How to call a Win32API function from Ruby?

Question

I want to lock a file as MS Office applications do from a Ruby program so that deletions won't be allowed because "the file is opened on another program".

The Ruby standard library does not seem to be able to do that - I just tried flock() - so I'm trying to invoke the LockFileEx function.

fd = File.open("locked.file", File::RDWR|File::CREAT, 0644)
fd.write "this file to be locked"
import_array = %w(p i i i i i)
wapi = Win32API.new('kernel32', 'LockFileEx', import_array, "i")
wapi.call(fd, 1, 0, 0, 0, 0)

The wapi.call fails with a TypeError exception "Can't convert File into String".

What should I use as first item in import_array to represent the file handle ?

How do I pack the file descriptor into a String ? Where do I get the file descriptor structure ?

I'm using Ruby 1.9.3.

Answer 1

First, you have to map the Ruby file descriptor to a C runtime file descriptor. I'm not familiar enough with Ruby source to know how to do that; it may be an identity transform.

Second, you have to map the C runtime descriptor to a Win32 file handle. For this, you need _get_osfhandle .

Third, you need to fix your call to LockFileEx so that you actually pass a valid OVERLAPPED structure; NULL will NOT work.

Answer 2

The Ruby file locking mechanism is co-operative and relies on all parties knowing the convention of the Ruby lock file. Microsoft Office will not co-operate.

Instead I suggest that you get the file system to enforce the lock. Simply open the file with an exclusive lock using standard Ruby file handling mechanisms.