MEMFS(2)

NAME

memfs − heap based filesystem

SYNOPSIS

include "sys.m";
include "memfs.m";
memfs := load MemFS MemFS->PATH;
init: fn(): string;
newfs: fn(maxsz: int): ref Sys->FD;

DESCRIPTION

The MemFS module implements a Styx file server where all files and filesystem data structures are stored in the local heap, not on disk.

init()
The init function should be called once to initialise the internal state of MemFS. Failure to initialise is reported by the returned string. A return value of nil indicates success.

newfs(maxsz)
The newfs function takes a single argument, maxsz defining the maximum number of bytes of the new filesystem. The return value is a file descriptor, serving Styx, suitable for passing to mount (see sys-bind(2)).

Multiple filesystems of different sizes can be supported by a single instance of the module. Each filesystem is constrained by maxsz, any filesystem operations that cause the limit to be exceeded are failed with the error message:

filesystem full

File data is allocated in 512 byte blocks. The actual filesystem data limit is always a multiple of this block size:

actualmaxsz := (maxsz / 512) ∗ 512;

SOURCE

/appl/lib/memfs.b

CAVEATS

Only file data blocks are considered in the determination of heap usage, the overhead of internal data structures cannot be accounted for. MemFS could store its own data structures in data blocks, but this would impede performance.

Newly created and empty files are pre-allocated a data block in an attempt to prevent directory entries from using heap without regard to the requested maxsz.

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