Fuse3(3pm)

📦 version 0.021, in ubuntu stonking

💖 Filesys::Fuse3 💖

okay bestie, so this is Fuse3(3pm) from the User Contributed Perl Documentation section of the manual, and it's literally the whole entire guide to writing your own filesystems in Perl. slay. let's get into it 💅

✨ NAME ✨

Filesys::Fuse3 — write filesystems in Perl using Fuse3 🦋

✨ SYNOPSIS ✨

use Filesys::Fuse3;

Filesys::Fuse3::main(
    mountpoint => '/mnt/my_fs',
    threaded   => 0,
    debug      => 1,

    getattr    => sub { ... }, # fetches attributes, like 'stat'
    getdir     => sub { ... }, # obtains directory listings
    open       => sub { ... }, # opens files
    statfs     => sub { ... }, # returns filesystem data
    read       => sub { ... }, # reads file contents

    # there are many more you can implement!
    # See "FUNCTIONS YOUR FILESYSTEM MAY IMPLEMENT" below.
);
not me showing you the whole vibe in five lines of code, she's that efficient 💀

✨ DESCRIPTION ✨

This module lets you implement filesystems in Perl, through the FUSE <http://fuse.sourceforge.net> (Filesystem in USErspace) kernel/lib interface.

FUSE expects you to implement callbacks for the various functions.

In the following definitions, "errno" can be 0 (for a success), -EINVAL, -ENOENT, -EONFIRE, any integer less than 1 really.

You can import standard error constants by saying something like:

use POSIX qw(EDOTDOT ENOANO);

Every constant you need (file types, open() flags, error values, etc) can be imported either from POSIX or from Fcntl, often both. See their respective documentations, for more information.

🎀 EXPORTED SYMBOLS 🎀

None by default. (tragic, i know)

You can request all exportable symbols by using the tag :all. 💖

You can request the extended attribute symbols by using the tag :xattr. This will export XATTR_CREATE and XATTR_REPLACE.

🌸 FUNCTIONS 🌸

💅 Filesys::Fuse3::main

Takes arguments in the form of hash key=>value pairs. There are many valid keys. Most of them correspond with names of callback functions, as described in section 'FUNCTIONS YOUR FILESYSTEM MAY IMPLEMENT'. A few special keys also exist:

  • debug => boolean — This turns FUSE call tracing on and off. Default is 0 (which means off).
  • mountpoint => string — The point at which to mount this filesystem. There is no default, you must specify this. An example would be '/mnt'.
  • mountopts => string — This is a comma separated list of mount options to pass to the FUSE kernel module.

At present, it allows the specification of the allow_other argument when mounting the new FUSE filesystem. To use this, you will also need 'user_allow_other' in /etc/fuse.conf as per the FUSE documentation

mountopts => "allow_other" or
mountopts => ""
  • threaded => boolean — This turns FUSE multithreading on and off. The default is 0, meaning your FUSE script will run in single-threaded mode. Note that single-threaded mode also means that you will not have to worry about reentrancy, though you will have to worry about recursive lookups. In single-threaded mode, FUSE holds a global lock on your filesystem, and will wait for one callback to return before calling another. This can lead to deadlocks, if your script makes any attempt to access files or directories in the filesystem it is providing. (This includes calling stat() on the mount-point, statfs() calls from the 'df' command, and so on and so forth.) It is worth paying a little attention and being careful about this.

Enabling multithreading will cause FUSE to make multiple simultaneous calls into the various callback functions of your perl script. If you enable threaded mode, you can enjoy all the parallel execution and interactive response benefits of threads, and you get to enjoy all the benefits of race conditions and locking bugs, too. (iconic honestly) Please also ensure any other perl modules you're using are also thread-safe.

(If enabled, this option will cause a warning if your perl interpreter was not built with USE_ITHREADS, or if you have failed to use threads or threads::shared.)

  • nullpath_ok => boolean — This flag tells Filesys::Fuse3 to not pass paths for functions that operate on file or directory handles. This will yield empty path parameters for functions including read, write, flush, release, fsync, readdir, releasedir, fsyncdir, truncate, fgetattr and lock. If you use this, you must return file/directory handles from open, opendir and create. Default is 0 (off). Only effective on Fuse 2.8 and up; with earlier versions, this does nothing.
  • utimens_as_array => boolean — This flag causes timestamps passed via the utimens() call to be passed as arrays containing the time in seconds, and a second value containing the number of nanoseconds, instead of a floating point value. This allows for more precise times, as the normal floating point type used by Perl (double) loses accuracy starting at about tenths of a microsecond.
  • nopath => boolean — Flag indicating that the path need not be calculated for the following operations:

read, write, flush, release, fsync, readdir, releasedir, fsyncdir, ftruncate, fgetattr, lock, ioctl and poll

Closely related to nullpath_ok, but if this flag is set then the path will not be calculated even if the file wasn't unlinked. However the path can still be defined if it needs to be calculated for some other reason.

Only effective on Fuse 2.9 and up.

  • utime_omit_ok => boolean — Flag indicating that the filesystem accepts special UTIME_NOW and UTIME_OMIT values in its "utimens" operation.

If you wish to use these constants, make sure to include the ':utime' flag when including the Fuse module, or the ':all' flag.

Only effective on Fuse 2.9 and up.

🫶 Filesys::Fuse3::fuse_get_context

use Filesys::Fuse3 'fuse_get_context';

my $caller_uid = fuse_get_context()->{uid};
my $caller_gid = fuse_get_context()->{gid};
my $caller_pid = fuse_get_context()->{pid};

Access context information about the current Filesys::Fuse3 operation.

⭐ Filesys::Fuse3::fuse_version

Indicates the fuse version in use; more accurately, indicates the version of the fuse API in use at build time. If called in scalar context, the version will be returned as a decimal value; i.e., for fuse API v2.6, will return "2.6". If called in array context, an array will be returned, containing the major, minor and micro version numbers of the fuse API it was built against.

🧚 Filesys::Fuse3::fuse_buf_size

Computes the total size of a buffer vector. Applicable for "read_buf" and "write_buf" operations.

🧚 Filesys::Fuse3::fuse_buf_copy

Copies data from one buffer vector to another. Primarily useful if a buffer vector contains multiple, fragmented chunks or if it contains an FD buffer instead of a memory buffer. Applicable for "write_buf".

🔥 Filesys::Fuse3::notify_poll

Only available if the Fuse module is built against libfuse 2.8 or later. Use fuse_version() to determine if this is the case. Calling this function with a pollhandle argument (as provided to the "poll" operation implementation) will send a notification to the caller poll()ing for I/O operation availability. If more than one pollhandle is provided for the same filehandle, only use the latest; you can send notifications to them all, but it is unnecessary and decreases performance.

⚠️ ONLY supply poll handles fed to you through "poll" to this function. Due to thread safety requirements, we can't currently package the pointer up in an object the way we'd like to to prevent this situation, but your filesystem server program may segfault, or worse, if you feed things to this function which it is not supposed to receive. If you do anyway, we take no responsibility for whatever Bad Things(tm) may happen. 😭

🔥 Filesys::Fuse3::pollhandle_destroy

Only available if the Fuse module is built against libfuse 2.8 or later. Use fuse_version() to determine if this is the case. This function destroys a poll handle (fed to your program through "poll"). When you are done with a poll handle, either because it has been replaced, or because a notification has been sent to it, pass it to this function to dispose of it safely.

⚠️ ONLY supply poll handles fed to you through "poll" to this function. Due to thread safety requirements, we can't currently package the pointer up in an object the way we'd like to to prevent this situation, but your filesystem server program may segfault, or worse, if you feed things to this function which it is not supposed to receive. If you do anyway, we take no responsibility for whatever Bad Things(tm) may happen. 💀

👑 FUNCTIONS YOUR FILESYSTEM MAY IMPLEMENT 👑

getattr

Arguments: filename.

Returns a list, very similar to the 'stat' function (see perlfunc). On error, simply return a single numeric scalar value (e.g. "return -ENOENT();").

FIXME: the "ino" field is currently ignored. I tried setting it to 0 in an example script, which consistently caused segfaults. 🫠

Fields (the following was stolen from perlfunc(1) with apologies):

(
 $dev, $ino, $mode, $nlink, $uid, $gid, $rdev, $size,
 $atime, $mtime, $ctime, $blksize, $blocks
) = getattr($filename);

Here are the meaning of the fields:

 0 dev      device number of filesystem
 1 ino      inode number
 2 mode     file mode  (type and permissions)
 3 nlink    number of (hard) links to the file
 4 uid      numeric user ID of file's owner
 5 gid      numeric group ID of file's owner
 6 rdev     the device identifier (special files only)
 7 size     total size of file, in bytes
 8 atime    last access time in seconds since the epoch
 9 mtime    last modify time in seconds since the epoch
10 ctime    inode change time (NOT creation time!) in seconds
            since the epoch
11 blksize  preferred block size for file system I/O
12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If you wish to provide sub-second precision timestamps, they may be passed either as the fractional part of a floating-point value, or as a two-element array, passed as an array ref, with the first element containing the number of seconds since the epoch, and the second containing the number of nanoseconds. This provides complete time precision, as a floating point number starts losing precision at about a tenth of a microsecond. So if you really care about that sort of thing... and honestly, valid ✨

Arguments: link pathname.

Returns a scalar: either a numeric constant, or a text string.

This is called when dereferencing symbolic links, to learn the target.

example rv: return "/proc/self/fd/stdin";

mknod

Arguments: Filename, numeric modes, numeric device

Returns an errno (0 upon success, as usual).

This function is called for all non-directory, non-symlink nodes, not just devices. she contains multitudes

mkdir

Arguments: New directory pathname, numeric modes.

Returns an errno.

Called to create a directory.

Arguments: Filename.

Args: a pathname. Returns an errno.

This is the one that deletes a file, a device, or a symlink. Gone bestie, poof. 💨

rmdir

Args: a pathname. Returns an errno.

Called to remove a directory. Yes queen, tear down the whole folder. 🗂️

Args: existing filename, symlink name. Returns an errno.

Called to create a symbolic link — the little shortcut king, lowkey doing the most. ✨

rename

Args: old filename, new filename. Returns an errno.

Called to rename a file, and/or move a file from one directory to another. Glow-up relocation era. 💅

Args: existing filename, hardlink name. Returns an errno.

Called to create hard links. Two names, one inode, no cap. 🔗

chmod

Args: pathname, numeric modes. Returns an errno.

Called to change permissions on a file/directory/device/symlink. Setting boundaries, iconic behaviour tbh. 🚧

chown

Args: pathname, numeric uid, numeric gid. Returns an errno.

Called to change ownership of a file/directory/device/symlink. It's giving "new owner, new me". 👑

truncate

Args: pathname, numeric offset. Returns an errno.

Called to truncate a file, at the given offset. ✂️

utime

Args: pathname, numeric actime, numeric modtime. Returns an errno.

Called to change access/modification times for a file/directory/device/symlink.

open

Args: pathname, numeric flags (which is an OR-ing of stuff like O_RDONLY and O_SYNC, constants you can import from POSIX), fileinfo hash reference.

Returns an errno, a file handle (optional).

The fileinfo hash reference contains information from the fuse open() call which may be modified by the module.

The only fileinfo fields presently supported are:

  • direct_io (version 2.4 onwards)
  • keep_cache (version 2.4 onwards)
  • nonseekable (version 2.8 onwards)

Your open() method needs only check if the operation is permitted for the given flags, and return 0 for success. Optionally a file handle may be returned, which will be passed to subsequent read, write, flush, fsync and release calls.

read

Args: pathname, numeric requested size, numeric offset, file handle.

Returns a numeric errno, or a string scalar with up to $requestedsize bytes of data.

Called in an attempt to fetch a portion of the file. 📖

write

Args: pathname, scalar buffer, numeric offset, file handle. You can use length($buffer) to find the buffersize. Returns length($buffer) if successful (number of bytes written).

Called in an attempt to write (or overwrite) a portion of the file. Be prepared because $buffer could contain random binary data with NULs and all sorts of other wonderful stuff. 😭

statfs

Args: none.

Returns any of the following:

  • -ENOANO()

or

  • $namelen, $files, $files_free, $blocks, $blocks_avail, $blocksize

or

  • -ENOANO(), $namelen, $files, $files_free, $blocks, $blocks_avail, $blocksize

flush

Args: pathname, file handle.

Returns an errno or 0 on success.

Called to synchronise any cached data. This is called before the file is closed. It may be called multiple times before a file is closed. 🚿

release

Args: pathname, numeric flags passed to open, file handle, flock_release flag (when built against FUSE 2.9 or later), lock owner ID (when built against FUSE 2.9 or later).

Returns an errno or 0 on success.

Called to indicate that there are no more references to the file. Called once for every file with the same pathname and flags as were passed to open.

fsync

Args: pathname, numeric flags.

Returns an errno or 0 on success.

Called to synchronise the file's contents. If flags is non-zero, only synchronise the user data. Otherwise synchronise the user and meta data. 💾

setxattr

Args: pathname, extended attribute's name, extended attribute's value, numeric flags (which is an OR-ing of XATTR_CREATE and XATTR_REPLACE).

Returns an errno or 0 on success.

Called to set the value of the named extended attribute.

If you wish to reject setting of a particular form of extended attribute name (e.g.: regexps matching user\..* or security\..*), then return -EOPNOTSUPP.

If flags is set to XATTR_CREATE and the extended attribute already exists, this should fail with -EEXIST. If flags is set to XATTR_REPLACE and the extended attribute doesn't exist, this should fail with -ENOATTR.

XATTR_CREATE and XATTR_REPLACE are provided by this module, but not exported by default. To import them:
use Filesys::Fuse3 ':xattr';

or:

use Filesys::Fuse3 ':all';

getxattr

Args: pathname, extended attribute's name.

Returns an errno, 0 if there was no value, or the extended attribute's value.

Called to get the value of the named extended attribute. 🔍

listxattr

Args: pathname.

Returns a list: 0 or more text strings (the extended attribute names), followed by a numeric errno (usually 0).

removexattr

Args: pathname, extended attribute's name.

Returns an errno or 0 on success.

Removes the named extended attribute (if present) from a file. 🧹

opendir

Args: pathname of a directory. Returns an errno, and a directory handle (optional).

Called when opening a directory for reading. If special handling is required to open a directory, this operation can be implemented to handle that. 🎀

readdir

Args: pathname of a directory, numeric offset, (optional) directory handle.

Returns a list of 0 or more entries, followed by a numeric errno (usually 0). The entries can be simple strings (filenames), or arrays containing an offset number, the filename, and optionally an array ref containing the stat values (as would be returned from getattr()).

This is used to read entries from a directory. It can be used to return just entry names like getdir(), or can get a segment of the available entries, which requires using array refs and the 2- or 3-item form, with offset values starting from 1. If you wish to return the parameters to fill each entry's struct stat, but do not wish to do partial entry lists/entry counting, set the first element of each array to 0 always.

Note that if this call is implemented, it overrides getdir() ALWAYS.

releasedir

Args: pathname of a directory, (optional) directory handle.

Returns an errno or 0 on success.

Called when there are no more references to an opened directory. Called once for each pathname or handle passed to opendir(). Similar to release(), but for directories. Accepts a return value, but like release(), the response code will not propagate to any corresponding closedir() calls.

fsyncdir

Args: pathname of a directory, numeric flags, (optional) directory handle.

Returns an errno or 0 on success.

Called to synchronize any changes to a directory's contents. If flag is non-zero, only synchronize user data, otherwise synchronize user data and metadata. 💫

init

Args: none.

Returns (optionally) an SV to be passed as private_data via fuse_get_context().

destroy

Args: (optional) private data SV returned from init(), if any.

Returns nothing.

access

Args: pathname, access mode flags.

Returns an errno or 0 on success.

Determine if the user attempting to access the indicated file has access to perform the requested actions. The user ID can be determined by calling fuse_get_context(). See access(2) for more information. 🚪

create

Args: pathname, create mask, open mode flags.

Returns errno or 0 on success, and (optional) file handle.

Create a file with the path indicated, then open a handle for reading and/or writing with the supplied mode flags. Can also return a file handle like open() as part of the call. 🌱

lock

Args: pathname, numeric command code, hashref containing lock parameters, (optional) file handle.

Returns errno or 0 on success.

Used to lock or unlock regions of a file. Locking is handled locally, but this allows (especially for networked file systems) for protocol-level locking semantics to also be employed, if any are available.

See the Fuse documentation for more explanation of lock(). The needed symbols for the lock constants can be obtained by importing Fcntl. 🔐

utimens

Args: pathname, last accessed time, last modified time.

Returns errno or 0 on success.

Like utime(), but allows time resolution down to the nanosecond. By default, times are passed as "numeric" (internally these are typically represented as "double"), so the sub-second portion is represented as fractions of a second. If you want times passed as arrays instead of floating point values, for higher precision, you should pass the "utimens_as_array" option to "Filesys::Fuse3::main".

Note that if this call is implemented, it overrides utime() ALWAYS.

bmap

Args: pathname, numeric blocksize, numeric block number.

Returns errno or 0 on success, and physical block number if successful.

Used to map a block number offset in a file to the physical block offset on the block device backing the file system. This is intended for filesystems that are stored on an actual block device, with the 'blkdev' option passed. 🧱

ioctl

Args: pathname, ioctl command code, flags, data if ioctl op is a write, (optional) file handle.

Returns errno or 0 on success, and data if ioctl op is a read.

Used to handle ioctl() operations on files. See ioctl(2) for more information on the fine details of ioctl operation numbers. May need to h2ph system headers to get the necessary macros; keep in mind the macros are highly OS-dependent.

Keep in mind that read and write are from the client perspective, so read from our end means data is going out, and write means data is coming in. It can be slightly confusing. 🙈

poll

Args: pathname, poll handle ID (or undef if none), event mask, (optional) file handle.

Returns errno or 0 on success, and updated event mask on success.

Used to handle poll() operations on files. See poll(2) to learn more about event polling. Use IO::Poll to get the POLLIN, POLLOUT, and other symbols to describe the events which can happen on the filehandle. Save the poll handle ID to be passed to "notify_poll" and "pollhandle_destroy" functions, if it is not undef. Threading will likely be necessary for this operation to work.

There is not an "out of band" data transfer channel provided as part of FUSE, so POLLPRI/POLLRDBAND/POLLWRBAND won't work.

Poll handle is currently a read-only scalar; we are investigating a way to make this an object instead. 👀

write_buf

Args: pathname, offset, buffer vector, (optional) file handle.

Write contents of buffer to an open file.

Similar to the "write" method, but data is supplied in a generic buffer. Use fuse_buf_copy() to transfer data to the destination if necessary.

read_buf

read_buf 🌈

Args it gets handed: Pathname, size, offset, buffer vector, and a maybe-there file handle.

It grabs data from an open file and parks it in a buffer, bestie. Think of it as the "read" method's cooler cousin — same energy, except the data gets stored and handed back inside a generic buffer instead.

And here's the slay part: no actual copying of data has to take place 💅. You can literally just drop the source file descriptor into the 'fd' member of the buffer access hash (and OR the 'flags' member with FUSE_BUF_IS_FD) and let somebody else retrieve it later.

Also — and this is the detail people miss — if you OR FUSE_BUF_FD_SEEK with 'flags', then the 'pos' member needs to hold the offset (in bytes) to seek to inside that file descriptor. Don't skip that, it's giving necessary.

If instead you're actually reading data in, the read data goes in the 'mem' member of the buffer access hash, and you should update the 'size' member if you ended up reading less than what was requested. Periodt.

flock 🔒

Args: pathname, a maybe-there file handle, unique lock owner ID, operation ID.

Performs BSD-style file locking operations. Very demure, very mindful.

The operation ID is gonna be one of LOCK_SH, LOCK_EX or LOCK_UN. And if it's a non-blocking lock request, that's shown by having LOCK_NB OR'd into the value.

More deets live in the flock(2) manpage. For the lock symbols themselves, do:

use Fcntl qw(flock);

Locking gets handled locally, but this still lets protocol-level locking semantics get employed too — especially for networked file systems — if any of those happen to be available. iykyk 🌸

fallocate ✨

Args: pathname, a maybe-there file handle, mode, offset, length.

Allocates space for an open file. Iconic behaviour honestly.

What it does: makes sure the required space is actually allocated for the specified file. And the guarantee is the good part — if this function returns success, then any subsequent write request to the specified range is guaranteed not to fail because of a lack of space on the file system media 👑

🎀 EXAMPLES 🎀

There's a lil handful of example scripts chilling in the examples/ subdirectory. Behold:

  • example.pl — a simple "Hello world" type of script 🌷
  • loopback.pl — a filesystem loopback-device. Like fusexmp from the main FUSE dist, it just recurses file operations into the real filesystem. Unlike fusexmp, though, it only re-shares files under the /tmp/test directory 👀
  • rmount.pl — an NFS-workalike that tunnels through SSH. It needs an account on some ssh server (obviously) with public-key authentication enabled (ngl, if you have to type in a password, you don't have this — see man ssh_keygen for more info). Copy rmount_remote.pl to your home directory on the remote machine, make a subdir somewhere, and then run it like:
./rmount.pl host /remote/dir /local/dir
  • rmount_remote.pl — a ripoff of loopback.pl, meant to be used as a backend for rmount.pl 🙈

💀 BUGS 💀

  • fuse3 "copy_file_range", "lseek", "statx" callbacks are not implemented. Not a vibe, but we said what we said.

👑 AUTHOR 👑

Original Fuse author: Mark Glines, mark@glines.org.

Ported to fuse3 by Dominique Dumont, domi.dumont@free.fr<gt>.

🌈 SEE ALSO 🌈

perl, the FUSE documentation <https://libfuse.github.io/doxygen/index.html>.

Okay bestie that's the whole page, we ate and left no crumbs 🫶 this one's from perl v5.42.3, dated 2026-08-20, and it's Fuse3(3pm) — stay locked in and go build something cute 💖✨

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