opendisk, Disk – generic disk device interface

#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)

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 directory 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 control 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 initialized 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 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. 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.


floppy(3), sd(3)