You might think that the
function would tell you whether a SID is well-known, but it doesn't.
it tells you whether a SID exactly matches the well-known SID
For example, you can ask,
"Is this the Authenticated Users SID?"
"Is this the Everyone SID?"
But you can't ask,
"Is this any type of well-known SID?"
I guess you could enumerate through all the well-known SIDs, and check if your SID matches any of them, but that's getting kind of ugly.
If what you're interested in is whether this is a machine-relative SID (or a domain-relative SID, which is the special case where the machine is the domain controller), as opposed to a universal SID, you can check whether the SID format is S-1-5-21-#-#-#-#. All machine-relative SIDs have that form.
#define SECURITY_NT_NON_UNIQUE (0x00000015L) // decimal 21 #define SECURITY_NT_NON_UNIQUE_SUB_AUTH_COUNT (3L)
If you want to exclude
other predefined machine-relative SIDs,
you can verify that the last number
(the RID) is greater than or equal to 1000.
#define SECURITY_OTHER_ORGANIZATION_RID (0x000003E8L)