Answer Posted / raj kumar

Readers: read data, never modify it
Writers: read data and modify it
criteria:
– Each read or write of the shared data must happen within
a critical section.
– Guarantee mutual exclusion for writers.
– Allow multiple readers to execute in the critical section
at once.
In this example, semaphores are used to prevent any writing
processes from changing information in the database while
other processes are reading from the database.
semaphore mutex = 1; // Controls access to
the reader count
semaphore db = 1; // Controls access to
the database
int reader_count; // The number of
reading processes accessing the data

Reader()
{
while (TRUE) { // loop forever
down(&mutex); // gain access
to reader_count
reader_count = reader_count + 1; // increment the
reader_count
if (reader_count == 1)
down(&db); // if this is
the first process to read the database,
// a down on db
is executed to prevent access to the
// database by a
writing process
up(&mutex); // allow other
processes to access reader_count
read_db(); // read the database
down(&mutex); // gain access
to reader_count
reader_count = reader_count - 1; // decrement
reader_count
if (reader_count == 0)
up(&db); // if there are
no more processes reading from the
// database,
allow writing process to access the data
up(&mutex); // allow other
processes to access reader_countuse_data();
// use the data
read from the database (non-critical)
}

Writer()
{
while (TRUE) { // loop forever
create_data(); // create data
to enter into database (non-critical)
down(&db); // gain access
to the database
write_db(); // write
information to the database
up(&db); // release
exclusive access to the database
}

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