Re: File descriptors and local variables

[Patrick Donnelly <batrick@...>]

On Tue, Nov 5, 2013 at 5:21 PM, Leo Razoumov <slonik.az@gmail.com> wrote:
> On 11/5/13, Paul K <paulclinger@yahoo.com> wrote:
>> Hi All,
>>
>> I've written on several occasions something that looks like this:
>>
>> local exists = io.open("/somefile") ~= nil
>>
>> What happens to the returned file handle in those cases when
>> "/somefile" exists? The documentation states that the file is closed
>> when the handle is garbage collected after going out of scope, but
>> does it go out of scope immediately (as it's not assigned to anything)
>> or only after reaching the "end" statement for the current scope?
>> Thank you.
>>
>> Paul.
>>
>
> Paul,
> GC is non-deterministic. The only guarantee it provides is that the
> variable will not be collected while it is
> still referenced. A more deterministic approach would be to close it
> immediately yourself.
> The following helper function does just that:
>
> function file_exists(filename)
>     local file = io.open(filename)
>     if file then
>             io.close(file)
>             return true
>     else
>             return false
>     end
> end

It's not quite so simple. You need to check that the error is indeed
ENOENT (the third return value of io.open). Of course, the value of
ENOENT is implementation-defined. :(

[It would be nice if the io/os library provided fields in the io/os
table for each of these (ANSI only) errors. Of course, if you're doing
something complicated you might as well use a POSIX-y library.]

-- 
Patrick Donnelly