# General Information

## Getting started

A file system is implemented by subclassing the `pyfuse3.Operations` class and implementing the various request handlers. The handlers respond to requests received from the FUSE kernel module and perform functions like looking up the inode given a file name, looking up attributes of an inode, opening a (file) inode for reading or writing or listing the contents of a (directory) inode.

By default, pyfuse3 uses asynchronous I/O using Trio, and most of the documentation assumes that you are using Trio. If you’d rather use asyncio, take a look at asyncio Support. If you would like to use Trio (which is recommended) but you have not yet used Trio before, please read the Trio tutorial first.

An instance of the operations class is passed to `pyfuse3.init` to mount the file system. To enter the request handling loop, run `pyfuse3.main` in a trio event loop. This function will return when the file system should be unmounted again, which is done by calling `pyfuse3.close`.

All character data (directory entry names, extended attribute names and values, symbolic link targets etc) are passed as `bytes` and must be returned as `bytes`.

For easier debugging, it is strongly recommended that applications using pyfuse3 also make use of the faulthandler module.

## Lookup Counts

Most file systems need to keep track which inodes are currently known to the kernel. This is, for example, necessary to correctly implement the unlink system call: when unlinking a directory entry whose associated inode is currently opened, the file system must defer removal of the inode (and thus the file contents) until it is no longer in use by any process.

FUSE file systems achieve this by using “lookup counts”. A lookup count is a number that’s associated with an inode. An inode with a lookup count of zero is currently not known to the kernel. This means that if there are no directory entries referring to such an inode it can be safely removed, or (if a file system implements dynamic inode numbers), the inode number can be safely recycled.

The lookup count of an inode is increased by every operation that could make the inode “known” to the kernel. This includes e.g. `lookup`, `create` and `readdir` (to determine if a given request handler affects the lookup count, please refer to its description in the `Operations` class). The lookup count is decreased by calls to the `forget` handler.

## FUSE and VFS Locking

FUSE and the kernel’s VFS layer provide some basic locking that FUSE file systems automatically take advantage of. Specifically:

- Calls to `rename`, `create`, `symlink`, `mknod`, `link` and `mkdir` acquire a write-lock on the inode of the directory in which the respective operation takes place (two in case of rename).

- Calls to `lookup` acquire a read-lock on the inode of the parent directory (meaning that lookups in the same directory may run concurrently, but never at the same time as e.g. a rename or mkdir operation).

- Unless writeback caching is enabled, calls to `write` for the same inode are automatically serialized (i.e., there are never concurrent calls for the same inode even when multithreading is enabled).