tojblockd was designed to allow USB devices (such as smartphones) to act as mass storage devices and to let users manipulate their data files via mass storage access.
Normally, exporting a filesystem as USB mass storage has significant drawbacks: the filesystem in question has to be unmounted while it's exported, and it has to be a filesystem type that's suitable for most desktop computers (in practice, that's VFAT, with UDF as a contender).
This makes exporting the user's home directory problematic, and it means that files on the exported filesystem become invisible to apps while the mass storage mode is active.
tojblockd takes a directory tree and whips up a VFAT image that can be exported. It reads the directory tree via the normal filesystem interface, so there are no limitations on the target directory.
To save resources, the VFAT image is constructed only as needed. Files from the target directory are just mapped in and are not read until the USB host asks for them.
tojblockd effectively takes a snapshot of the directory tree and filenames when it starts up. If files disappear while the directory is exported, the USB host will get I/O errors when trying to read their data blocks. If new files are created while the directory is exported, they won't be visible in the image until tojblockd is restarted.
The intended configuration is that tojblockd is started whenever the USB cable is plugged in and the system is switched to mass storage mode, and tojblockd is stopped again whenever that stops being the case.
The simplest way to run tojblockd is to start it with a directory
name as argument. It will open /dev/nbd0 and configure it to
serve as a block device representing that directory tree.
For real use, it must be tied to the USB detection and mode switching systems. The details will depend on the platform it's running on. There's no published example setup yet.
tojblockd uses Linux-specific interfaces (particularly the network block device interface) and was developed for use with Nemo Mobile and Mer.
The kernel it runs on must have the nbd driver either compiled in
or inserted as a module.
tojblockd is under the GPLv2+. See the files LICENSE.md and COPYING for details.