[Mono-list] Does FileMode.Append have O_APPEND semantics on Unix?

Jonathan Pryor jonpryor at vt.edu
Wed May 3 07:08:57 EDT 2006

On Wed, 2006-05-03 at 08:43 +0200, Jaroslaw Kowalski wrote:
> My question is: is FileMode.Append supposed to have O_APPEND semantics on 
> Unix?

Sadly, no.  Mono's System.IO layer is layered upon io-layer, which
exports a Win32 API.  FileMode.Append is mapped to OPEN_ALWAYS for the
CreateFile() call, and OPEN_ALWAYS is mapped to O_CREAT on Unix.
O_APPEND never enters the picture.

> If not, what's the best way to open file with this append semantics in 
> mono? I've seen you have MonoIO class, but I'm not sure how it can be used. 

MonoIO is how managed code interacts with the runtime io-layer, which
uses Win32 API calls for its implementation.  You can't get append
semantics with MonoIO.

If you really want O_APPEND, you can always use Mono.Unix.UnixStream
within Mono.Posix.dll:

        int fd = Mono.Unix.Native.Syscall.open ("path",
        Stream s = new UnixStream (fd);
        // Write to `s'...

However, using O_APPEND isn't necessarily a panacea -- the open(2) man
page mentions:

        O_APPEND may lead to corrupted files on  NFS  file  systems  if
        more  than one process appends data to a file at once.  This is
        because NFS does not support appending to a file, so the client
        kernel  has  to simulate it, which can’t be done without a race

So using your existing named-mutex solution may be the better solution,
at least if your users will potentially use NFS.

 - Jon

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