Getting a file size (on Windows)

The first point I’d like to make is that you’re using a Microsoft Windows API, so you have already lost. You are just not quite aware of how much you have lost.

A quick look around and you say “Ahh… GetFileSize, that’s what I want to do!” Except, of course, you’re wrong. You don’t want to use GetFileSize at all. It has the following signature:

DWORD WINAPI GetFileSize(  __in       HANDLE hFile,

__out_opt  LPDWORD lpFileSizeHigh


Yes, it supports larger than 4GB files! How? A pointer to the high-order doubleword is passed in! So how do you know if this errored? Return -1? WRONG! Because the high word could have been set and your file length could legitimately be 0x1ffffffff. So to find out if you actually had an error, you must call GetLastError! Instead of one call, you now have two.

The Microsoft documentation even acknowledges that this is stupid: “Because of this behavior, it is recommended that you use GetFileSizeEx instead.”

GetFileSizeEx is presumably named “Ex” as in “i broke up with my ex because their API sucked.”

You now have something that looks like this:

BOOL WINAPI GetFileSizeEx(  __in   HANDLE hFile,

__out  PLARGE_INTEGER lpFileSize


Which starts to look a little bit nicer. For a start, the return code of BOOL seems to indicate success or failure.

You now get to provide a pointer to a LARGE_INTEGER. Which, if you missed it, a LARGE_INTEGER is:

typedef union _LARGE_INTEGER {  struct {

DWORD LowPart;

LONG HighPart;


struct {

DWORD LowPart;

LONG HighPart;

} u;




Why this abomination? Well… ” If your compiler has built-in support for 64-bit integers, use the QuadPart member to store the 64-bit integer. Otherwise, use the LowPart and HighPart members to store the 64-bit integer.”

That’s right kiddies… if you’ve decided to loose from the get-go and have a compiler that doesn’t support 64-bit integers, you can still get the file size! Of course, you’re using a compiler that doesn’t have 64bit integer support… and the Microsoft documentation indicates that the GetFileSizeEx call requires Windows 2000… so it’s post y2k and you’re using a compiler without 64-bit ints? You have already lost.

Oh, but you say something about binary compatibility for apps written in the old days (handwave something like that). Well… let’s see… IRIX will give you 64bit numbers in stat (stat64) unless you build with -o32 – giving you the old ABI. I just can’t see a use for GetFileSize….. somebody please enlighten me.

Which header would you include? Any Linux/UNIX person would think of something logical – say sys/stat.h (Linux man page says sys/types.h, sys/stat.h and unistd.h). No, nothing sensible like that. It’s “Declared in WinBase.h; include Windows.h”.

So… you thought that obviously somebody went through the API and gave you all this Ex function goodness to get rid of mucking about with parts of a 64bit int? You were wrong. Let me say it with this:

DWORD WINAPI GetCompressedFileSizeTransacted(
  __in       LPCTSTR lpFileName,
  __out_opt  LPDWORD lpFileSizeHigh,
  __in       HANDLE hTransaction

I’ll now tell you that this was introduced in Vista/Server 2008.

Obviously, you want to be able to use Transaction NTFS on Windows Vista with a compiler that doesn’t have 64 bit ints. Oh, and you must then make another function call to see if something went wrong?

But you know what… perhaps we can get away from this complete and utter world of madness and use stat()…. or rather… perhaps _stati64().

Well… you’d be fine except for the fact that these seem to lie to you (at least on Windows Server 2003 and Vista) – it seems that even Explorer can lie to you.

But perhaps you’ve been barking up the wrong tree… you obviously don’t want to find the file size at all – what you want is to FindFirstFile! No, you don’t want FindFirstFileEx (in this case, Ex is for Extremely complicated). It’s meant to be faster too… you know, maybe.

So remember kids, smoke your crack pipe – you’re going to need it if using this thing called the Microsoft Windows File Management Functions.

3 thoughts on “Getting a file size (on Windows)

  1. Maybe this is why money is charged for most software on windows. No one would want to do it for fun, so it has to be done for money. And it requires /so/ much sanity to be forcefully removed that no one would want to do it without suitable recompense.

    Stu: Is there a reason why the label for the subscribe button below is blank?

  2. SS: thanks for taking the time to post this. I was pulling my hair out with GetFileSize, trying to understand why on earth it kept resulting in an invalid handle error. It was clearly getting a valid file handle because my app is able to successfully write to that file! FindFirstFile works beautifully. Micro$oft’s APIs have become a convoluted mess.. I feel your pain!

  3. “Any Linux/UNIX person would think of something logical – say sys/stat.h (Linux man page says sys/types.h, sys/stat.h and unistd.h”
    Actually posix, or worse, linux doesn’t make any much sense too.
    It’s only logical because you already knew it.
    Except perhaps for the close similarity with C STD API, all those file APIs are a mess, now give me my C++ STL and hide all this shit from me.

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