum; // Video and Audio interface object
HRESULT hr = CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&pDevEnum)); // Creates the system device enumerator
if (SUCCEEDED(hr))
{
// Create an enumerator for the category.
hr = pDevEnum->CreateClassEnumerator(category, ppEnum, 0); // Enumeration of 'category' objects
if (hr == S_FALSE)
{
hr = VFW_E_NOT_FOUND; // The category is empty. Treat as an error.
}
pDevEnum->Release(); // Deletes Enumeration object
}
return hr;
}
void main(void)
{
IGraphBuilder *pGraph = NULL;
IMediaControl *pControl = NULL; // contains methods for stopping and starting the graph
IMediaEvent *pEvent = NULL; // methods for getting events from the Filter Graph Manager
IPin *m_pCamOutPin;
IBaseFilter *m_pDF=NULL;
IMoniker *pM;
IEnumMoniker *pEnum; // Enumerator object
// Initialize the COM library.
HRESULT hr = CoInitialize(NULL);
if (FAILED(hr))
{
printf("ERROR - Could not initialize COM library");
return;
}
// Create the filter graph manager and query for interfaces.
hr = CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER,
IID_IGraphBuilder, (void **)&pGraph);
if (FAILED(hr))
{
printf("ERROR - Could not create the Filter Graph Manager.");
return;
}
// Bind
hr = EnumerateDevices(CLSID_VideoInputDeviceCategory, &pEnum);
pEnum->Next(1, &pM, NULL);
pM->BindToObject(0, 0, IID_IBaseFilter, (void**)m_pDF);
pM->Release();
hr=pGraph->AddFilter(m_pDF, L"Video Capture");
CComPtr<IEnumPins> pinEnum;
m_pDF->EnumPins(&pinEnum);
hr = pinEnum->Reset();
hr = pinEnum->Next(1, &m_pCamOutPin, NULL);
if (FAILED(hr))
return;
// control
hr = pGraph->QueryInterface(IID_IMediaControl, (void **)&pControl);
hr = pGraph->QueryInterface(IID_IMediaEvent, (void **)&pEvent);
// Build the graph. IMPORTANT: Change this string to a file on your system.
hr = pGraph->Render(m_pCamOutPin);
if (SUCCEEDED(hr))
{
// Run the graph.
hr = pControl->Run();
if (SUCCEEDED(hr))
{
// Wait for completion.
long evCode;
pEvent->WaitForCompletion(INFINITE, &evCode);
// Note: Do not use INFINITE in a real application, because it
// can block indefinitely.
}
}
SAFE_RELEASE(pControl);
SAFE_RELEASE(pEvent);
SAFE_RELEASE(pGraph);
CoUninitialize();
}
Any ideas on what am I doing wrong and what do I have to do to get this to work?

Answer 1

m_pDF is a pointer to IBaseFilter:
IBaseFilter *m_pDF=NULL;
BindToObject last parameter is a pointer to a pointer. Which means this functioncall is wrong:
pM->BindToObject(0, 0, IID_IBaseFilter, (void**)m_pDF);
Instead you need to pass the address of m_pDF to get a pointer to a pointer:
pM->BindToObject(0, 0, IID_IBaseFilter, (void**)&m_pDF);
It is not a problem to cast IBaseFilter** to void**, but you can't cast IBaseFilter* to void**.

Answer 2

The crash should be related m_pDF, it not be a valid pointer.
you should add check for pM->BindToObject(0, 0, IID_IBaseFilter, (void**)m_pDF); to know that m_pDF is initialize sucess or not.
and you should also check hr=pGraph->AddFilter(m_pDF, L"Video Capture"); hr is ok or not.
if fail, you can get the error code know the reason.