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UNIX and LINUX פעמי עתידים
מר דב בלום מכללת עמל, באר שבע
1 History of UNIX
2 Overview of UNIX
3 Processes in UNIX
4 Memory management in UNIX
5 Input/output in UNIX
6 The UNIX file system
7 Security in UNIX
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Sockets
• Used in I/O networking (from Berkeley
UNIX)
• Created dynamically
• Allow process to connect to a stream or
device
• Like a phone socket – standard connection
• Creating a socket returns FD (file
descriptor)
• Creates something like a “pipe”
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Sockets
• Each socket supports a specific kind of
network
– Reliable byte stream
– Reliable packet stream
– Unreliable packet stream
• Protocols:
– TCP Transmission Control Protocol (byte stream)
– UDP User Datagram Protocol (unreliable packet stream)
– Various reliable packet protocols (Sequin, bridges )
– Both are above IP
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Sockets
• Each socket has an address
– 32 bit integers – 4 bytes or
– 8 hexanumbers IPv4 dotted quad 168.92.23.4)
• IPv6 128 bits = 16 bytes
– 2001:0db8:85a3:0000:0000:8a2e:0370:7334
– Or 2001:db8: 85a3:: 8a2e: 370:7334
– 2001:db8:85a3::8a2e:370:7334
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Sockets
• After socket has an address
and is on both sides (both computers)
– One side makes a listen call on the socket
which creates a buffer and blocks
– The other side makes a connect call gives the
FD of the local socket and the address of the
remote;
– The remote accepts the call and the connection
is made ( it works like a pipe)
– Read and write using the FD
– Close
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Networking
Use of sockets for networking
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Terminal Management
The main POSIX calls for managing the terminal
4 calls for setting speeds get/set i/o speed
2 calls for the special chars to erase a char or a line,
interrupt
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O (1)
1. 1 driver for each device type
2. major device number
3. purpose: separate most of OS from
hardware
4. OS has common interface with drivers
5. minor device number – if a few identical
devices
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O 2
• When a user accesses a special file, the file
system determines the major and minor
device numbers and whether it is a block or
character special file
• Major device number is used to index into
either bdevsw1 array for block special or
cdevsw for character special files
• These structures contain pointers to the
procedures to open the device, read, write
etc.
• Next slide - some of the fields of a typical
cdevsw table are shown
1 Block device switch array
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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9
UNIX I/O (3)
Some of the fields of a typical cdevsw table (char devices)
bdevsw is the table for (block devices)
Major device # is index for table
Adding a device in UNIX means adding a table entry
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O (4)
Driver has 2 parts:
• Top part runs in caller’s mode and
interfaces to system
• Bottom part runs in kernel mode
and interfaces with hardware
• Drivers can make calls to kernel
procedures
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O (5)
• Buffer Cache to minimize block reads
1. When any block (inode, directory, file )
has to be read, the cache is checked
2. If block is not in cache, read from disk
• Algorithm to control cache (limited space)
1. Move newest read to head of cache
2. Remove tail
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O (6)
The UNIX I/O system in BSD
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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UNIX I/O (7)
• Buffer Cache used for writes , too
• Any write is to the cache in RAM, not the
disk!
• Every 30 seconds Linux pdflush daemon will
write “dirty” blocks to disk
• pdflush daemon will flush block to disk if it
stays too long (even if clean)
What about char devices?
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Streams - BSD
• c-list is a block of 64 chars, a count and a
pointer to next block
• When a user reads from /dev/tty (StdIO) the
chars got from c-list to kernel code called
“line discipline” which can filter it into a
“cooked” stream
• Writes are also “cooked” (tab -> spaces;
LF/CR
• Process can ask for “raw” (eg to send data)
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Streams - System V
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Input/Output Implementation in Linux
Figure 10-22. The Linux I/O system showing one file system in detail.
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Linux Disk read scheduler
• I/O scheduler in Linux to minimize disk
head movements
• Purpose: to reorder read/write requests
• Linus Elevator scheduler :
– Doubly linked list
– New entries put into sorted place in list
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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Linux Disk read scheduler
• Linus Elevator scheduler :
– Doubly linked list
– New entries put into sorted place in list
• New Linux scheduler has “deadlines”
because old one could cause starvation
(0.5 seconds for read, 5 sec for write are
defaults)
• if read/write deadline is almost up, it takes
priority over linked list
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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IO Modules in Unix and Linux
• Unix had staticly linked drivers in the
kernel from the boot, since the IO
devices didn’t change much
• Linux world – many more device
available
• Linux – “loadable modules” can be
added while kernel is running
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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IO Modules in Linux
• What needs to be done when an IO module
is added?
–Memory Relocation
–set resources
–set interrupts
–add new device to major table
–initialize device
Unix and Linux IO (Section 5) Mr. Dov Bloom Amal College of Technology
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