department of computer science southern illinois university edwardsville spring, 2010 dr. hiroshi...
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Department of Computer ScienceSouthern Illinois University Edwardsville
Spring, 2010
Dr. Hiroshi FujinokiE-mail: [email protected]
CS 547/490 Network Programming
Semaphore and Shared Memory
IPC2.PPT/001
CS 547/490 Network Programming
What is a problem in “pipe”?
Not flexible - only FIFO queue is implemented
FIFO
Other Data Structures
• Stack • Tree • Heap
• Binary-Tree
• B-Tree
• List • Table
• Link-ListIPC2.PPT/002
CS 547/490 Network Programming
What is a problem in “pipe” (continued)?
Not flexible - only FIFO queue is implemented
A solution: Semaphore + Shared Memory
= similar to pipe, but almost any data structure can be implemented by “semaphore + shared memory”
IPC2.PPT/003
CS 547/490 Network Programming
Shared Memory:
• Shared memory is a piece of memory space that does not belong to any particular user process.
• Shared memory can be accessed (shared) by multiple processes.
• Shared memory is created by OS.
• Two processes can send and receive data through shared memory
• No limitation in the order data is accessed (and even bi-directional)
• Race condition is there (users are responsible for avoiding race condition)
IPC2.PPT/004
CS 547/490 Network Programming
OS
Allocate/Manage
Shared Memory
WRITE
READ
Process 2Process 1
Can access any addresswithin a shared memory
Property of Shared Memory (1) “Random Access” is possible
IPC2.PPT/005
Not FIFO any more
CS 547/490 Network Programming
OS
Allocate/Manage
Shared Memory
READ
WRITE
Process 2Process 1
Property of Shared Memory (2) No limitation in “direction”
WRITE
READ
IPC2.PPT/006
CS 547/490 Network Programming
OS
Shared Memory
Process 2Process 1
READ
WRITE
WRITE
READ
OS does NOT provideprotection against
race-condition
Property of Shared Memory (3) No protection from “Race Condition”
IPC2.PPT/007
Time Server
OS
CS 547/490 Network Programming
Application Example
WriterProcess
TimeKeeper
(Delta Time)
Client 1
Client 2
Client n
RemoteClient 1
RemoteClient 2
RemoteClient n
Shared Memory
12:59:59
IPC2.PPT/008
CS 547/490 Network Programming
(1) Multiple readers can simultaneously access to the clock
(2) When the time keeper (= writer) updates the time, no client can access
(3) Once the time keeper leaves, multiple clients can access to the clock
Access Rules to the Shared Memory
IPC2.PPT/009
CS 547/490 Network Programming
START
Writer?
accept
create shared memory
folk
TimeKeeper
YES
folk
Client
NO
IPC2.PPT/010
CS 547/490 Network Programming
IPC2.PPT/011
Time Keeper (Writer) Time Client (Reader)
reader_count = reader_count +1;
if (reader_count = 1) wait (writer);
wait (mutex);
signal (mutex);
reader_count = reader_count -1;if (reader_count = 0) signal (writer);
wait (mutex);
signal (mutex);
wait (writer);
signal (writer);
The Time KeeperUpdates the current time A reader reads
the current time
writer, mutex = binary (mutual exclusion) semaphore