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.TH DISK 2

.SH NAME

opendisk, Disk \- generic disk device interface

.SH SYNOPSIS

.nf

.ft L

#include <u.h>

#include <libc.h>

#include <disk.h>

.ft

.PP

.ft L

typedef struct Disk {

	char *prefix;

	char part[NAMELEN];

	int fd, wfd, ctlfd, rdonly;

	int type;

	vlong secs, secsize, size, offset;

	int c, h, s;

} Disk;

.ft

.PP

.B

Disk* opendisk(char *file, int rdonly, int noctl)

.SH DESCRIPTION

These routines provide a simple way to gather

and use information about 

.IR floppy (3)

and

.IR sd (3)

disks and disk partitions,

as well as plain files.

.PP

.I Opendisk

opens

.I file

for reading and stores the file descriptor in

the 

.B fd

field of the

.B Disk

structure.

If 

.I rdonly

is not set, 

.I opendisk

also opens

.I file

for writing and stores that file descriptor in

.BR wfd .

The two file descriptors are kept separate to

help prevent accidents.

.PP

If

.I noctl

is not set, 

.I opendisk

looks for a

.B ctl

file in the same directory as the

disk file;

if it finds one, it declares

the disk to be

an

.I sd

device,

setting the

.B type

field in the

.B Disk

structure

to

.BR Tsd .

If the passed

.I file

is named

.BI fd n disk \fR,

it looks for a file

.BI fd n ctl \fR,

and if it finds that,

declares the disk to be

a floppy disk, of type

.BR Tfloppy .

If either

control

file is found, it is opened for reading

and writing, and the resulting file descriptor

is saved as 

.BR ctlfd .

Otherwise the returned disk

has type

.BR Tfile .

.PP

.I Opendisk

then stats the file and stores its length in

.BR size .

If the disk is an

.I sd

partition, 

.I opendisk

reads the sector size from the

control

file and stores it in 

.BR secsize ;

otherwise the sector size is assumed to be 512,

as is the case for floppy disks.

.I Opendisk

then stores the disk size measured in sectors in

.BR secs .

.PP

If the disk is an

.I sd

partition, 

.I opendisk 

parses the

control

file to find the partition's offset

within its disk;

otherwise it sets

.B offset

to zero.

If the disk is an ATA disk,

.I opendisk

reads

the disk geometry (number of cylinders, heads, and sectors)

from the 

.B geometry

line in the

.I sd

control file;

otherwise it sets these to zero as well.

.B Name

is initialized with the base name of

the disk partition, and is useful for forming messages to the

.I sd

control file.

.B Prefix

is set to the passed filename without

the 

.B name

suffix.

.PP

The IBM PC BIOS interface allocates

10 bits for the number of cylinders, 8 for 

the number of heads, and 6 for the number of sectors per track.

Disk geometries are not quite so simple

anymore, but to keep the interface useful,

modern disks and BIOSes present geometries

that still fit within these constraints.

These numbers are still used when partitioning

and formatting disks.

.I Opendisk

employs a number of heuristics to discover this

supposed geometry and store it in the

.BR c ,

.BR h ,

and

.B s

fields.

Disk offsets in partition tables and

in FAT descriptors are stored in a form

dependent upon these numbers, so

.I opendisk

works hard to report numbers that

agree with those used by other operating

systems; the numbers bear little or no resemblance

to reality.

.SH SOURCE

.B /sys/src/libdisk/disk.c

.SH SEE ALSO

.IR floppy (3),

.IR sd (3)