DISK(2)DISK(2)NAME
opendisk, Disk - generic disk device interface
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <disk.h>
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;
Disk* opendisk(char *file, int rdonly, int noctl)
DESCRIPTION
These routines provide a simple way to gather and use information about
floppy(3) and sd(3) disks and disk partitions, as well as plain files.
Opendisk opens file for reading and stores the file descriptor in the
fd field of the Disk structure. If rdonly is not set, opendisk also
opens file for writing and stores that file descriptor in wfd. The two
file descriptors are kept separate to help prevent accidents.
If noctl is not set, opendisk looks for a ctl file in the same direc‐
tory as the disk file; if it finds one, it declares the disk to be an
sd device, setting the type field in the Disk structure to Tsd. If the
passed file is named fdndisk, it looks for a file fdnctl, and if it
finds that, declares the disk to be a floppy disk, of type Tfloppy. If
either control file is found, it is opened for reading and writing, and
the resulting file descriptor is saved as ctlfd. Otherwise the
returned disk has type Tfile.
Opendisk then stats the file and stores its length in size. If the
disk is an sd partition, opendisk reads the sector size from the con‐
trol file and stores it in secsize; otherwise the sector size is
assumed to be 512, as is the case for floppy disks. Opendisk then
stores the disk size measured in sectors in secs.
If the disk is an sd partition, opendisk parses the control file to
find the partition's offset within its disk; otherwise it sets offset
to zero. If the disk is an ATA disk, opendisk reads the disk geometry
(number of cylinders, heads, and sectors) from the geometry line in the
sd control file; otherwise it sets these to zero as well. Name is ini‐
tialized with the base name of the disk partition, and is useful for
forming messages to the sd control file. Prefix is set to the passed
filename without the name suffix.
The IBM PC BIOS interface allocates 10 bits for the number of cylin‐
ders, 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. Opendisk employs a number of
heuristics to discover this supposed geometry and store it in the c, h,
and s fields. Disk offsets in partition tables and in FAT descriptors
are stored in a form dependent upon these numbers, so opendisk works
hard to report numbers that agree with those used by other operating
systems; the numbers bear little or no resemblance to reality.
SOURCE
/sys/src/libdisk/disk.c
SEE ALSOfloppy(3), sd(3)DISK(2)
