Lazarus: How to check if file is in use?

Question

I need to check if a file is in use before I try to get access to it. Is there way to do it in Lazarus environment?

Answer 1

Open the file with FileOpen(FileName, fmOpenReadWrite or fmShareExclusive) and check the result.

Update (thanks to Ken White's comment). This is not the direct answer to your question as FileOpen actually gets access to the file, but you shouldn't perform the check and then open the file, otherwise you'll get a race condition. You should open the file and then check if the open succeeded.

Opening a file with FileOpen and accessing the file through its handle may seem unfamiliar. You can achieve the same goal when opening a file with e.g. Rewrite inside a try-except block. I am not sure about Lazarus, but in Delphi using try-except with high-level file routines requires explicit resetting IOResult in case of exception (put SetInOutRes(0) into except section), otherwise the following file operation fails.

Answer 2

You don't. You try to open it, and handle the exception if and when you can't.

Checking to see if it's in use first serves no purpose.

Your code to see if it's in use says it's not
--->>> Another app opens the file, locking it
Your code to open file fails

Instead, use a normal try..except block:

try
  FS := TFileStream.Create(YourFileName, fmOpenReadWrite, fmShareExclusive);
  // Code to use file here
except
  // Handle failure to access file
end;