Understand the working on FltGetVolumeGuidName() - windows

Question

I want to know the implementation sequence of the function FltGetVolumeGuidName(), I basically want to get the Guid of all volumes from my system ?
Below is the code, which I tried, any help will be greatly appreciated , thanks in advance.
volumeContext->GUIDinfo.Buffer = NULL; //kernel crash here <<<<======
volumeContext->GUIDinfo.Length = 0;
volumeContext->GUIDinfo.MaximumLength = 0;
//fetching correct size
(void) FltGetVolumeGuidName(pVolumeList, &volumeContext->GUIDinfo, &BufferSizeNeeded);
//Allocating space
if (NULL == volumeContext->GUIDinfo.Buffer) {
status = STATUS_INSUFFICIENT_RESOURCES;
DbgPrint("\n STATUS_INSUFFICIENT_RESOURCES");
break;
}
//Memory allocation
volumeContext->GUIDinfo.Buffer = (PWCHAR)ExAllocatePoolWithTag(PagedPool, BufferSizeNeeded, MEMTAG_VOL_GUID);
volumeContext->GUIDinfo.Length = 0;
ASSERT(BufferSizeNeeded <= UNICODE_STRING_MAX_BYTES);
volumeContext->GUIDinfo.MaximumLength = (ULONG)BufferSizeNeeded;
ntStatus = FltGetVolumeGuidName(pVolumeList, &volumeContext->GUIDinfo, &BufferSizeNeeded);
if (ntStatus == STATUS_BUFFER_TOO_SMALL) {
DbgPrint("\n STATUS_BUFFER_TOO_SMALL");
}

Answer 1

Eryk, thank you very much. Agreed that each of the PFLT_VOLUME I get a valid pointer. Now when I call FltGetVolumeGuidName(PFLT_VOLUME (pVolumeList) &volumeContext->GUIDinfo, BufferSizeNeeded) as mentioned above and print the values in the &volumeContext->GUIDinfo = (Null) and BufferSizeNeeded = 96.
With the literature around on FltGetVolumeGuidName, I understand 1st call FltGetVolumeGuidName to get the BufferSizeNeeded and use this size and allocate memory and call again FltGetVolumeGuidName to fetch Guid.
I have a problem (kernel panic) when I initialize as below
volumeContext->GUIDinfo.Buffer = NULL; //kernel crash here <<<<======
volumeContext->GUIDinfo.Length = 0;
volumeContext->GUIDinfo.MaximumLength = 0;