BASIC: How to check for an open file

Hello
How do I check in BASIC that a file is not already open by another thread when I try to open it for writing other than trapping the error “File is already open”?

This thread does not own the current file handle.
OS_Find generates the error
XOS_Find returns a number which is not the current file handle

Ron

If you know the full canonicalised path ¹, then…

• If you’re only supporting RISC OS 5.17 or later, use OS_FSControl 58 to scan the open file handles.

• Otherwise use OS_Args 254 to scan handles from 255 to 1 ² looking for those which do not have bit 11 set ³ in the Stream Status Word ⁴.

Then when you have a valid file handle, call OS_Args 7 to get the pathname associated with a file handle.

Then, it’s a simple comparison. ;-)

PS: If you need it, OS_FSControl 37 will canonicalise a filename.

¹ This means the full unambiguous path like SDFS::MyDisc.$.ThisDir.ThisFile instead of something like <My$Dir>.ThisFile.

² This means making assumptions about valid file handles, assumptions which hold… for now…

³ Bit 11 set (or =2048) means the handle is unallocated (not in use).

⁴ The Stream Status Word doesn’t appear to be documented in the wiki?!

Cheers Rick.
My Stronghelp doesn’t list FS_Control 58 (stops at 57).
OK, that’s better than having to check every handle.

but… I was hoping for something a little bit more Atomic. The time this takes, the file could have opened and closed several times from multiple requests so there’s still no guarantee of state by the time I try to open it, or even caching the file handle as this could have changed.

Any way I can trap the Open request without the error and repeatedly wait until it opens ?

Ron

I just call OS_Args,7 with 255 to 1, and if V isn’t set the name is in the supplied buffer.

My Stronghelp doesn’t list FS_Control 58 (stops at 57).

Mine is 3.38. http://www.riscos.info/downloads/stronghelp/manuals/
Ooh, there’s now a 3.39. ;-)

The time this takes, the file could have opened and closed several times from multiple requests

Are you worried about network files? Because it should be atomic if used on the local machine by software that scans the files without polling.

As for networking, that’s a big complication, because if you can’t trust the file to be opened between checking the handles and opening the file, then that pretty much holds true whether you check one file or two hundred.

Any way I can trap the Open request without the error and repeatedly wait until it opens ?

Yeah, don’t use OPENIN or OPENOUT. Directly call the XOS_Find SWI and just keep doing it until it returns a file handle and not an error.

The simple way from BASIC is to handle the error. e.g.

DEF FNopenup(file$)
errno%=0
ON ERROR LOCAL errno%=ERR: =0
I%=OPENUP(file$)
=I%