2. INTRODUCTION TO PROBLEM
• Multiple reader/writer activities can run simultaneously.
• At any time, a reader activity may wish to read data.
• Or a writer activity may wish to write or modify the data at any instant.
• Simultaneous reading and writing to the data can cause inconsistency.
• Therefore, to avoid this situation, a set of rules is followed,
• Any number of readers can access the data simultaneously.
• But during the time a writer is writing, no other reader or writer may access
the shared data.
3. SHARING OF DATA
•Readers: only read the data set they do not perform any updates
•Writers: can both read and write
6. I. First readers/writers problem (reader priority):
No reader will wait (for other readers to finish) even if a writer
is waiting
Writer starvation possible
Semaphore solution
II. Second reader/writers problem (writer priority):
No new readers allowed once a writer has asked for access
Solution can be implemented using monitors
7. SEMAPHORE
Definition:
Semaphore is non-negative integer variable (It’s denoted by
‘S’).
Types of semaphore:
1) Binary Semaphore(value between 0 & 1)
2) Counting Semaphore(value are 1,2,3…..)
Functions are used to modify the value of semaphore variable.
1) wait(S) = S – 1
2) Signal(S) = S + 1
8. Here we have used three semaphore variables for solving this
problem:
1)Mutex initialize to 1 (Binary Semaphore)
2)Wrt initialize to 1 (Binary Semaphore)
3)Readcount initialize to 0 (Counting Semaphore
SOLUTION USING SEMAPHORE
Mutex: Used to lock critical section for both readers and writers.
Wrt: Used to block the writers from entering the critical section.
Read count : Helps counting the number of readers in the critical
section and permits the writer to enter when it become zero.
9. STRUCTURE OF READER PROCESS
wait (mutex);
readcount ++;
if (readcount == 1)
wait (wrt);
signal(mutex);
. .READ THE OBJECT . .
wait(mutex);
readcount --;
if readcount == 0)
signal (wrt); s
signal(mutex);
• The mutex semaphore ensures mutual exclusion
and wrt handles the writing mechanism and is
common to the reader and writer process code.
• As soon as readcount becomes 1, wait operation is
used on wrt.
• This means that a writer cannot access the object
anymore. After the read operation is done,
readcount is decremented. When readcount
becomes 0, signal operation is used on wrt. So a
writer can access the object now.
10. STRUCTURE OF WRITER PROCESS
• If a writer wants to access the
object, wait operation is
performed on wrt.
• After that no other writer can
access the object.
• When a writer is done writing
into the object, signal operation
is performed on wrt.
do {
// writer requests for critical section
wait(wrt);
// performs the write
// leaves the critical section
signal(wrt);
}while(true);
