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Is there any way to create a block device via user space in OSX (10.8+), without 3rd party libraries (FUSE, etc)?

I am trying to create a userspace tool. The idea is the user supplies a file to the tool and the tool creates a virtual interface. Whenever the interface is written to, the tool applies an operation to the data and then writes to the original file. Whenever the interface is read from, the tool reads from the original file and applies the inverse operation.

I don't necessarily need a block device. The tool needs to create some kind of virtual interface that can be treated as a file, i.e. it can be opened and saved to by another application.

I looked at the userspace routines of the I/O Kit, but nothing seemed aplicable, as creating a virtual USB/FireWire/MMC/SCSI device seems excessive.

Ethan Reesor
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1 Answers1

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The closest thing I can think of without going into kernel space is a fifo or named pipe - this is essentially a pipe with a file name. Unfortunately, being a pipe, you can't seek, etc. - you can just open the fifo for reading in one process and for writing in another. You create these either with the mkfifo command line utility or the mkfifo() C function.

There are also UNIX domain sockets, which are similar to IP sockets but again are identified by a file name, not a networking construct. These can be read and written from both ends, but again no seeking.

As for actually implementing the ops functions for a vnode (the in-kernel representation of a file in OSX) I believe you do have to drop to the kernel. Note that the I/O kit isn't strictly necessary for creating a block device - if the BSD device nodes are sufficient, and you don't need support for hardware or ejecting volumes etc. you can simply create a node with bdevsw_add(), supplying the ops vector as a parameter. (for an I/O Kit based storage device, the IOMediaBSDClient does this automatically, along with creating the character device with cdevsw_add_with_bdev()) Another, rather more elaborate option is to implement your own file system.

pmdj
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  • Hi, in your answer you mention the option to build filesystem. If I only care about the low level operations just as "read page/write page/page-in/page-out", is it possible to mount the standard filesystem (HFS+) on top of the device file (`/dev/my_device`), and enjoy both worlds ? – Zohar81 Feb 13 '18 at 09:21
  • @Zohar81 I don't see why not as VFS happens at the BSD layer, but I haven't tried it. If you want partition table support and such you will definitely need to go with an IOKit block storage device. – pmdj Feb 13 '18 at 11:42