Working with the Unix OS

145
I/O Subsystem
Close only for the last close of the device
- kernel searches the file table to make sure that no other processes still have the device open
- several process may access the device via different file table entry
- several device files may specify the same device different inodes but same device
- for block devices, kernel searches the mount table to make sure that the device does not contain a mounted file
system
- the kernel searches the buffer cache for blocks marked "delayed write" and writes them before
invoking the device close procedure.
- kernel releases inode of the device file
Kernel algorithms for read and write are similar to those of a regular file.
The kernel can transmit data directly between address space and the device, or device drivers may buffer data
internally e.g. terminal drivers use clist to buffer data.
- memory mapped - status registers
- programmed I/O - execute instructions
- direct memory access (DMA) - used for bulk data transfer in parallel to CPU operations.
- transfer data between device and user's address space faster (one less copy, no kernel buffers).
! Strategy interface
To transmit data between the buffer cache and a device. The process must be locked in memory until the I/O
transfer is complete.
! ioctl(fd, command, arg)
Device specific
! Interrupt handlers
Many physical devices can be associated with one interrupt vector entry, the driver must be able to resolve which
device caused the interrupt.
Disk Drivers
Partitioning the disk into sections, means that some
sections can be read-only, some read-write, and some
unmounted (no access)
Section Name Start Length
Block in Blocks
0 0s0 0 64000
1 0s1 64000 192000
2 0s2 256000 256000
3 0s3 0 512000
Sections may overlap, but file systems must not.
$ ls -1 Idev/dsk15 Idev/rdsk15
br------- 2 root root 0,21 Feb 12 15:40
/dev/dsk15
crw-rw--- 2 root root 7,21 May 7 09:29
/dev/rdsk15
The kernel loops internally 4 times to read 4096 bytes
Programs that read and write the disk directly can
destroy the consistency of the file system data.
Therefore "fsck" should not run on active file system.
Figure 31. Reading Disk Data - block & raw interface
#include "fcntl.h" #include
main()
{
char buf1[4096], buf2[4096];
int fdl, fd2, i;
}
if (((fdl =open("/dev/dsk5", O_RDONLY)) == -1) ||
((fd2=open("/dev/rdsk5", O_RDONLY)) == -1))
{
printf("failure on open\n");
exit();
}
lseek(fdl, 8192L, 0);
lseek(fd2, 8l92L, 0);
if ((read(fdl, bufl, sizeof(buf1)) == -1) ||
(read(fd2, buf2, sizeof(buf2)) == -1))
{
printf("failure on read\n");
exit(0);
}
for (i = 0; i < sizeof(buf1); i++)
if (buf[I] != buf2[i])
{
printf("different at offset %d\n", i);
exit(0);
}
printf("reads match\n");