Why do many Win32 Strucs contain their size as a member-CodePudding

This is not strictly a programming question but more of a design question.

Working with some strucs from the WinAPI I noticed that many of them contain a field called dwSize that simpally stores their size e.g THREADENTRY32 or BLUETOOTH_DEVICE_SEARCH_PARAMS

typedef struct tagTHREADENTRY32 {
DWORD dwSize;
DWORD cntUsage;
DWORD th32ThreadID;
DWORD th32OwnerProcessID;
LONG  tpBasePri;
LONG  tpDeltaPri;
DWORD dwFlags;
} THREADENTRY32;

Thinking about this design i find it intriguing why can't the code that using those structures use sizeof?

What is the actual thinking behind this?

CodePudding user response：

This is to support future versions of that structure that might have new fields added to it.