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I'm trying to get an old-style VB6 application to read a large (greater than 2GB) binary file. I declare a buffer as:

Dim TCBuffer as String
TCPBuffer = String(4096, Chr(0))

And read the data in a loop using:

Get #FileNum, , TCPBuffer

But once the current position reaches 2GB, the line above raises an error.

Run-time error '63':

Bad record number

So I wondered about using Scripting.FileSystemObject instead. However, it appears this object is very limited. For one thing, it appears you can only create and open text files.

Any suggestions on how I can read a large binary file from VB6?

UPDATE:

Thinking about this some more, another option would be to use API functions. But one of the real limitations of VB6 here is the lack of unsigned data types. So even using API functions would be a trick.

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  • VB6 is so old that when it was current there were no files allowed larger than 2GB, and I don't believe it supported more than 32-bit integers (which would max out at 2GB). No numeric support for the value + no large file support in the OS (Win95 at the time) = no files > 2GB in VB6. You might try a memory mapped file, but I'm not sure how you would use it from that dinosaur of a language. Commented May 24, 2018 at 0:18
  • @KenWhite: Yes, correct. Functions like FileLen and LOF return a negative number, which I can convert to the correct value. But there is nothing I can do with the error described above. That's why I wondered about using something like Scripting.FileSystemObject, which I assume is much newer. Commented May 24, 2018 at 0:20
  • Hmm... I don't know if that would work or not, given the inherent VB6 limitations. Might be worth a shot. If not, can you use something else to split the large file into smaller segments that VB6 can use? (I'm assuming you have a reason you have to use VB6, but can you split the task?) Commented May 24, 2018 at 0:26
  • @KenWhite: Well, that's a thought if I can't find anything better. But, for that matter, I don't think VB can do that either! Commented May 24, 2018 at 0:28
  • My thought would have been to see if there's a way to work with the API and a memory-mapped file, but I have no idea how I would do that in VB6. I'm also not sure that would work, either, because of the integer range issues. Commented May 24, 2018 at 0:33

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I ended up creating a file class that uses the Windows API internally.

Any function that accepts or returns a file size or offset is of type Double. A little code converts that value to and from an unsigned long, even though it must be stored in a signed Long when being passed to the API (because VB6/VBA doesn't support unsigned longs).

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You best bet is probably to use the Windows API to memory map the file, using the CreateFile, CreateFileMapping, and MapViewOfFile functions. Refer to the Microsoft documentation on "Creating a File Mapping object" for more information on how the API works, though obviously you'll need to translate the specifics to VB. I did a little searching and found some old articles still around with some pointers, and an archive of sample code from Visual Basic Programmer's Journal (see "The Persistence Of Memory (July 1996)"), so you're not the only person who's needed to do this and it seems to be an established technique. Keep in mind that for a file that big in a 32-bit process you'll need to map just part of the file at a time into your address space.

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I don't understand what benefit comes from a memory mapped file. Why not just use the API file functions to read the file? Memory mapped files will take extra effort, and I don't see the win.
@JonathanWood Mainly so that you can map just part of the file into memory at a time. Your process only has 32 bits of address space to work with. Though if you only need sequential access to the file, and there's an API you can use that won't try to load the whole file at once, maybe there is a better option. I just generally think of memory mapping when trying to read part of a file at once.
Yes, ReadFile() reads just the requested number of bytes. This is the approach I've started. There are a few issues, but I think I have them worked out. We'll see how it goes.

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