[fpc-pascal] Is System.IOResult thread-safe? (If not, I need to replace it.)

Jonas Maebe jonas.maebe at elis.ugent.be
Wed Nov 10 13:39:57 CET 2010

On 10 Nov 2010, at 13:26, MegaBrutal wrote:

> I'm new on the list. I have a problem. I've been trying to make my
> application thread-safe by using lockfiles. I used System.Rewrite to
> lock the file, and System.IOResult to check the result. But I've
> encountered problems, and such problems are most likely occur, when
> more than one thread's trying to lock the file at the same time. I've
> been thinking what might be the problem, and I wondered if
> System.IOResult is thread-safe or not. I suppose not. :(

It is thread safe, but your function is not.

> Not a big
> deal, though, I may use an other function to lock the file in a
> thread-safe manner.
> This is my function (that tries to lock the file):
> function TSpoolObjectReader.Open: boolean;
> begin
>   {$I-} Rewrite(LockFile, 1); {$I+}

Rewrite does not cause an error if a file already exists. It simply  
overwrites it. You cannot first check whether it exists and then  
create it if it doesn't, because in between another thread could have  
created the lock file already.

I think the easiest way to get cross-platform lockfile behaviour, is to
a) create the lockfile at the start of your program (e.g., a rewrite  
followed by a close)
b) whenever you want to lock it, use

systutils.fileopen(lockfilename,fmOpenWrite or fmShareExclusive);

If that function returns -1, acquiring the lock failed.


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