The first call to GetTokenInformation (). What for?

Question

A look at the MSDN documentation for GetTokenInformation () and Obtaining an example SID login , GetTokenInformation () needs to be called twice. The first call is to get the size of the buffer.

So, buffer size for what? Just say that I use TokenUser as the second parameter, I see that dwReturnLength returned by the first call is not the size of the TOKEN_USER structure.

Thanks in advance

Answer 1

The structure TOKEN_USERcontains pointers (in particular, a pointer to SID, which itself has a variable length). These pointers should point somewhere. The API function expects the buffer to be large enough to hold not only the structure TOKEN_USER, but also everything that the structure indicates. The function tells you how much memory it needs for everything. It will be in adjacent memory.

Answer 2

The full example from your second URL should clearly indicate how the length returned from the first call is used. You use this to allocate raw memory of this size - this is a variable ptg- and give it to PTOKEN_GROUPS for use in the second call.

// Get required buffer size and allocate the TOKEN_GROUPS buffer.

   if (!GetTokenInformation(
         hToken,         // handle to the access token
         TokenGroups,    // get information about the token groups 
         (LPVOID) ptg,   // pointer to TOKEN_GROUPS buffer
         0,              // size of buffer
         &dwLength       // receives required buffer size
      )) 
   {
      if (GetLastError() != ERROR_INSUFFICIENT_BUFFER) 
         goto Cleanup;

      ptg = (PTOKEN_GROUPS)HeapAlloc(GetProcessHeap(),
         HEAP_ZERO_MEMORY, dwLength);

      if (ptg == NULL)
         goto Cleanup;
   }

// Get the token group information from the access token.

   if (!GetTokenInformation(
         hToken,         // handle to the access token
         TokenGroups,    // get information about the token groups 
         (LPVOID) ptg,   // pointer to TOKEN_GROUPS buffer
         dwLength,       // size of buffer
         &dwLength       // receives required buffer size
         )) 
   {
      goto Cleanup;
   }

Answer 3

:

TokenInformation [out, optional]

. , TokenInformationClass .

TokenInformationLength [in]

, TokenInformation. TokenInformation - NULL, .

ReturnLength [out]

, , TokenInformation. TokenInformationLength, .

TokenInformationClass TokenDefaultDacl DACL, , ReturnLength, sizeof (TOKEN_DEFAULT_DACL) DefaultDacl TOKEN_DEFAULT_DACL NULL.

, № 2, API . .

, .

, Win32 API. .

Answer 4

SID , SID, .

Answer 5

You can do this work on your first call if your buffer is large enough. Most codes that use these methods will first try with a fixed-size buffer, and then allocate a larger buffer if the call indicates that it needs more memory.