I (Colin McCormack) have written a quick package to take advantage of the Linux directory modification facility.
What this means is that you can open a directory, register your interest, and have an asynchronous script invoked when a file is added to, modified in, deleted from that directory.
I have it in mind to use this under tclhttpd.
You can get the package here: http://sourceforge.net/projects/libtclpq/
Documentation available here: http://sourceforge.net/docman/display_doc.php?docid=11924&group_id=18733
Written with CriTcl.
AK - Now this is a useful thing. Note that the documentation about directory notification [L1 ] mentions that this can trigger when files in a directory change. In other words, this can be used to write a more efficient tail.
Also note that this is restricted to Linux kernel 2.4 and higher.
Regarding the described interface of dnotify I have one nit: That the caller is burdened with the task to extract the fd to watch out of a channel handle. A better interface (IMHO) would be to accept channel handles and have the C code determine the file handle (There are functions in the Tcl C API for this: Tcl_GetChannelHandle IIRC). Channel handles for non-files (like sockets) would simply cause the command to throw an error.
CL has a still-too-vague memory that much of this is possible in Windows, also. More on this, in time.