The filesystem is entirely maintained in memory, no external storage is used. File data is allocated in 512 byte blocks. If a maximum size is specified, the actual maximum is rounded down to the nearest whole number of blocks:

actualsize := (size / 512) * 512;

The root of the filesystem is owned by the user who invoked memfs and is created with Read, Write and Execute permissions for the owner and Read and Execute permissions for everyone else (8r755).

-s

Serve styx on file descriptor 0. The mountpoint and other mount options are ignored.

-r

The default option. The old mountpoint directory becomes a union directory consisting of just the root of the new filesystem.

-a

Add the root of the new filesystem to the end of the union directory represented by the mountpoint directory.

-b

Add the root of the new filesystem to the beginning of the union directory represented by the mountpoint directory.