Prerequisite – Monitors, Readers-Writers Problem
There is a shared resource that is accessed by multiple processes i.e. readers and writers. Any number of readers can read from the shared resource simultaneously, but only one writer can write to the shared resource at a time. When a writer is writing data to the resource, no other process can access the resource. A writer cannot write to the resource if there are any readers accessing the resource at that time. Similarly, a reader can not read if there is a writer accessing the resource or if there are any waiting writers.
The Reader-Writer problem using a monitor can be implemented using pthreads. The POSIX threads (or pthread) libraries are a standards-based thread API for C/C++. The library provides the following synchronization mechanisms:
-
Mutexes (pthread_mutex_t) – Mutual exclusion lock:
Block access to variables by other threads. This enforces exclusive access by a thread to a variable or set of variables.
-
Condition Variables – (pthread_cond_t):
The condition variable mechanism allows threads to suspend execution and relinquish the processor until some condition is true.
Implementation of Reader-Writer solution using pthread library:
Execute the program using the following command in your Linux system
$g++ -pthread program_name.cpp $./a.out or $g++ -pthread program_name.cpp -o object_name $./object_name
Code:
// Reader-Writer problem using monitors #include <iostream> #include <pthread.h> #include <unistd.h> using namespace std;
class monitor {
private :
// no. of readers
int rcnt;
// no. of writers
int wcnt;
// no. of readers waiting
int waitr;
// no. of writers waiting
int waitw;
// condition variable to check whether reader can read
pthread_cond_t canread;
// condition variable to check whether writer can write
pthread_cond_t canwrite;
// mutex for synchronization
pthread_mutex_t condlock;
public :
monitor()
{
rcnt = 0;
wcnt = 0;
waitr = 0;
waitw = 0;
pthread_cond_init(&canread, NULL);
pthread_cond_init(&canwrite, NULL);
pthread_mutex_init(&condlock, NULL);
}
// mutex provide synchronization so that no other thread
// can change the value of data
void beginread( int i)
{
pthread_mutex_lock(&condlock);
// if there are active or waiting writers
if (wcnt == 1 || waitw > 0) {
// incrementing waiting readers
waitr++;
// reader suspended
pthread_cond_wait(&canread, &condlock);
waitr--;
}
// else reader reads the resource
rcnt++;
cout << "reader " << i << " is reading\n";
pthread_mutex_unlock(&condlock);
pthread_cond_broadcast(&canread);
}
void endread( int i)
{
// if there are no readers left then writer enters monitor
pthread_mutex_lock(&condlock);
if (--rcnt == 0)
pthread_cond_signal(&canwrite);
pthread_mutex_unlock(&condlock);
}
void beginwrite( int i)
{
pthread_mutex_lock(&condlock);
// a writer can enter when there are no active
// or waiting readers or other writer
if (wcnt == 1 || rcnt > 0) {
++waitw;
pthread_cond_wait(&canwrite, &condlock);
--waitw;
}
wcnt = 1;
cout << "writer " << i << " is writing\n";
pthread_mutex_unlock(&condlock);
}
void endwrite( int i)
{
pthread_mutex_lock(&condlock);
wcnt = 0;
// if any readers are waiting, threads are unblocked
if (waitr > 0)
pthread_cond_signal(&canread);
else
pthread_cond_signal(&canwrite);
pthread_mutex_unlock(&condlock);
}
} // global object of monitor class M; void * reader( void * id)
{ int c = 0;
int i = *( int *)id;
// each reader attempts to read 5 times
while (c < 5) {
usleep(1);
M.beginread(i);
M.endread(i);
c++;
}
} void * writer( void * id)
{ int c = 0;
int i = *( int *)id;
// each writer attempts to write 5 times
while (c < 5) {
usleep(1);
M.beginwrite(i);
M.endwrite(i);
c++;
}
} int main()
{ pthread_t r[5], w[5];
int id[5];
for ( int i = 0; i < 5; i++) {
id[i] = i;
// creating threads which execute reader function
pthread_create(&r[i], NULL, &reader, &id[i]);
// creating threads which execute writer function
pthread_create(&w[i], NULL, &writer, &id[i]);
}
for ( int i = 0; i < 5; i++) {
pthread_join(r[i], NULL);
}
for ( int i = 0; i < 5; i++) {
pthread_join(w[i], NULL);
}
} |
Output: