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Diskblocks: Block, Disk, tempfile - temporary storage of variable-sized blocks


include "diskblocks.m";
diskblocks := load Diskblocks Diskblocks->PATH;

Block: adt {
   addr: big;   # address on file
   n:    int;   # size in bytes

Disk: adt {
   init:    fn(fd: ref Sys->FD, gran: int, maxblock: int): ref Disk;
   new:     fn(d: self ref Disk, n: int): ref Block;
   release: fn(d: self ref Disk, b: ref Block);
   read:    fn(d: self ref Disk, b: ref Block,
               a: array of byte, n: int): int;
   write:   fn(d: self ref Disk, b: ref Block,
               a: array of byte, n: int): ref Block;

init:     fn();
tempfile: fn(): ref Sys->FD;


Diskblocks manages a set of variable-sized blocks on a temporary file.

Init must be called before any other function in the module.

Each block has an address and a size in bytes, represented by a value of type Block.

Each file is represented by the type Disk, providing the following operations:

init(fd, gran, maxblock)
Initialises the file fd for use as temporary block storage and returns a reference to a Disk to describe it. Fd must be open for reading and writing, and must refer to a file that allows random access. Blocks are allocated in multiples of the granularity gran, in bytes; the largest possible block is maxblock bytes, which must be a multiple of gran.
Allocate a block of n bytes on Disk d and return a reference to it.
Free the Block b, making it available for reallocation.
d.write(b, a, n)
Write n bytes from array a to Block b on Disk d, returning a reference to the resulting Block. If b is nil or n exceeds b's current size, write allocates a new block (releasing b). Thus the returned value might differ from b, and must be used in subsequent IO requests.
d.read(b, a, n)
Read n bytes from Block b on Disk d into array a, returning the number of bytes read. N must not exceed b.n.

Tempfile returns a file descriptor referring to a newly-created temporary file, suitable for use by Disk.init. The file will be removed automatically when the file descriptor is closed.




A function that returns an integer returns -1 on error; a function that returns a reference returns nil on error. The system error string is set in either case.

DISKBLOCKS(2 ) Rev:  Thu Feb 15 14:43:27 GMT 2007