Daily Archives: 11 October, 2004

Federal Election

Looks like we are going to have at least another three years with The Rodent. It also looks like they will have a majority in the senate, which will reduce the senate’s effectiveness as a house of review.

We might not have John Howard for the entire term though, since he is of retirement age. NineMSN seems to think that Peter Costello is already the leader.

It also looks like The Democrats senators up for reelection got completely wiped out, with much of their support going to The Greens.

Gnome-VFS

Robert Collins wrote an interesting critique on the gnome-vfs API. I don’t agree with all the points, and there are some reasons why the API isn’t as elegant as it could be. Below are some responses.

Initialisation

One thing that gnome_vfs_init() does is to call g_thread_init(). Before this function is called, the locking APIs in glib are no-ops. You really want this function called early on if the app is going to use threads, otherwise you will end up with inconsistencies (eg. a lock() call might be a no-op, but the unlock() call might not be if g_thread_init() is called in between).

The other issue is that gnome_vfs_init() can fail. If it is called automatically, then any function that might invoke the initialisation routine now has a new failure mode. I don’t know whether this is a real problem or not though.

Calling Style – Inconsistent Ordering

One big difference between the out parameters in gnome_vfs_open() and gnome_vfs_read() is that the first function is essentially a constructor for a file handle, while the second is a method for a file handle that fills in a provided buffer.

I’ll agree that the calling conventions are not as nice as they could be though. If they were being designed today, I suspect that they would look more like this:

GnomeVFSHandle  *gnome_vfs_open (const gchar     *text_uri,
                                 GnomeVFSOpenMode open_mode,
                                 GError         **error);
GnomeVFSFileSize gnome_vfs_read (GnomeVFSHandle  *handle,
                                 gpointer         buffer,
                                 GnomeVFSFileSize bytes,
                                 GError         **error);

Unfortunately, the GError API was not developped til the 2.0 series, while these parts of the gnome-vfs API persist from the 1.x days.

Calling Style – Inconsistent Method Naming

I agree that the gnome_vfs_truncate() function name is inconsistent. My guess as to why they chose gnome_vfs_truncate and gnome_vfs_truncate_handle() was to match the underlying truncate() and ftruncate() C library calls. This was probably a case of balancing consistent APIs with ease of transition from libc APIs to gnome-vfs APIs.

Returning data to pointers

I agree that the existing calling convention is not as nice as it could be. As I said earlier, it would probably have been designed to use GError if it was being developed today.

The GError API has a number of benefits over errno style ones, including:

1. Automatically threadsafe. The place where the error is reported is on the stack. A global variable is a problem for multi threaded apps on systems without Linux 2.6 style thread local storage (you need to do tricks like making errno into a function that returns the appropriate variable for the current thread).
2. In Robert’s example, the actual error information is looked up from a file handle. What do you do if there is no file handle involved in the function call? Also, wouldn’t gnome_vfs_open() return a NULL file handle on error?
3. For the cases where there is a file handle to look up the error info on, what happens if two threads are working with the file handle at the same time?
4. GError is consistent with other Gnome APIs 🙂

The GError API also makes it easy to pass error data up a number of call frames similar to exceptions. If your function has a GError argument, you can simply pass that same error object to other functions when you call them. If those functions fail on an error, simply return immediately, and the caller can handle the error.

Streams Interface

There actually is a stream interface available in one of the libraries both GTK and gnome-vfs depend on: GIOChannel. I guess it would be nice if gnome-vfs provided a GIOChannel implementation for VFS file handles. The main thing that would be needed here would be the io_create_watch() implementation, which would probably require exposing a file descriptor to poll on (this could probably be implemented using a pipe pretty easily).

Doing this as GObject interfaces isn’t really an option, since GIOChannel is implemented in libglib which is below libgobject, and gnome-vfs file handles aren’t GObjects. I know that at one point Ian was planning to change the various handles to GObjects, but this didn’t happen. It would probably be possible to do this kind of change while only requiring changes to VFS methods, so it can’t be completely ruled out.

Directory Interface

You can asynchronously load a directory listing using gnome_vfs_async_load_directory(). I don’t blame you for missing it — the organisation of the APIs in the various headers is a bit confusing.

Language Bindings

There are a lot of things a language binding can do to make gnome-vfs nicer to use. Some of these things include:

• If the language provides exceptions, convert error GnomeVFSResult‘s to exceptions and change the calling conventions to something more sane.
• If the language allows for runtime type checking or multiple dispatch, don’t wrap the gnome_vfs_foo() and gnome_vfs_foo_uri() functions separately. Instead, just check if a string or a GnomeVFSURI was passed in and do the right thing.
• If the language has a standard file handle interface or convention, try to implement it in the binding.

The Python bindings do some of these things, and definitely make things easier to use.