Implementing an access method in practice

GnomeVFS - Filesystem Abstraction library
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Implementing an access method in practice

Implementing a new access method is really not difficult at all. This section explains how this is done.

Using shared libraries

Every module must be compiled as a shared library (i.e. a .so file).

The current way for accessing the right module for the right method is very simple, and is based on file names. In practice, a module implementing access method named foo must be named libfoo.so. For example, the module implementing the ftp: access method is called libftp.so, the module implementing #gzip access is called libgzip.so and so on.

This might change in the future.

The initialization/shutdown functions

Every shared library module must provide two functions:

GnomeVFSMethod *vfs_module_init (void);
void vfs_module_shutdown (GnomeVFSMethod *method);

These are the only functions that the VFS library will access directly. All the other symbols (i.e. functions and variables) in the module should be made static.

vfs_module_init() is called as soon as the module is loaded in memory. It will have to return a pointer to a GnomeVFSMethod object that will contain the pointers to the method's implementation functions. We will describe this later.

vfs_module_shutdown, instead, is called before the module is unloaded or the program that uses it dies. This functions should:

Deallocate all the memory allocated by the module.
Close all the file descriptors associated with the module.
Kill any external process spawned by the module.
In general, make sure that any operation that was going on before this function was called will be interrupted correctly, as soon as possible and without any leaks.

The GnomeVFSMethod object

This object is contains pointers to the module implementation functions.

GnomeVFSResult (* open)              (GnomeVFSMethodHandle **method_handle_return,
                                      GnomeVFSURI *uri,
                                      GnomeVFSOpenMode mode
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* create)            (GnomeVFSMethodHandle **method_handle_return,
                                      GnomeVFSURI *uri,
                                      GnomeVFSOpenMode mode,
                                      gboolean exclusive,
                                      guint perm
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* close)             (GnomeVFSMethodHandle *method_handle
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* read)              (GnomeVFSMethodHandle *method_handle,
                                      gpointer buffer,
                                      GnomeVFSFileSize num_bytes,
                                      GnomeVFSFileSize *bytes_read_return
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* write)             (GnomeVFSMethodHandle *method_handle,
                                      gconstpointer buffer,
                                      GnomeVFSFileSize num_bytes,
                                      GnomeVFSFileSize *bytes_written_return
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* seek)              (GnomeVFSMethodHandle *method_handle,
                                      GnomeVFSSeekPosition  whence,
                                      GnomeVFSFileOffset    offset
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* tell)              (GnomeVFSMethodHandle *method_handle,
                                      GnomeVFSFileOffset *offset_return);

GnomeVFSResult (* truncate)          (GnomeVFSMethodHandle *method_handle,
                                      GnomeVFSFileSize where
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* open_directory)    (GnomeVFSMethodHandle **method_handle,
                                      GnomeVFSURI *uri,
                                      GnomeVFSFileInfoOptions options,
                                      const GList *meta_keys,
                                      const GnomeVFSDirectoryFilter *filter
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* close_directory)   (GnomeVFSMethodHandle *method_handle
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* read_directory)    (GnomeVFSMethodHandle *method_handle,
                                      GnomeVFSFileInfo *file_info
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* get_file_info)     (GnomeVFSURI *uri,
                                      GnomeVFSFileInfo *file_info,
                                      GnomeVFSFileInfoOptions options,
                                      const GList *meta_keys
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* get_file_info_from_handle)
                                     (GnomeVFSMethodHandle *method_handle,
                                      GnomeVFSFileInfo *file_info,
                                      GnomeVFSFileInfoOptions options,
                                      const GList *meta_keys
                                      GnomeVFSCancellation *cancellation);

gboolean       (* is_local)          (const GnomeVFSURI *uri
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* rename)            (GnomeVFSURI *uri, const gchar *new_name
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* make_directory)    (GnomeVFSURI *uri, guint perm
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* remove_directory)  (GnomeVFSURI *uri
                                      GnomeVFSCancellation *cancellation);

GnomeVFSResult (* unlink)            (GnomeVFSURI *uri
                                      GnomeVFSCancellation *cancellation);

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GNOME Virtual File System access method implementation		Handling cancellation